Ultrafast High Intensity Laser

Plasmonics produces extreme UV light

2011-10-24T09:59:00.000-05:00

An international team of researchers has invented a simple way of creating ultrashort pulses of extreme ultraviolet (EUV) light. The system uses a new 3D metallic waveguide, or "nanofunnel", that coverts pulses of infrared light to EUV.

EUV light has a wavelength of around 5–50 nm, which is about 100–10 times shorter than that of visible light. As a result, ultrashort pulses of EUV light are ideal for studying fundamental physics phenomena – such as how electrons move in atoms, molecules and solids.

However, it is difficult to produce EUV radiation using conventional methods that rely on using amplified light pulses from an oscillator (a source of laser light) to ionize noble gas atoms. The electrons liberated during this process are accelerated in the light field and their surplus energy is freed as attosecond (10–18 s) pulses of light of different wavelengths. The shortest wavelengths of light can then be "filtered out" to produce a single EUV pulse – a complicated process.
Simpler way of making pulses

Now, researchers at the Korea Advanced Institute of Science and Technology (KAIST), the Max Planck Institute of Quantum Optics (MPQ) in Germany and Georgia State University (GSU) in the US have come up with a different – and much simpler – way of doing things.

The new technique works by converting femtosecond (10–15 s) infrared pulses into femtosecond EUV pulses. The process exploits surface-plasmon polaritons (SPPs), which are particle-like collective oscillations that occur when light interacts with a metal's conduction electrons.

The nanofunnel made by the KAIST-MPQ-GSU team was devised so that it concentrated incident infrared light pulses into a spot that is smaller than the wavelength of the incident light. The funnel is a metallic nanostructure made of silver that contains a hollow hole shaped like a tapered cone. The cone is just a few micrometres long and filled with xenon gas. The tip of the funnel is around 100 nm across.

The researchers sent infrared light pulses (at a rate of 75 MHz) into the funnel, which is designed so that it contains patches of metal that are positively charged, followed by patches that are negatively charged. This arrangement produces electromagnetic fluctuations on the inside walls of the funnel, which result in the creation of SPPs. These particles then travel towards the tip, where the conical shape of the funnel concentrates their fields.

"The field on the inside of the funnel can become a few hundred times stronger than the field of the incident infrared light," explains Mark Stockman of GSU. "This enhanced field results in the generation of EUV light in the Xe gas."

An important feature of the nanofunnel is that it can be produced at frequencies of up to about 75 MHz. Seung-Woo Kim, team leader at KAIST, where the experiments were carried out, adds: "Due to their short wavelength and potentially short pulse duration, EUV light pulses can be an important tool for exploring electron dynamics in atoms, molecules and solids. Electrons move very fast – on the attosecond timescale – and light flashes that are shorter than attoseconds long are therefore needed to image these particles. Although scientists routinely use attosecond light flashes for such studies, they have much lower frequencies. Our new nanofunnel could change all this."

The results are detailed in Nature Photonics.

Brightest gamma ray on Earth -- for a safer, healthier world

2011-09-20T11:15:00.004-05:00

Physicists have discovered that ultra-short duration laser pulses can interact with ionised gas to give off beams that are so intense they can pass through 20 cm of lead and would take 1.5 m of concrete to be completely absorbed.

The ray could have several uses, such as in medical imaging, radiotherapy and radioisotope production for PET (positron emission tomography) scanning. The source could also be useful in monitoring the integrity of stored nuclear waste.

In addition, the laser pulses are short enough- lasting a quadrillionth of a second- to capture the response of a nucleus to stimuli, making the rays ideal for use in lab-based study of the nucleus.

The device used in the research is smaller and less costly than more conventional sources of gamma rays, which are a form of X-rays.

The experiments were carried out on the Gemini laser in the Central Laser Facility at the Science and Technology Facilities Council's Rutherford Appleton Laboratory. Strathclyde was also joined in the research by University of Glasgow and Instituto Superior Técnico in Lisbon.

Professor Dino Jaroszynski of Strathclyde, who led the research, said: "This is a great breakthrough, which could make the probing of very dense matter easier and more extensive, and so allow us to monitor nuclear fusion capsules imploding.

"To prove this we have imaged very thin wires - 25 microns thick - with gamma rays and produced very clear images using a new method called phase-contrast imaging. This allows very weakly absorbing material to be clearly imaged. Matter illuminated by gamma rays only cast a very weak shadow and therefore are invisible. Phase-contrast imaging is the only way to render these transparent objects visible.

"It could also act as a powerful tool in medicine for cancer therapy and there is nothing else to match the duration of the gamma ray pulses, which is also why it is so bright.

"In nature, if you accelerate charged particles, such as electrons, they radiate. We trapped particles in a cavity of ions trailing an intense laser pulse and accelerated these to high energies. Electrons in this cavity also interact with the laser and pick up energy from it and oscillate wildly - much like a child being pushed on a swing. The large swinging motion and the high energy of the electrons allow a huge increase in the photon energy to produce gamma rays. This enabled the gamma ray photons to outshine any other earthbound source.

"The accelerator we use is a new type called a laser-plasma wakefield accelerator which uses high power lasers and ionised gas to accelerate charged particles to very high energies - thus shrinking a conventional accelerator, which is 100m long, to one which fits in the palm of your hand."

The peak brilliance of the gamma rays was measured to be greater than 1023 photons per second, per square milliradian, per square millimetre, per 0.1% bandwidth.

The research was supported by the Engineering and Physical Sciences Research Council, the Science and Technology Facilities Council, the Laserlab-Europe Consortium and the Extreme Light Infrastructure project. It is linked to SCAPA (Scottish Centre for the Application of Plasma-based Accelerators), which is based at Strathclyde and is run through the Scottish Universities Physics Alliance.

The research has been published in the journal Nature Physics.

Researchers at NIF moving closer to fusion ignition point

2011-09-10T11:08:00.002-05:00

(PhysOrg.com) -- Researchers at the U.S. National Ignition Facility (NIF) report that they are growing ever closer to reaching the ignition point with their laser generated nuclear fusion project. The facility, part of the Lawrence Livermore National Laboratory has been doing research to find out if very high powered lasers could be used to create nuclear fusion that could then be used to drive steam turbines to make electricity. In related news, officials for UK companies AWE and the Rutherford Appleton Laboratory have announced that they are joining forces with the research team working on the NIF project, adding years of expertise in both nuclear fusion and laser technology.

To achieve inertial confinement fusion, researchers at the NIF project shoot multiple (192) very high powered lasers at a single pellet comprised of the hydrogen isotope deuterium, which causes it to compress to a fraction of its original size and fuse into helium atoms -releasing neutrons. The neutrons could then, in theory, be used to heat water to drive steam turbines. The only problem is, thus far, the power consumed by the lasers (some shots use more power than the whole rest of the United States) exceeds the power produced by firing them at the pellet. But, that appears to be changing.

At a meeting this past week, sponsored by London’s Royal Society, representatives from the US facility and its two new British partners met to announce the terms of agreement between them all. NIF Director Ed Moses told the group (according to the BBC) that one shot of the NIF recently produced, for just the tiniest fraction of a second, more power than all the rest of the world was consuming. And while that is certainly impressive, it’s still just a fraction of what is needed to achieve ignition; the point where a self-sustaining chain reaction occurs (required for energy gain). Moses added that he believes the group will achieve ignition within the next couple of years. Part of the reason for his optimism is the advances that have been made in high power laser diodes over the ten years since the NIF was first designed.

One possible issue the group will certainly need to discuss is the enormous number of hydrogen pellets that would be needed to feed a facility that was actually engaged in producing electricity; some estimates range to 10 pellets a second, or a million every day.

If the team is successful in its endeavor, the enormous amounts of tax dollars spent will be more than made up for in energy production. Just 1300 pounds of water, for example, could provide as much electricity as 2 million metric tons of coal.

White laser pulses with precisely tailored waveform enable control of electrons in microcosm

2011-09-10T10:57:00.004-05:00

(PhysOrg.com) -- An expedition through the fast-paced microscopic world of atoms reveals electrons that spin around at enormous speeds and have gigantic forces are acting on them. Monitoring the ultrafast motion of these electrons requires ultrashort flashes of light. However, in order to control them, the structure of these light flashes, or light pulses, needs to be tamed as well.

This type of control over light pulses has now, for the first time, been achieved by a team of physicists lead by Eleftherios Goulielmakis and Ferenc Krausz of the Laboratory of Attosecond Physics at the Max Planck Institute of Quantum Optics (MPQ) and the Ludwig-Maximilians-University Munich in Garching, along with collaborators from the Center of Free-Electron Laser Science (DESY Hamburg) and the King Saud University (Saudi Arabia). Additionally, the researchers were able to make their pulses shorter than a complete light oscillation, thereby creating for the first time isolated sub-optical-cycle flashes of light. These novel tools allow for the precise control of electron motion in atoms and molecules.

The motion of electrons in the microcosm occurs on an attosecond time scale, where one attosecond is a billionth of a billionth of a second. On such a short scale, only light itself is able to keep up with the motion. Because of the fast oscillations of its electromagnetic field, light can act somewhat like a pair of tweezers on electrons, influencing their motions and interactions. The time it takes light, generated by modern laser sources, to complete one full oscillation amounts to around 2.6 femtoseconds, where one femtosecond is a thousand attoseconds, or one millionth of a billionth of a second.

This is the reason why light is a promising tool for controlling electron dynamics in the microcosm. Yet, before this can become reality, the light’s field oscillations have to be tamed, i.e. its field has to be precisely and completely controllable on a time scale which is shorter than one full oscillation cycle. In order to achieve this lofty goal, researchers first have to learn how to develop and perfect these extraordinary tweezers.

The international team at MPQ around Eleftherios Goulielmakis and Ferenc Krausz has now mastered a big step towards this ambitious aim, managing to sculpt wave forms of laser pulses with sub-cycle precision. Furthermore, the researchers were able to make their pulses shorter than a complete light oscillation, thereby creating for the first time isolated sub-optical-cycle flashes of light.

Extremely short and intense pulses of light with a new waveform

In order to control light pulses on a sub-cycle time scale, it is necessary to use white laser light, as it contains wavelengths (light colours) ranging from the near-ultraviolet through to the visible and all the way to the near infrared region of the electromagnetic spectrum. The physicists have created these light pulses and sent them into a newly developed “light field synthesizer”. The light field synthesizer is analogous to a sound synthesizer, as used by electronic musicians. Just as the sound synthesizer, which superimposes sound waves of different frequencies to create different sounds and beats, the light field synthesizer superimposes optical waves of different colours and phases to create various field shapes. The apparatus first splits the incident white laser light into red, yellow and blue colour channels. After manipulating the properties of the individual colours, they are recombined to form the synthesized wave form. Several components of this novel device, e.g. its mirrors and its elaborate beam splitters, were developed in the service centre of the Munich Centre for Advanced Photonics (MAP) located at the LMU.

Utilizing this technology, the scientists achieved the generation of completely new isolated waveforms. Furthermore, in doing so they managed to compose the shortest pulses ever measured in the visible spectral range, lasting only 2.1 femtoseconds. These pulses are more intense than the ones commonly afforded by current femtosecond light sources because all the energy of the electromagnetic field is confined into a tiny temporal window. Thus, also the strength of the electromagnetic forces increases.

It is precisely these powerful and specially tailored electromagnetic forces which are necessary to control electrons in atoms and molecules, as they are similar in strength to the forces occurring in such microscopic systems. However, to steer electron motion on a microscopic scale, strength is not the only prerequisite because precision is also needed. This level of desired precision is provided by the well-controlled wave forms of the synthesized light pulses.

Electronics which can be controlled with light waves comes close

Thanks to these latest results, the scientists have accomplished a major step towards the control of the microcosm. “These newly developed tools allow us to initiate, control and therefore further understand inner-atomic processes. With these devices, we can master the fine structuring of ultrashort light fields and reliably measure the newly formed light”, explains Dr. Adrian Wirth, Postdoctoral Fellow in the research team of Eleftherios Goulielmakis, leader of the ERC-research group “Attoelectronics”.

As a matter of fact, the physicists have already applied this novel technique in an experiment. By shining the newly designed light pulses onto krypton atoms, the outermost electron was ripped away within less than 700 attoseconds, marking the fastest electronic process which has been initiated by optically visible light. Similar processes can certainly be achieved in more complex systems such as molecules, solids and nano-particles.

This new technology may very well lead the way towards light-based electronics in the future. Light fields are expected to drive electrons not only in isolated systems such as atoms or molecules, but even on microscopic circuits so as to perform logic operations at unprecedented speeds” said Goulielmakis, whose group is exploring the principles of electronics on these extreme time scales. “We are progressively increasing our understanding of the principles in the microcosm and learning how to control it”, adds Ferenc Krausz.

More information: Adrian Wirth, Mohammed Th. Hassan, Ivanka Grguraš, Justin Gagnon, Antoine Moulet, Tran T. Luu, Stefan Pabst, Robin Santra, Zeyad A. Alahmed, Abdallah M. Azzeer, Vladislav S. Yakovlev, Volodymyr Pervak, Ferenc Krausz & Eleftherios Goulielmakis, Synthesized Light Transients
Science Express September 6, 2011

Bending light the 'wrong' way

2011-08-20T11:18:00.000-05:00

(PhysOrg.com) -- Scientists have tried this with sophisticated meta-materials, but at the Vienna University of Technology (TU Vienna) it has now been done with simple metals; materials with a negative refractive index bend light the "wrong" way.

The effect can be seen just by poking a stick into the water; at the water surface, the light changes its direction, the stick appears to be bent. This tilt is described by the refractive index. For years, scientists have been trying to create special materials with a negative refractive index – their optical properties are quite different from those of normal materials. Researchers at the TU Vienna could now show that even common metals can have a negative refractive index, if they are placed in a magnetic field.

Different Kind of Diffraction for Better Optics

When we drive a car into the snow at the edge of the road, the wheels on the road may turn faster than the wheels on the snow. This changes the direction of the car and it starts skidding. Something quite similar happens to beams of light that travel through the interface between two materials, in which light travels at different speeds – such as air and glass. "The refractive index measures, how strongly the light is deflected", explains Andrei Pimenov, Professor at the Institute for Solid State Physics at the TU Vienna. For years there have been speculations about the properties of possible materials with a negative refractive index. Entering such a material, light would bend in the opposite direction. Scientists believe that this could lead to completely new optical effects and technologies.

Metal Bends Light

It was believed that these effects can only be achieved using so called "meta-materials". Such materials are constructed from small intricate structures, which diffract the light in special ways on a microscopic level. At the TU Vienna, scientists found out that with simple tricks even quite common metals such as cobalt or iron can exhibit a negative refractive index. "We place the metal in a strong magnetic field and irradiate it with light of precisely the correct wavelength", Andrei Pimenov explains. He uses microwave radiation, which can penetrate thin foils of metal. Due to magnetic resonance effects in the metal, the light is bent drastically at the surface. Within the metal, it turns into the other direction, as if there was a mirror inside the metal.

The Perfect Lens

Recently, materials with a negative diffractive index have attracted a great deal of attention, because their peculiar behavior could allow for completely new kinds of optical lenses. The resolution of regular lenses is limited by the wave length of light. With long radar waves, it is impossible to take a picture of a butterfly, with visible light, nobody can depict an atom. "But using a material with a negative refractive index, one could theoretically get infinitely high resolution", says Andrei Pimenov. Being able to use simple metals instead of complicated meta-materials makes things a lot easier. However, before optical lenses with a negative refractive index can be built, scientists have to find ways to compensate for the absorption of the light in the material.

New tool may yield smaller, faster optoelectronics

2011-08-14T11:07:00.001-05:00

The steady improvement in speed and power of modern electronics may soon hit the brakes unless new ways are found to pack more structures into microscopic spaces. Unfortunately, engineers are already approaching the limit of what light—the choice tool for "tweezing" tiny features—can achieve. But there may be a way of reaching beyond this so-called "diffraction limit" by precisely steering, in real time, a curve-shaped beam of weird "virtual particles" known as surface plasmons.

This technique, described in the Optical Society's (OSA) journal Optics Letters, opens the possibility of even smaller, faster communications systems and optoelectronic devices. Examples of optoelectronic devices used today include photodiodes such as solar cells, integrated optical circuits used in communications, and charged coupled imaging devices at the heart of cell phone cameras and receivers on the world's most advanced telescopes. This method also may yield new, important tools for research in chemistry, biology, and medicine.

The key to this innovation is the ability—for the first time—to actively manipulate a blended stream of light and plasma, known as a plasmonic Airy beam. The beam, owing to the laws of electromagnetism, travels, not in a straight line like the beams of light to which we are accustomed, but rather in an arc. "It's an odd thing for sure, as light is supposed to travel in a straight line," says Peng Zhang a member of the research team with the National Science Foundation (NSF) Nanoscale Science and Engineering Center of the University of California, Berkeley and Department of Physics and Astronomy at San Francisco State University (SFSU). "That's why people are so crazy about these kinds of interesting beams."

As the beam first strikes a metal surface (typically at an irregular feature called a grating structure), it stirs up small waves of electrons at the metal-insulator interface. These waves, which can be thought of as "virtual particles" known as surface plasmon polaritons (SPPs), then follow the curved trajectory of the Airy beams (see Fig. 1). And, just as ocean waves move objects on the surface of the water, the SPPs can be directed to manipulate ultrafine-scale features on the surface of a metal.

SPPs are already essential elements in the design and manufacture of optoelectronic devices. The reason they're so critical is that they can affect extremely small-scale objects, smaller than the diffraction limit, or half of the wavelength of light used to create SPPs.

The current systems, however, have a significant drawback: they required fixed, permanent nanostructures to direct the SPPs. This lack of flexibility severely limits their uses in nano-system design and manufacture. But by being able to manipulate the Airy beam, and therefore the SPPs, in real time, the new design gives scientists on-the-fly control (see Fig. 2).

"We have demonstrated a new way of routing the flow of surface plasmons without any guiding structures," says Xiang Zhang, who led this research and is the director of the NSF Nanoscale Science and Engineering Center at Berkeley and a faculty scientist with the Materials Sciences Division of the Lawrence Berkeley National Laboratory.

The lack of guiding structures, according to Xiang Zhang, is the critical innovation in their design. Currently, to manipulate surface plasmons over two-dimensional metal surfaces, different elements such as waveguides, lenses, beam splitters, and reflectors need to be created. This is done by either structuring metal surfaces (fabricating some permanent nanostructures) or placing insulators on metals. These permanent guiding structures cannot be reconfigured; once the structure is fabricated it cannot be changed in real time.

By using computer-controlled optics, however, the research team has developed a way to steer and manipulate the beams, precisely directing their trajectories to specific spots on an optical surface and adjusting them as needed. Due to their unique arc-shaped paths, the beams have the added ability to bypass surface roughness and defects, or even vault over obstacles.

"These on-the-fly adjustments are extremely desirable," says Zhigang Chen, a principal investigator with the Department of Physics and Astronomy at SFSU. "They enable reconfigurable optical interconnections in ultra-compact integrated photonic circuits, which are at the core of many high-speed computing technologies. They also would enable on-chip nanoparticle manipulations for chemical, medical, or biological research purposes."

The Airy beams used to direct the flow of plasmons also remain coherent, not fanning out or distorting as they travel along their curved trajectories, much in the same way that laser light remains coherent even after traveling great distances.

To create the Airy beams, the researchers used a laser beam and modulated its phase, or wave front, with a spatial light modulator (a device similar to a miniature liquid crystal display) controlled by a personal computer. By continuously changing the specially designed patterns in the computer, they were able to dynamically control the trajectories of the beam in real time.

"These results point out a new direction for dynamically routing surface energies without any permanent guiding structures," says Peng Zhang, "which could inspire researchers from different areas to develop new technologies or tools for a variety of applications." For example, in nano-photonics, researchers may design practical reconfigurable plasmonic devices for ultra-compact integrated photonic circuits. In biology and chemistry, researchers may establish new tools for dynamically manipulating nanoparticles or molecules, and improving the performance of sensors.

"The ultrafine wavelength nature of surface plasmons makes them a promising tool for future nanolithography or nanoimaging applications," says research team member Sheng Wang, also of the NSF Nanoscale Science and Engineering Center. "Now, with the dynamic tunable plasmonic Airy beams, researchers may also shed new light on ultrahigh resolution bioimaging. For example, by bypassing obstacles and directly shining a beam on a target sample, background noise can be greatly reduced, which would enable more accurate imaging."

"This method may also encourage researchers in other fields to manipulate the surface waves in other low-dimensional systems, including graphenes, topological insulators, and magnetic thin films," says fellow team member Yongmin Liu of the NSF Nanoscale Science and Engineering Center.

More information: The paper, titled "Plasmonic Airy beams with dynamically controlled trajectories," was authored by Peng Zhang, Sheng Wang, Yongmin Liu, Xiaobo Yin, Changgui Lu, Zhigang Chen, and Xiang Zhang. It appears in the Aug. 15 issue of Optics Letters (vol. 36, issue 16, pp. 3191-3193).

Physicists create a living laser

2011-06-13T10:39:00.002-05:00

To date, lasers have been built from inanimate materials, such as purified gases, synthetic dyes or semiconductors. But now physicists in the US have shown how to induce lasing in a single living biological cell. By shining intense blue light onto fluorescent protein molecules in a cell, the team made the molecules generate intense, monochromatic, directional green light. This phenomenon could potentially be used to distinguish cancerous cells from healthy cells, claim the researchers.

The material used in the latest work is the green fluorescent protein (GFP), which is found in the jellyfish Aequorea victoria and has been used to image live cells since the 1960s. By combining the gene that encodes GFP with the DNA of any other protein, the GFP can be attached to that protein. The light it gives off can then be used to track the protein in living cells.

The natural fluorescence of GFP is incoherent, just like the light emitted by a normal light bulb. But physicists Malte Gather and Seok Hyun Yun, at the Massachusetts General Hospital and Harvard Medical School in Boston, thought it might be possible to amplify the protein’s light and so build a biological laser. A tantalizing prospect because almost any organism, from a bacterium to a cow, can be programmed to synthesize GFP.

Gather and Yun put human embryonic kidney cells into a Petri dish and then added the DNA that encodes for GFP to the cells. They then attached a drop of solution containing these re-programmed cells onto a mirror with a diameter of about 3 cm. They placed another, equal-sized, mirror above the solution, leaving a gap of about 200 μm between the mirrors. They then focused nanosecond-long blue laser pulses onto the space between the mirrors and moved the mirrors around, with the aid of a microscope, until they were able to shift a single cell into the beam's focus.

With the cell in place, the researchers gradually increased the power of the blue laser and watched how the green fluorescence changed as a result. Above a certain threshold – when the blue pulses had an energy of about 1 nJ – the energy of the emitted green light increased sharply and its spectrum narrowed to just a few well-defined peaks. This, the researchers say, is a clear signature of lasing because above this threshold there are enough protein molecules in an excited state to generate stimulated rather than spontaneous emission. The emitted green light is amplified as it bounces back and forth between the mirrors, as occurs in a conventional laser cavity.

Gather says that, to the best of his knowledge, this is the first time that a laser has been made from a living material. He mentions that scientists have previously mixed dead tissue with inorganic laser materials and seen coherent emission from the composite. But this latest material is made entirely from living tissue, and this remains alive even after emitting hundreds of laser pulses.

Gather believes that the latest work could eventually have important practical applications. Conventional machines, called cytometers, that analyse large numbers of cells usually provide just one parameter for each cell – brightness. More can be learned by studying cells under a microscope, but the long exposures required mean that this is a time-consuming process. In the GFP cell-laser, variations in intercellular structure, which introduce slight changes to the refractive index of the cell, alter both the spatial output of the laser light and its spectrum. Gather says that this additional information "might make it easier to distinguish between a cancerous cell and a benign cell, or a cell that has become infected with a virus".

The next step, says Gather, is to shrink the mirror cavity so that it is small enough to fit inside a cell, the typical diameter of which is between 10 and 20 μm. This may then allow imaging of cell-lasers inside a living animal, rather than having to extract cells for investigation in the lab. In this case the pumping laser could be supplied either from the outside by shining it through the body or by injecting light through optical fibres inserted into the body.

However, Gather emphasizes that it is difficult to predict precisely what applications could follow and adds that the motivation for the experiment was "largely basic scientific curiosity". The researchers were trying to answer the basic question, why do lasers not exist in nature? "Some astronomers claim there are star clusters that produce coherent light," Gather says, "but as far as I know, there is nothing on Earth that does so."

Writing in a "News and Views" commentary piece to accompany the paper, Steve Meech, a chemist at the University of East Anglia in the UK, says that "it is currently unclear what applications lie in store for cellular lasers". But he adds that "whatever the eventual applications, the advent of GFP in photonics certainly marks an exciting new avenue of research for this extraordinarily versatile protein".

The research appears on the website of Nature Photonics.

SACLA X-ray free electron laser sets new record

2011-06-13T10:28:00.000-05:00

RIKEN and the Japan Synchrotron Radiation Research Institute (JASRI) have successfully produced a beam of X-ray laser light with a wavelength of 1.2 Angstroms, the shortest ever measured. This record-breaking light was created using SACLA, a cutting-edge X-ray Free Electron Laser (XFEL) facility unveiled by RIKEN in February 2011 in Harima, Japan. SACLA (SPring-8 Angstrom Compact free electron LAser) opens a window into the structure of atoms and molecules at a level of detail never seen before.

The use of ultra high-intensity X-ray free electron laser light to explore the miniature structure of matter, until recently inconceivable, is today transforming how we visualize the atomic world.. By providing much shorter wavelengths and higher intensities than other lasers, XFEL enables researchers to directly observe and manipulate objects on an unrivalled scale, opening new research opportunities in fields ranging from medicine and drug discovery to nanotechnology.

One of only two facilities in the world to offer this novel light source, SACLA has the capacity to deliver radiation one billion times brighter and with pulses one thousand times shorter than other existing X-ray sources. In late March, the facility marked its first milestone with beam acceleration to 8GeV and spontaneous X-rays of 0.8 Angstroms.

Only three months later, SACLA has marked a second milestone. On June 7, SACLA successfully increased the density of the electron beam by several hundred times and guided it with a precision of several micrometers to produce a bright X-ray laser with a record-breaking wavelength of only 1.2 Angstroms (a photo energy of 10 keV). The new measurement far exceeds the previous record of 1.5 Angstroms set in 2009 at the only other operational XFEL facility in the world, the Linac Coherent Light Source (LCLS) in the United States.

With experiments soon to commence and user operations at the facility to begin by the end of fiscal 2011, this new record offers a taste of things to come with SACLA's powerfulbeam, the world's most advanced X-ray free electron laser.

Researchers discover way to create true-color 3-D holograms

2011-04-08T14:49:00.002-05:00

Satoshi Kawata, Miyu Ozaki and their team of photonics physicists at Osaka University in Japan, have figured out a way to capture the original colors of an object in a still 3-D hologram by using plasmons (quantums of plasma oscillation) that are created when a silver sheathed material is bathed in simple white light. The discovery marks a new milestone in the development of true 3-D full color holograms. In their paper, published in Science magazine, the researchers show a rendered apple in all its natural red and green hues.

Holograms, of course, have been around for years, with the first images created in the 60’s. Back then the technique was to fire a laser at an object and then record the patterns of interference in the light waves onto a photo sensitive material. Later, rainbow type holograms (such as those used on credit cards) were, and still are, created by using a technique whereby white light is reflected off a silver backing through a plastic film that contains several different images of a single object.

The team at Osaka took another approach, they use both lasers and white light. They first fire a laser at an object, say an apple, to create an interference pattern, but instead of just one laser color, they actually use three; red, green and blue. The interference pattern is then captured on a light sensitive material which is coated with silver (because it contains electrons that are easily excited by white light) and silicon dioxide (to help steer the waves). They then shine a steady white light on the metal sheathed material exciting the free electrons, causing the creation of surface plasmons, which results in the regeneration of the captured image as a true-color 3-D hologram; one that can be viewed from almost any angle and is the same colors as the original object.

Currently, the technique has only been shown to work on still images, and the results displayed on a very small surface area (about as big as a baseball card), but the results of research is nonetheless a very big step towards creating not just more realistic holograms, but true animated 3-D technology.

More information: "Surface-Plasmon Holography with White-Light Illumination," by M. Ozaki et al., Science 8 April 2011: Vol. 332 no. 6026 pp. 218-220. DOI: 10.1126/science.1201045

Ultrafast Laser Scribing Cuts Cost, Hikes Solar Cell Efficiency

2011-03-28T14:09:00.002-05:00

An advancement that will improve solar cell efficiency and reduce manufacturing costs involves using an ultrafast pulsed laser scribing technique to create more precise microchannels.

Microchannels are critical to cost and efficiency because they are needed to interconnect a series of solar panels into an array capable of generating usable amounts of power, said Yung Shin, a professor of mechanical engineering and director of Purdue University's Center for Laser-Based Manufacturing. Conventional scribing methods, which create the channels mechanically with a stylus, are slow and expensive and produce imperfect channels, impeding solar cells' performance.

"Production costs of solar cells have been greatly reduced by making them out of thin films instead of wafers, but it is difficult to create high-quality microchannels in these thin films," Shin said. "The mechanical scribing methods in commercial use do not create high-quality, well-defined channels. Although laser scribing has been studied extensively, until now we haven't been able to precisely control lasers to accurately create the microchannels to the exacting specifications required."

"The efficiency of solar cells depends largely on how accurate your scribing of microchannels is," Shin said. "If they are made as accurately as possible, efficiency goes up."

The work, funded by a three-year, $425,000 grant from the National Science Foundation, is led by Shin and Gary Cheng, an associate professor of industrial engineering. A research paper demonstrating the feasibility of the technique was published in Proceedings of the 2011 NSF Engineering Research and Innovation Conference in January. The paper was written by Shin, Cheng and graduate students Wenqian Hu, Martin Yi Zhang and Seunghyun Lee.

Research results have shown that the fast-pulsing laser accurately formed microchannels with precise depths and sharp boundaries. The laser pulses last only a matter of picoseconds. Because the pulses are so fleeting, the laser does not cause heat damage to the thin film, instead removing material in precise patterns in a process called cold ablation.

"It creates very clean microchannels on the surface of each layer," Shin said. "You can do this at very high speed, meters per second, which is not possible with a mechanical scribe. This is very tricky because the laser must be precisely controlled so that it penetrates only one layer of the thin film at a time, and the layers are extremely thin. You can do that with this kind of laser because you have a very precise control of the depth, to about 10 to 20 nanometers."

Traditional solar cells are usually flat and rigid, but emerging thin-film solar cells are flexible, allowing them to be used as rooftop shingles and tiles or building facades, or as the glazing for skylights. Thin-film solar cells account for about 20 percent of the photovoltaic market globally in terms of watts generated and are expected to account for 31 percent by 2013.

The researchers plan to establish the scientific basis for the laser ablation technique by the end of the three-year period. The work is funded through NSF’s Civil Mechanical and Manufacturing Innovation division.

For more information, visit: www.purdue.edu

Laser-Driven Electrons Observed in Real Time

2011-03-15T15:17:00.000-05:00

The discovery will advance the development of new x-ray sources, the resolution of which will be much higher than current devices allow, according to physicists at the Laboratory of Attosecond Physics (LAP) at Max Planck Institute for Quantum Optics (MPQ) and Ludwig Maximilians University of Munich (LMU), in cooperation with colleagues from Friedrich Schiller University Jena.

In the researchers' experiments, when short laser pulses irradiate helium atoms, their structure is heavily disturbed. If the light is strong enough, electrons are pulled out of the atoms, and the helium atoms become ions. In this mixture, the electrons are much lighter than the helium ions and, as a result, are pushed aside.

Although the laser pulse sweeps across the system, the ions remain stationary and the released electrons oscillate around one location. Together, the particles form wave structures (electron plasma waves). In laser physics, this process and these waves are used under special conditions to rapidly accelerate a small number of the electrons to close to the speed of light and to control them.

In the plasma wave, gigantic electric fields are formed, which are 1000 times stronger than those generated in the world’s largest particle accelerators. A small number of the electrons take advantage of these fields, flying as a swarm behind the laser pulse in its slipstream and accelerating to close to the speed of light. In this process, every accelerated electron has almost the same energy.

Physicists have long been aware of this phenomenon, and it has been demonstrated in earlier experiments, but until now, it has been possible to individually observe only the electron swarm or the whole plasma wave with reduced resolution.

The laser physicists, including Ferenc Krausz and his employees Laszlo Veisz and Alexander Buck of LAP, succeeded in recording both phenomena with a high-resolution image of the plasma wave. The process was documented in snapshots with the same light pulse responsible for accelerating the electrons. The physicists previously split the laser pulse so that a small portion of it illuminated the system of free electrons and ions perpendicularly to the electron beam. The periodic structure of the plasma wave refracts and partially deflects the light.

"We observe the deflection and thereby image the plasma wave as a modulation of brightness onto a camera," said Veisz, the research group leader of the LAP team.

In doing so, the researchers can achieve a unique spatial and temporal resolution in the femtosecond range. The electron swarm produces strong magnetic fields that they also can record to determine its position and duration. Eventually, a film describing the acceleration of the electrons results from the combination of both measurement methods.

"The obtained improved knowledge about laser-driven electron acceleration helps us in the development of new x-ray sources of unprecedented quality, not only for basic research, but also for medicine," Krausz said.

The physicists describe their results in the scientific journal Nature Physics.

Laser heats up fusion quest

2011-03-12T15:06:00.001-06:00

Physicists at the $3.5bn National Ignition Facility (NIF) say they have taken an important step in the bid to generate fusion energy using ultra-powerful lasers. By focusing NIF's 192 laser beams onto a tiny gold container, researchers have achieved the temperature and compression conditions that are needed for a self-sustaining fusion reaction – a milestone that they hope to pass next year.

Located at the Lawrence Livermore National Laboratory in California and officially opened last year, NIF will provide data for nuclear weapons testing as well as carry out fundamental research in astrophysics and plasma physics. The facility will also aim to fuse the hydrogen isotopes deuterium and tritium in order to demonstrate the feasibility of laser-based fusion for energy production.

These hydrogen isotopes will be contained within peppercorn-sized spheres of beryllium, which will be placed in the centre of an inch-long hollow gold cylinder – known as a hohlraum. By heating the inside of the hohlraum, NIF's laser beams will generate X-rays that cause the beryllium spheres to explode and, due to momentum conservation, the deuterium and tritium to rapidly compress. A shockwave from the explosion will then increase the temperature of the compressed matter to the point where the nuclei overcome their mutual repulsion and fuse.

One of the main aims of NIF is to achieve "ignition", which means that the fusion reactions generate enough heat to become self-sustaining. Researchers hope that by burning some 20–30% of the fuel inside each sphere the reactions will yield between 10 and 20 times as much energy as supplied by the lasers.

NIF first began testing the laser beams last year and now two groups at Lawrence Livermore have shown that they can obtain the desired conditions inside the hohlraum. They did this by using plastic spheres containing helium, rather than actual fuel pellets, since these were easier to analyse, and by combining their experimental measurements with computer simulations, the researchers found that the hohlraum converted nearly 90% of the laser energy into X-rays and that it heated up to some 3.6 million degrees Celsius. They also found that the sphere was compressed very uniformly, its diameter shrinking from around two millimetres to about a tenth of a millimetre.

"These results are better than we were hoping," says NIF boss Edward Moses. "People were concerned that we wouldn't be able to achieve the desired temperature and implosion shape, but those fears have proved unfounded." Moses says that the next step will be to replace the plastic spheres with beryllium ones containing unequal quantities of deuterium and tritium, in order to study how hydrodynamic stabilities might lead to asymmetrical implosions. The final step will then be to switch over to actual fuel pellets, which will contain equal quantities of the two hydrogen isotopes, and which, it is hoped, will ignite.

Moses says he hopes that ignition will take place in 2012. But he is keen not to raise expectations, having had to deal with many technical problems since construction started on NIF back in 1997. Indeed, he and his colleagues had predicted last January that ignition would be achieved by the end of 2010. "We might be able to reach ignition around spring or summertime next year," he says. "But there's a lot of physics that can run us off course in the meantime."

David Hammer, a plasma physicist at Cornell University in New York, says that the latest results are encouraging. However, he warns that the study was done without fully understanding the interactions taking place between the laser beams and plasma inside the hohlraum and that such interactions could wreck the very precise symmetry of the implosion needed for ignition.

The work is described in Phys. Rev. Lett. 106 085003 and 106 085004.

Ultra high speed film: Nano-scientists take snapshots of electronic states

2011-03-11T15:28:00.000-06:00

German scientists in the team of Professor Michael Bauer, Dr. Kai Roßnagel and Professor Lutz Kipp from the Institute of Experimental and Applied Physics, together with colleagues from the University of Kaiserslautern and the University of Colorado in Boulder, U.S.A., are following the course of electronic switching processes which occur within fractions of a second (femtoseconds). The results of their research may trigger future developments of custom-made and ultra fast opto-electronic components in order to increase data transmission rates or to accelerate optical switches, to name just one example of potential areas of application.
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Two still frames recorded from the newly developed imaging method. The time interval betweenthe two frames is only 0.00000000000007 seconds. Recording: Rohwer et al., Copyright: CAU
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"These techniques that we have developed enables us to record films of extremely fast processes in a much more comprehensive manner than it was previously possible with similar techniques", Bauer explains. "We are able to, for example, directly track phase transitions in solids or catalytic reactions on surfaces."

To record the films, the Kiel scientists used ultra short flashes of light in the soft x-ray spectral region generated with a specific laser system. Bauer: "The amount of information gained from our pictures when played back in slow motion is vast. We will get completely new insights into most relevant electronic properties of solids which are important for a variety of current and future technologies, for example, in telecommunications."

More information: http://www.nature. … /nature09829

Light turns insulator into a superconductor

2011-01-17T07:50:00.002-06:00

The team from Oxford, Germany and Japan are said to have observed conclusive signatures of superconductivity after hitting a non-superconductor with a strong burst of laser light. (D. Fausti et al., "Light-Induced Superconductivity in a Stripe-Ordered Cuprate", Science v.331, p.189 (2011)).

Abstract: One of the most intriguing features of some high-temperature cuprate superconductors is the interplay between one-dimensional “striped” spin order and charge order, and superconductivity. We used mid-infrared femtosecond pulses to transform one such stripe-ordered compound, nonsuperconducting La1.675Eu0.2Sr0.125CuO4, into a transient three-dimensional superconductor. The emergence of coherent interlayer transport was evidenced by the prompt appearance of a Josephson plasma resonance in the c-axis optical properties. An upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which is significantly faster than expected. This places stringent new constraints on our understanding of stripe order and its relation to superconductivity.

‘We have used light to turn a normal insulator into a superconductor,’ said Prof Andrea Cavalleri of the Department of Physics at Oxford University and the Max Planck Department for Structural Dynamics, Hamburg. ‘That’s already exciting in terms of what it tells us about this class of materials. But the question now is can we take a material to a much higher temperature and make it a superconductor?’

The material the researchers used is closely related to high-temperature copper oxide superconductors, but the arrangement of electrons and atoms normally act to frustrate any electronic current.

In the journal Science, they describe how a strong infrared laser pulse was used to perturb the positions of some of the atoms in the material. The compound, held at a temperature just 20 degrees above absolute zero, almost instantaneously became a superconductor for a fraction of a second, before relaxing back to its normal state.

Superconductivity describes the phenomenon where an electric current is able to travel through a material without any resistance.

High-temperature superconductors can be found among a class of materials made up of layers of copper oxide, and typically superconduct up to a temperature of around –170°C. They are complex materials where the right interplay of the atoms and electrons is thought to ‘line up’ the electrons in a state where they collectively move through the material with no resistance.

‘We have shown that the non-superconducting state and the superconducting one are not that different in these materials, in that it takes only a millionth of a millionth of a second to make the electrons ‘synch up’ and superconduct,’ said Professor Cavalleri. ‘This must mean that they were essentially already synched in the non-superconductor, but something was preventing them from sliding around with zero resistance. The precisely tuned laser light removes the frustration, unlocking the superconductivity.’

The advance immediately offers a new way to probe with great control how superconductivity arises in this class of materials.

The researchers are hopeful it could also offer a new route to obtaining superconductivity at higher temperatures. If superconductors that work at room temperature could be achieved, it would open up many more technological applications.

‘There is a school of thought that it should be possible to achieve superconductivity at much higher temperatures, but that some competing type of order in the material gets in the way,’ said Prof Cavalleri. ‘We should be able to explore this idea and see if we can disrupt the competing order to reveal superconductivity at higher temperatures. It’s certainly worth trying.’

NRL begins field tests of laser acoustic propagation

2011-01-16T12:25:00.001-06:00

An NRL research team led by physicist, Dr. Ted Jones, Plasma Physics Division, performed the first successful long distance acoustic propagation and shock generation demonstration of their novel underwater photo-ionization laser acoustic source. These tests, performed at the Lake Glendora Test Facility of Naval Surface Warfare Center-Crane, expanded on their earlier laboratory research on pulsed laser propagation through the atmosphere.

Using a pulsed Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) 532 nanometer wavelength laser housed in a floating platform, pulses were directed by steering mirrors down through a focusing lens and into the water surface. Each laser pulse produced an acoustic pulse with a sound pressure level of approximately 190 decibels (dBs), which was detected and measured by boat-mounted hydrophones at distances up to 140 meters, roughly the length and a half of a football field. Prior laboratory acoustic propagation distances were limited to about three meters.

"The goal of this laser acoustic source development is to enable efficient remote acoustic generation from compact airborne and ship-borne lasers, without the need for any source hardware in the water," said Jones. "This new acoustic source has the potential to expand and improve both Naval and commercial underwater acoustic applications."

The driving laser pulse has the ability to travel through both air and water, so that a compact laser on either an underwater or airborne platform can be used for remote acoustic generation. A properly tailored laser pulse has the ability to travel many hundreds of meters through air, remaining relatively unchanged, then quickly compress upon entry into the water. Atmospheric laser propagation is useful for applications where airborne lasers produce underwater acoustic signals without any required hardware in the water, a highly useful and efficient tool for undersea communications from aircraft.

Few femtosecond, few kiloampere electron bunch produced by a laser–plasma accelerator

2011-01-10T07:35:00.000-06:00

Particle accelerators driven by the interaction of ultraintense and ultrashort laser pulses with a plasma can generate accelerating electric fields of several hundred gigavolts per metre and deliver high-quality electron beams with low energy spread, low emittance and up to 1 GeV peak energy. Moreover, it is expected they may soon be able to produce bursts of electrons shorter than those produced by conventional particle accelerators, down to femtosecond durations and less. Here we present wide-band spectral measurements of coherent transition radiation which we use for temporal characterization. Our analysis shows that the electron beam, produced using controlled optical injection, contains a temporal feature that can be identified as a 15 pC, 1.4–1.8 fs electron bunch (root mean square) leading to a peak current of 3–4 kA depending on the bunch shape. We anticipate that these results will have a strong impact on emerging applications such as short-pulse and short-wavelength radiation sources, and will benefit the realization of laboratory-scale free-electron lasers.

An ultrashort and ultraintense laser pulse (red) is focused on a gas jet in which a plasma wave is excited. Electrons are injected into the plasma wave during the collision with the injection pulse (green), which arrives at a relative angle.

(via Nature Physics doi:10.1038/nphys1872)

Dutch researchers build affordable alternative to mega-laser X-FEL

2010-12-23T07:44:00.001-06:00

Stanford University in the USA has an X-FEL (X-ray Free Electron Laser) with a pricetag of hundreds of millions. It provides images of 'molecules in action', using a kilometer-long electron accelerator. Dutch researchers at Eindhoven University of Technology (TU/e) have developed an alternative that can do many of the same things. However this alternative fits on a tabletop, and costs around half a million euro. That's why the researchers have jokingly called it 'the poor man's X-FEL'.

It's one of the few remaining 'holy grails' of science: a system that allows you to observe the extremely high-speed molecular processes at an atomic scale. You could call it an ultra-fast video microscope. Instead of visible light this kind of system uses X-rays or electrons, because it requires radiation with a wavelength of less than a nanometer. The X-rays or electrons have to be emitted in ultra short pulses, so that the exposure time is extremely short. However these pulses are not easy to generate. An X FEL uses X-ray pulses for this purpose, generated by accelerating electrons in an accelerator of a kilometer, or longer. These electrons are then converted into X-rays. An installation of this kind is very costly, uses large amounts of energy and needs a whole team to operate it. A European X-FEL, which will cost a billion euro, is currently under construction in Hamburg (Germany).

TU/e doctoral candidate ir. Thijs van Oudheusden has developed a machine that in many respects can compete with this billion-euro facility, based on ideas from his co supervisor dr.ir. Jom Luiten. The essence of their 'poor man's X-FEL' is that it uses electrons instead of X-rays. "Why convert electrons into X-rays if you can use the electrons themselves?", asks Van Oudheusden. "As well as that you only need to give the electrons a low energy, so you can accelerate them in just a centimeter. That's why the whole system fits on a tabletop."

The physical barrier that Van Oudheusden had to overcome is that the electrons in electron bunches repel each other. This causes the electron bunches to expand, making them longer than the desired 100 femtoseconds (1 femtosecond is 10-15 second), which in turn would make the 'video microscope' too slow. Jom Luiten thought of a solution to prevent the undesired expansion. The key was to create bunches of exactly the right shape, so they can be controlled and focused by means of electrical fields into bunches of the desired type and length. All with a number of electrons (1 million) that is sufficient to create a diffraction pattern in just a single shot.

Supervisor prof.dr. Marnix van der Wiel believes that half to three-quarters of the kind of research that can be done on an X-FEL can also be done with the 'poor man's X_FEL'. But this doesn't immediately mean that the latter is automatically a lot cheaper in relation to the scientific output that can be generated with it. "The X-FEL at Stanford works non-stop, all year round, and is used by thousands of research groups over several decades. So if you're allocated time on the system you have to take all your equipment to the USA, where you have to stick to a very strict schedule. Our finding is a good alternative for people who want to have the freedom to do research in their own labs. As far as the costs are concerned, it depends on the user if our system will turn out to be cheaper on a per publication basis."

TU/e spin-off AccTec BV intends to build the machine developed by Van Oudheusden and Luiten and to sell it to scientific users. AccTec expects the total price to be below half a million euro.

Thijs van Oudheusden gained his PhD on 13 December with his doctoral thesis entitled 'Electron source for sub-relativistic single-shot femtosecond diffraction'.

FEMTOLASERS: An Ultrafast Success Story

2010-04-16T21:44:00.005-05:00

When lasers were discovered in 1960, it was unclear just how this new kind of light could be used. Similar uncertainty faced Ferenc Krausz and his fellow scientists in 1994 when, as a young postdoc at Vienna University of Technology in Austria, he set up a company to market femtolasers, FEMTOLASERS Produktion.

For the first time it became possible to generate light pulses with a duration of around 10 femtoseconds.

The success story of FEMTOLASERS began ....

At the end of the 1980s, a revolution was underway in the field of laser technology. In the early part of the decade, scientists had discovered titanium ions as a crystalline gain medium in lasers. Then along came a new technology to generate short pulses, "Kerr-lens mode locking," courtesy of University of St Andrews (UK) physicist Wilson Sibbet. This young era of new inventions was set to take over from femtosecond laser technology, which worked with liquid dyes as a gain medium.

By the early 1990s, the field of ultrashort laser physics was gripped by a pioneering spirit. Ferenc Krausz, his PhD recently completed at Vienna University of Technology (TU Vienna), was enthusiastic about the new possibilities promised by solid-state laser technology.

The first working group Krausz led at TU Vienna in 1994 was dedicated to the study of new types of solid-state and short-pulse lasers. This team was driven by a desire to significantly shorten the duration of light pulses, using Ti:Sa lasers. The 100-femtosecond pulse barrier had already been cracked. But the broadband laser material titanium-doped sapphire offered an opportunity to get much lower than that.

The greatest challenge to be faced was how to counteract the dispersion that became more and more noticeable as pulse duration shortened. Dispersion occurs in all materials. It delays the colour components in broadband light to varying degrees and thus leads to a lengthening of the pulse. The shorter the pulse, the wider its bandwidth, an effect which causes the pulse to disperse even faster.
Krausz' working group solved the problem by inventing "chirped mirrors" and using them to control the dispersion of femtosecond Ti:Sa oscillators, in cooperation with Robert Szipöcs and Kárpát Ferencz of Budapest.

Chirped mirrors (CM) are made from alternating nanometer-scale layers of two different transparent optical materials with different indices of refraction. They can reflect up to 99.9% of the incident light over a wide bandwidth. In addition, they let light of different wavelengths penetrate to differing depths before it is reflected.

As a result, it now became possible to control dispersion across wide bandwidths and thereby support the formation of pulses of hitherto unachieved shortness.

The journal Optics Letters (Vol. 19, No. 3) published two separate articles on the subjects of the invention of chirped mirrors (U.S. patent 5,734,503) and the concurrent development of a Ti:Sa oscillator which could generate pulses lasting just 11 femtoseconds.

Femtosecond Pioneers

Spurred on by this success, Krausz, together with Andreas Stingl and Christian Spielmann, both scientists in his team at TU Vienna in 1994, decided to set up a company, FEMTOLASERS Produktion in Vienna.

In the first two years, the team produced a small number of Ti:Sa laser systems with chirped mirrors as a kit. These were the very first chirped mirrors ever made.

In 1997, the first commercial laser with a pulse duration of less than 12 femtoseconds was ready for market. Demand for these laser systems was rising steadily, and FEMTOLASERS was the only company serving the market.

Ultrafast Atoms

Although in the beginning it was not yet clear just what the unusually short pulses could be used for, it quickly became apparent that they could open up spectacular new perspectives in biology and physics in particular.

Ever shorter pulses permitted the observation of ever faster processes, for example, in nature. Here, time-resolved femtosecond spectroscopy has done much to further the understanding of elementary processes that occur in chemical reactions. With the aid of this technique, it is possible to track-in real time-the ultrafast movement of atoms in chemical reactions.

The laser pulses are used, for example, to capture momentary snapshots of molecular arrangements. These individual snapshots can then be put together, like an animated film, to depict the time sequence of an event. In 1999, Ahmed Zewail was awarded the Nobel Prize in Chemistry for this development.

Competition to FEMTOLASERS was not long in coming, and soon the only way to survive in a tough market environment was to keep improving the technology of short pulse lasers. FEMTOLASERS developed new amplifiers which injected a million times higher energy into the light pulses.

This development opened the way to the generation of coherent radiation; in addition to pulses in the visible spectrum it was now also possible to generate femtosecond pulses in the ultraviolet and x-ray ranges.

To date, FEMTOLASERS has sold well over 500 systems and is one of the market leaders in this area.

Further development in femtosecond pulse technology since 2001 has opened the door to the attosecond time dimension, 1000 times shorter than femtoseconds. At that time, the team at TU Vienna was the first group worldwide to succeed in generating individual attosecond pulses with a length of 650 as.

Attosecond flashes arise when electrons in an inert gas are excited by femtosecond laser pulses of a few wave cycles. Today femtosecond pulse technology is capable of generating light flashes which carry over 70% of their energy in a single oscillation cycle.

Thanks to these advances, the team succeeded in 2008 in generating light pulses shorter than 100 attoseconds.

By then, the work was being continued at the Max Planck Institute of Quantum Optics (MPQ) in Garching (Germany) by a working group led by Krausz. The group had moved there from Vienna in 2004. Krausz is director of the MPQ, and he also holds a chair in Experimental Physics at the Ludwig-Maximilians-Universität (LMU) of Munich.

Attosecond flashes below 100 as lift the veil on previously invisible electron movements. In particular it becomes possible to observe interactions between individual electrons in real time, because within and between atoms, these particles generally move on an attosecond timescale.

Attosecond Flashes

For attosecond technology to emerge, it was necessary both to shorten the femtosecond laser pulses to around the ultimate limit set by the oscillation period and to precisely control the wave form of the oscillating light field. These wave-form-controlled few-cycle light pulses, which Krausz and his group demonstrated for the first time in cooperation with Nobel Prize winner Theodor Hänsch and his team in 2003 (and which FEMTOLASERS was the first to market a little later), are not just essential for being able to consistently generate and measure individual attosecond flashes. They also open up for the first time the way to controlling the movement of atoms in atomic systems.

Imaging Potential

The potential significance of this is enormous. It may become possible, for example, to control biological processes at a molecular level, or even accelerate microelectronics to its ultimate limits, which are defined by light frequencies.

In many other ways, too, short pulse laser technology will open up brand new insights into the microcosm in the future. For example, it will improve imaging techniques such as multiphoton microscopy by generating non-linear optical effects. In non-linear microscopy, a femtosecond laser excites biomolecules. The light emitted by them is used for imaging and enables a 3D representation.

Work is also proceeding on optimizing coherence tomography. Medical applications are mainly in ophthalmology and early diagnosis of skin cancer.

Terahertz imaging also has great potential and could benefit from the revolution in femtosecond laser technology. In the terahertz frequency range (100 GHz to 10 THz) materials like paper and many plastics become transparent and can be probed. Water and metals, however, have strong absorption lines at these frequencies. This offers potential applications in the examination of solid-state materials, plastics, and biomedical textiles and also in environmental monitoring, quality control, package testing, and security checks.

Future of Ultrafast Lasers

In addition to short pulse technology, Krausz and his team devote special attention to the further development of chirped mirrors. For this purpose in 2009, the team formed a new company in Garching, UltraFast Innovations (www.ultrafast-innovations.com), a spinoff from the excellence cluster, Munich-Centre for Advanced Photonics (www.munich-photonics.de). MPQ and LMU each have a 50% share in the company, which designs and manufactures custom optics.

The optical components offered by UltraFast Innovations are suitable for almost all areas of laser technology. The scientists test the new developments in their own research work, an aspect customers very much appreciate.

We can only guess at the enormously diverse range of potential applications that will emerge from new developments in ultrashort laser pulses and improvements in optical components in laser technology. Almost every new technological advance brings with it new applications, expanding ever further the reach of our understanding.

SPIE Professional April 2010: Open access article

Landmarks: Lasing with Electrons

2010-03-22T22:09:00.003-05:00

Traditional lasers have been essential in science and technology, but each one is limited in the wavelengths at which it can operate. Two papers published in Physical Review Letters in 1976 and 1977 described a wholly new kind of laser that could in principle operate over a wide range of wavelengths. Today, these so-called free electron lasers provide high intensity from microwaves all the way up to x rays, with applications ranging across biology, chemistry, and physics.

Conventional lasers rely on the process of stimulated emission, described by Albert Einstein. Electromagnetic radiation of the correct wavelength triggers atoms or molecules in an excited state to emit more radiation of the same wavelength, and the emitted radiation is in phase (or "coherent") with the triggering radiation [see 2005 Focus story, Invention of the Maser and Laser].

In 1971, John M. J. Madey of Stanford University in California showed theoretically that stimulated emission can also occur with bremsstrahlung, the radiation that a charged particle emits when forced to follow a curved path [1]. He considered a beam of electrons traveling close to the speed of light through a magnetic field oriented perpendicular to its path. He assumed a field that varies periodically in a way that forces the electrons to wiggle from side to side (or up and down), and as they wiggle, they emit radiation, concentrated in the forward direction.

Madey showed that if radiation of the right frequency travels along the same axis as the electron beam, it can stimulate additional bremsstrahlung radiation with the same frequency and phase. Unlike a conventional laser, however, where the operating frequency is a fixed property of the atoms or molecules at hand, the frequency of stimulated emission from "free electrons" depends only on the energy of the electrons and the magnetic field periodicity. So the wavelength can in principle be adjusted over a very wide range, just by varying the electron beam energy.

It was five years before Madey and several Stanford colleagues demonstrated the effect and reported it in Physical Review Letters. To provide a periodic magnetic field with the necessary strength and structure, the team built a superconducting electromagnet just over 5 meters long, with a period of 3.2 centimeters. Through the center of this magnet they directed a pulsed beam of electrons with energy of about 24 MeV from the Stanford Linear Accelerator Center (SLAC), along with laser light with a 10.6-micron wavelength. They observed a 7 percent amplification of the laser light when the electron beam energy had the right value to allow stimulated emission at the laser frequency.

The following year, Madey and colleagues published their account of the first free electron laser (FEL). It used the same apparatus as the earlier experiment, with a beam energy of 43.5 MeV, but now enclosed between two mirrors and without an external light source. Radiation generated spontaneously by the electron beam reflected back and forth between the mirrors, stimulating further emission to form a strong beam at an infrared wavelength of 3.4 microns.

Although Madey couched his theory in quantum mechanical terms, the working of FELs is now almost always described classically, says Joe Frisch of SLAC's Linac Coherent Light Source (LCLS), an FEL operating at x-ray wavelengths. In this picture, there is an interaction between the sideways motion of electrons and the radiation's sideways electric field that ultimately causes the electrons to form into "microbunches" spaced just a wavelength apart. Each bunch is at the same point in the wave, so their wiggling creates additional waves that are guaranteed to be in synch with the radiation that is already present.

At the LCLS, says Frisch, the electron-wiggling magnets (the undulators) have a 3-centimeter period, and electrons passing through them form into microbunches over about the first 100 periods. During the rest of their trip, the bunches generate coherent x rays with enough intensity that mirrors aren't necessary--a good thing, as no such mirrors exist for x rays. LCLS was the first FEL to generate x rays of sub-nanometer wavelength, with an energy of about 10 keV. Brief, intense pulses of x rays can image individual atoms, making it possible to follow the progress of chemical reactions. FELs are also invaluable in generating high power at infrared frequencies and below, with uses ranging from studies of molecular structure to medical diagnostics.

References:

[1] John M. J. Madey, "Stimulated Emission of Bremsstrahlung in a Periodic Magnetic Field," J. Appl. Phys. 42, 1906 (1971).

Graphene makes ultrafast laser

2010-03-20T22:04:00.002-05:00

Researchers at the University of Cambridge in the UK and CNRS in Grenoble, France, have fabricated an ultrafast "mode-locked" graphene laser. The result – which comes as quite a surprise, given the absence of a band gap in graphene – paves the way to photonic devices based on the material.

Since its discovery in 2004, graphene has continued to amaze scientists thanks to its unique electronic and mechanical properties that make it useful for a host of device applications. The "wonder material", as it is called, might even replace silicon as the electronic material of choice in the future. Graphene consists of a planar single sheet of carbon arranged in a honeycombed lattice and electrons travel through the material at extremely high speeds thanks to the fact that they behave like relativistic, or "Dirac", particles with no rest mass.

Now, Andrea Ferrari and colleagues say that graphene might be used in optoelectronics applications too, by demonstrating an ultrafast laser made from the material.

Ultrafast lasers are widely used in science and technology, and there is an increasing demand for compact, tunable laser sources. Today, the dominating technology in so-called mode-locked lasers – that is, lasers that produce ultrashort pulses at a very high repetition rate – is based on semiconductor saturable absorber mirrors (SESAMs). However, such devices are complicated and expensive to make, and are severely limited in their bandwidth.

Graphene mode-lockers

The new ultrafast laser exploits graphene and graphene layers as mode-lockers.

"In principle, this is quite a surprising result because graphene has no band gap, which is a key requirement for mode-locking in SESAMs," said Ferrari.

The team studied how light is absorbed in graphene and how photo-excited charge carriers behave in the material. In particular, they highlighted the key role of "Pauli blocking" in saturating the light absorption. Because of the Pauli exclusion principle, when pumping of electrons in the excited state is quicker than the rate at which they relax, the absorption saturates. This is because no more electrons can be excited until there is "space" available for them in the excited state.

Since the Dirac electrons in graphene linearly disperse, this means that it is the most wideband saturable light absorber ever, far out-passing the bandwidth provided by any other known material.

The researchers made their laser by starting with a graphene-polymer composite, obtained from a solution of graphene. Next, they placed this composite between two optical fibres in a laser cavity.

"Graphene is the ideal wideband saturable absorber, able to operate from the UV to visible and far-infrared wavelengths," Ferrari told our sister website nanotechweb.org. "Our graphene-based ultrafast laser, which harnesses the wideband optical nonlinearity of graphene, with no need for band gap engineering, extends the practical application of this novel material from nanoelectronics to optoelectronics and integrated photonics."

The team are now in the process of optimizing a fully functioning wideband tunable laser based on graphene, as well as trying similar experiments with graphene oxide.

The work was reported in ACS Nano.

Disk Laser Technology

2010-01-22T14:19:00.005-06:00

A disk laser or active mirror is a type of solid-state laser characterized by a heat sink and laser output that are realized on opposite sides of a thin layer of active gain medium. It was introduced in the 1990s by the group of Adolf Giesen at the University of Stuttgart, Germany.

The gain medium of a thin-disk laser is a laser crystal (often Yb:YAG) in the form of a disk with a thickness of 100-200 µm, which is fixed on a water-cooled heat sink. The cooled end face has a dielectric coating that reflects both the laser radiation and the pump radiation.

The heat is extracted dominantly through the cooled end face, and because the disk thickness is considerably smaller than the laser beam diameter, the heat flow is largely in the direction of the beam, rather than in a transverse direction, as for a laser rod. As a consequence, thermal lensing is weak. The figure 1 illustrates the difference between the two types. Hence, the beam quality achievable with Disk Lasers can be much higher than that of a rod system, improving the Beam Parameter Product (BPP) up to 6 times.

The small disk thickness, as required to limit the heating, leads to incomplete pump absorption in a double pass. Therefore, one usually uses some multipass pumping scheme, which can be realized with very compact optics.

Due to improvements in the area of semiconductor pumping diodes the potential of Disk Lasers is not exhausted. While the first generation "only" extracted 1kW of laser power out of one disk, today's generation already generates 2kW out of one disk crystal. Still, the potential for this technology is not limited and expected to increase to 4kW per disk towards the end of 2008. Further, by combining several individual disk cavities, as illustrated in Figure 2, the total available laser power of a Disk Laser is virtually unlimited. The pumping beam from diode pumping stacks is reflected multi-fold via mirrors inside the cavity to pass up to 20 times through the disk. The disk "converts" the optical pumping light into a laser beam for processing. Based on an existing 4-cavity design, a laser power of 16kW will soon be available. The beauty of this Disk Laser principle over the fiber laser principle is that there are no losses in beam quality when scaling up laser power. These improvements in beam quality and power also lead to significant advantages for the design of processing optics and allowed the development of high-power scanner optics.

It is hardly necessary to mention that indispensable features known from conventional lamp-pumped lasers have not changed: Disk Lasers offer closed-loop power control, are insensitive against back reflections returning from the workpiece, their availability (uptime) is greater than 99 per cent and due to their modular construction all components can be replaced and maintained in the field. Last, but not least, for users of Disk Laser this means that not only the performance of such devices improves, but prices for say a 4 kW Disk Laser are falling because less cavities are required to generate the same laser power.

Q switching is possible with high pulse energies but not with very short pulses because the laser gain is quite limited.

Two Photon Absorption

2010-01-10T16:25:00.004-06:00

The amount of light absorbed by a substance under normal single photon conditions is given by Beer’s Law, in which the amount of absorbed light is (in the weak absorption limit) proportional to the absorption cross-section of the molecule, s, the pathlength, l, and the concentration of absorbing species, C. In Two Photon Absorption (2PA), the absorption is proportional to the square of the light intensity. As a consequence, 2PA occurs only for very intense light, which, in common application, occurs at the focus of a laser beam. The photo at right shows fluorescence of a dye following 1PA and 2PA. The laser at the top of the cuvette is exciting the dye by 1PA causing yellow fluorescence emission. The emission can be seen clearly along the whole focussed laser path. The laser at the bottom of the cuvette is exciting the dye by 2PA, which causes the same yellow fluorescence. This time emission only occurs at the focal point of the laser because of the (intensity)2 dependence of the 2PA.

Simulated effects of excitation wavelength and numerical aperture on the dimensions of the 2PA volume. a, Normalized distributions of laser intensity-squared in the x-y and x-z plane for three different numerical apertures of water-immersion objective lenses at an excitation wavelength of 850 nm. Intensities-squared at lateral (x,y,0) and axial (x,0,z) positions were calculated using an ellipsoidal Gaussian approximation to the diffraction limited focus7,11 and expressed as the fraction of the intensity-squared at the focal point [I(0,0,0)2=1]. Color-coded contour plots depict isointensity lines in the x-z and x-y plane at the levels of 0.1, 0.3, 0.5, 0.7, and 0.9 of I(0,0,0)2. Note different scales for each panel. b, Dependence of 2PA volume on numerical aperture of the objective lens and illumination wavelength. Values were obtained by approximating the intensity-squared distribution as a three-dimensional Gaussian volume.12 For all calculations, it is assumed that the objective lens is uniformly illuminated (overfilled) and that no saturation of the fluorescence excitation process occurs. NA indicates numerical aperture.

Figure right provides a simplified illustration of the difference between single photon and two-photon activated processing. A material is polymerized along the trace of the moving laser focus, thus enabling fabrication of any desired polymeric 3D pattern by direct “recording” into the volume of photosensitive material. In a subsequent processing step the material, which was not exposed to the laser radiation, and therefore, stayed unpolymerized, is removed and the fabricated structure is revealed. The material sensitive in the UV range (λUV) can be polymerized by irradiation with the infra-red light of approximately double wavelength (λIR=2λUV), under the condition that the intensity of the radiation is high enough to initiate two-photon absorption.

First Pump-Probe Experiment at Linac Coherent Light Source Completed

2009-11-25T21:13:00.000-06:00

(PhysOrg.com) -- The first experiment using the Linac Coherent Light Source to illuminate molecules via a "pump-probe" technique has been completed by an international team of more than 30 scientists from institutions including Lawrence Berkeley National Laboratory, LCLS and the joint SLAC/Stanford PULSE Institute. Ryan Coffee, physicist with the LCLS Laser Group, presented initial results in a seminar at SLAC on Wednesday, November 18.

Pump-probe experiments use one laser pulse, in this case an infrared pulse, to pump energy into a sample and then probe it with another laser pulse, in this case an LCLS X-ray pulse. Such experiments are ideal for looking at atomic and molecular interactions, which take place in tiny fractions of a second. The LCLS probe pulses were as short as a few quadrillionths of a second and a billion times brighter than any X-ray source produced in a laboratory.
Coffee and his colleagues looked at the quantum behavior of electrons in nitrogen molecules, N2. The results represent a step toward a fundamental understanding of how nature converts light into chemical energy and might one day help revolutionize solar power, Coffee said.
Nitrogen atoms distribute their electrons between a lower and a higher energy shell. Using X-rays, the team picked off two electrons from the lower level, allowing a higher shell electron to descend and fill the vacancy. The energy released during this downward plunge ejected another electron from the atom, a phenomenon known as the Auger effect.
The team wanted to study how the nitrogen molecules' orientation affected this reaction. To do this, they used the infrared laser to line up the nitrogen molecules so that they were all facing the same direction.
"In a sense, we tried to make the gas act a little bit like a crystal," Coffee said.
After hitting the nitrogen with X-rays, the researchers detected electrons flying off and measured how the molecules' alignment with respect to the X-rays influenced the Auger effect. They observed numerous features that had strong dependence on the molecules’ direction. The results are currently being prepared for publication.
Future work will focus on how atomic bonds change as molecules either break apart or rearrange. Coffee thinks such work will lead to a deeper understanding of how nature converts light into energy. Ultimately, he hopes the results will lead to technology that will help humans generate power from the sun.
"I'm going for the solar power revolution, though I don't know where it will come from," he said. His gut feeling is that the important atoms to look at are carbon, nitrogen and oxygen.
"That's where energy in nature comes from," he said.
Coffee added that the team owes a debt of gratitude to the LCLS Controls, Accelerator, and Laser Groups, who made the experiment's success possible.

Laser creates record-breaking protons

2009-11-09T21:25:00.001-06:00

An international group of physicists working at the Los Alamos Laboratory in the US has used a laser to generate 67.5 MeV protons – the highest-energy protons yet produced in this way. Their work points the way to new laser-based devices for proton therapy, which would be far smaller and cheaper than existing particle-accelerator sources.

When a high-energy proton beam travels through the human body it deposits most of its energy within a small volume, the size and location of which can be calculated to great precision. As a result, protons offer a distinct advantage over other forms of radiation used to destroy tumour cells because they cause less damage to surrounding healthy tissue. Unfortunately, the accelerators needed to generate the protons can cover thousands of square metres and cost some $100m. This has limited the number of proton-therapy facilities available and patients often have to travel considerable distances to be treated in this way.

Some physicists believe that a laser-based proton generator could be made for about one tenth of the cost of a conventional accelerator and be small enough to be contained within a classroom-sized laboratory. The idea is that ultra-powerful laser pulses knock electrons out of the atoms within a tiny target, causing the electrons to accumulate on the target's rear surface. This sets up an electric field across the target, accelerating the resultant ions and forcing them to leave the material as a very high-energy beam.

Energy is a problem

In practice, however, some of the world's most powerful petawatt (10^15 W) lasers have only been able to generate protons with a maximum energy of about 58 megaelectronvolts (MeV). While tumours of the eye can be treated using protons of 60–70 MeV, deeper tumours require energies of about 300 MeV.

The latest breakthrough was carried out by Kirk Flippo of Los Alamos, Sandrine Gaillard of the Forschungszentrum Dresden–Rossendorf research centre (FZD) in Germany and colleagues, who used Los Alamos' Trident laser to generate 67.5 MeV protons. The work relies on a novel target design – an anvil-shaped piece of copper comprising a cone around 100 µm long with a 100 µm flat disc across perched on its tip. Flippo's team directed the laser beam to the inside of the cone, liberating electrons that were guided to the tip and which set up an electric field that accelerated protons away from the disc. The researchers claim that this arrangement is far more efficient than the thin films used in previous experiments – they used 80 J laser pulses, whereas the previous record of 58 MeV involved 450 J laser pulses.

Team-member Michael Bussmann of the FZD says that this significant step forward in maximum proton energy was also made possible by increasing the intensity of the main part of each pulse relative to the "pre-pulse", which precedes the main pulse and can damage the target.

Not enough protons

However, it might take a decade or more before laser-generated protons can be used to combat cancer. Another major challenge is that Trident and the other more intense lasers simply require too much energy to be able to function at the roughly 10 Hz pulse rate needed to produce enough protons for cancer therapy.

According to Bussmann, reaching the sought-after high production rates will be a matter of getting the target right. One possibility will be some kind of refinement of the anvil shape, he says. Others, however, believe that the answer lies in reducing the size of the target, allowing electrons to be heated and ejected from the target much more quickly and therefore with a more uniform energy distribution, in other words leading to fewer low-energy electrons. "We already have enough energy in our lasers, the question is how can we use it more efficiently," says Bussmann. "Nobody has the final idea right now," he said, "but we are in a position to test all these different theories and see which works best."

Looking beyond cancer therapy, Flippo believes that such proton sources could also be used to create medical isotopes and employed to generate neutrons for research in condensed-matter physics and other areas of science. They might also be used to search for nuclear materials inside cargo, given that the characteristics of a proton beam are altered in a well defined way by radioactive substances.

The research was presented at the annual meeting of the Division of Plasma Physics of the American Physical Society, held in Atlanta on 2–6 November.

Electron self-injection into an evolving plasma bubble

2009-11-02T21:35:00.001-06:00

Just five years ago, experimentalists finally demonstrated that such laser-plasma accelerators could produce monoenergetic, collimated electron beams with quality comparable to conventional accelerators. The secret was for the laser to produce a "bubble" almost completely devoid of electrons in its immediate wake that captured electrons from the surrounding plasma and accelerated them in an exceptionally uniform way. Yet the precise mechanism by which the bubble captured these electrons and accelerated them with such uniformity has remained one of the outstanding mysteries of this field.

Now new theoretical work by scientists from the University of Texas and Commissariat à l'Énergie Atomique (CEA, France), to be reported at the 2009 APS Division of Plasma Physics Annual Meeting, has shed light on this mystery. Formation of the exceptional quality electron beam is attributed to the evolution of the bubble shape which, in turn, is directly associated with the nonlinear evolution of the driving laser pulse (nonlinear focusing and defocusing).

The basic premise of this work is that the size of the bubble—the cavity of electron density traveling over the positive ion background with nearly the speed of light—is determined by the spot size of the driving laser pulse. Plasma nonlinearities cause the laser to focus and defocus in the course of propagation. Once the laser diffracts, the bubble expands. Electrons that constitute a dense electron shell surrounding the bubble move with relativistic speeds and thus have high inertia. As a consequence, some of them become too heavy to follow the expanding shell; they fall inside the bubble, stay inside till the end of the plasma (i.e. get trapped) and finally gain multi-GeV energy.

The trapped charge is proportional to the bubble growth rate. Once the laser becomes self-guided, and the spot size oscillations saturate, the injection process clamps. Simultaneously, longitudinal non-uniformity of the accelerating gradient equalizes the trapped electron energy. This scenario of self-injection and monoenergetic bunch formation is discovered and explored in fine detail in the 3-D particle-in-cell simulations. This is fundamentally different from the previous work which concentrated on either one-dimensional models of electron trapping or on the reduced description of transverse plasma wave breaking in planar 2-D geometry.

The discussed mechanism of electron self-injection is very robust in experiments with the high-power laser (tens of terawatts to petawatt). In addition, an appropriate modification of the plasma density (e.g. using a thin dense slab as a nonlinear lens for the laser) may cause the laser to self-focus and defocus faster, which results in a single self-injection event. This kind of laser beam manipulation may lead to the generation of a 2.5 GeV mono-energetic (~1% energy spread) electron bunch containing ~1010 electrons in a future experiment with the recently commissioned Texas Petawatt (TPW) laser - the most powerful laser in the world. Electrons with 2.5 GeV of energy are traveling at 99.999998% of the speed of light. Electron beams with such unique properties are clearly beneficial for medical applications, radiation physics, material science, and homeland security.
Source: American Physical Society

Laser recreates X-rays emitted by a black hole

2009-10-26T21:39:00.001-05:00

Physicists have used high-power lasers to recreate X-ray spectra emanating from some black holes and neutron stars. Conclusions drawn from the experiment appear to conflict with previous interpretations of astronomical data, suggesting that we may have to rethink our view of the structure surrounding black holes and neutrons stars.

Large quantities of X-rays are produced when a black hole or neutron star sucks in matter from a companion star, creating a ring of matter known as an accretion disc. As matter spirals into the black hole or neutron star, gravitational energy is converted into kinetic energy and heat. The intense radiation that is released travels outwards (in the form of photons) and ionizes material closer to the outer edge of the accretion disc – creating an X-ray emitting plasma.

Interpreting the X-ray spectrum of such a plasma is key to understanding the physics of such systems, because it is impossible for astronomers to directly measure its temperature, density and pressure. It has also proven very difficult to recreate such a "photo-ionized" plasma here on Earth because it requires an extremely hot source of radiation.

But now researchers in Japan, Korea and China are helping to address this weakness by studying the spectra of plasmas created in the lab. Such spectra are very similar to that produced by Cygnus X-3, a black hole and a companion star with highly ionized silicon ions on its surface. A similar X-ray spectrum has also been recorded from Vela X-1, a neutron-star binary system.

The researchers produced their X-ray spectra at the GEKKO-XII laser facility, which is located at Osaka University, Japan. The system combines a 10 TW laser that is capable of producing nanosecond pulses from twelve beams with a 10 PW laser that can deliver four picosecond beams.

"We used 12 nanosecond laser beams with wavelength, energy and pulse duration of 0.53 µm, 4 kJ in total and 1.2 ns [respectively]," explained Shinsuke Fujioka from Osaka University, who proposed and organized the experiment.

The beams are fired at a tiny plastic capsule, causing it to implode. "As it shrinks, a hot and dense plasma core forms inside the capsule," says Fujioka. The radiation produced then photo-ionizes a nearby sample of cold silicon gas.

Fujioka says that the shape of their X-ray spectra is quite similar to that recorded by astronomers. However, interpretations of the origin of characteristic lines emissions differ.

Astrophysicists claim that an X-ray peak at 1.84 keV stems from a forbidden transition of silicon ions. But Fujioka says that calculations performed by his team – which consider experimental measurements of the temperature and density of the plasma – suggest that the peak is associated with a different resonance transition of silicon ions. However, the researchers admit that they cannot provide a definite explanation for the origin of this peak. That is because the radiation flux produced in the laboratory lasts for tiny fractions of a second, while that produced by compact astrophysical objects is continuous.

The work is reported in Nature Physics and, writing in a companion piece, Paul Drake of the University of Michigan described the technique as having "great potential for further development," because it allows the energy of the photon source to be varied over a wide range while allowing a great deal of control over the photo-ionized material. However, Drake also cautions that more work must be done in terms of characterizing the physical properties of the resulting plasmas.

Fujioka says that the team may now turn its attention to investigations of the absorption of intense beams of X-rays. It is widely believed that the X-ray absorption rate in materials and plasmas is independent of the intensity of the beam, but they suspect that a plasma may become transparent in incredibly intense X-ray beams. If this is the case, it will modify our understanding of how plasmas behave in supernovae.

Airborne laser ready for flight tests

2009-09-13T21:00:00.002-05:00

IT SHOULD be the moment of truth for the Airborne Laser (ABL). In the coming months, the multibillion-dollar laser built into a customised Boeing 747 will try to shoot a ballistic missile as it rises above the clouds.

Don't expect instant reports of success, though. Instead, if all goes to plan, we're likely to hear about a series of incremental improvements.

Developed by the US Department of Defense's Missile Defense Agency (MDA), the ABL aims to focus a beam of laser energy in the megawatt range for several seconds onto a missile at a "militarily significant distance" - more than 100 kilometres.

So far, the laser has only operated at near full power on the ground. On 18 August it was fired successfully from the air, but at reduced power. That, however, was no mean feat: aircraft vibrations play havoc with the precisely aligned optical components needed to generate a laser beam.

Firing at full power poses other challenges too. At powers high enough to destroy missiles, any surface contamination or tiny flaw in the laser optics can absorb so much heat that they crack or shatter.

High-power laser beams also heat the air they pass through, creating perturbations that can disperse or divert the beam. To counteract those effects, the ABL uses an adaptive system that senses atmospheric changes along its path and makes optical adjustments to compensate.

To test that system, the MDA plans a series of increasingly powerful shots at modified ballistic missiles loaded with sensors to measure the distribution of laser power on the target. Engineers will assess each shot's performance and use the results to fine-tune the adaptive optics. Once this is done, the MDA will test the laser again in varying conditions, and attempt to destroy actual missiles. The first of these tests is planned to take place late this year, with two more to follow in early 2010, according to an MDA spokeswoman.

A sister project, the Advanced Tactical Laser, which aims to use an airborne high-powered laser to hit targets on the ground, recently completed its first successful test. With future funding dependent upon the success of these tests, the pressure is on the ABL team to prove its efficacy.

Lasers Generate Sound in H2O

2009-09-08T20:53:00.001-05:00

A new technology that uses flashes of laser light to remotely create underwater acoustics is being developed by scientists at the Naval Research Laboratory. The new acoustic source has the potential to expand and improve both Naval and commercial underwater acoustic applications, including undersea communications, navigation and acoustic imaging.

Dr. Ted Jones, a physicist in the Plasma Physics Division, is leading a team of researchers from the Plasma Physics, Acoustics, and Marine Geosciences Divisions in developing this acoustic source.

Efficient conversion of light into sound can be achieved by concentrating the light sufficiently to ionize a small amount of water, which then absorbs laser energy and superheats. The result is a small explosion of steam, which can generate a 220 decibel pulse of sound. Optical properties of water can be manipulated with very intense laser light to act like a focusing lens, allowing nonlinear self-focusing (NSF) to take place.

In addition, the slightly different colors of the laser, which travel at different speeds in water due to group velocity dispersion (GVD), can be arranged so that the pulse also compresses in time as it travels through water, further concentrating the light. By using a combination of GVD and NSF, controlled underwater compression of optical pulses can be attained.

The driving laser pulse has the ability to travel through both air and water, so that a compact laser on either an underwater or airborne platform can be used for remote acoustic generation. Since GVD and NSF effects are much stronger in water than air, a properly tailored laser has the ability to travel many hundreds of meters through air, remaining relatively unchanged, then quickly compress upon entry into the water. Atmospheric laser propagation is useful for applications where airborne lasers produce underwater acoustic signals without any required hardware in the water, such as undersea communications from aircraft.

Also, commercially available, high-repetition-rate pulsed lasers, steered by a rapidly movable mirror, can generate arbitrary arrays of phased acoustic sources. On a compact underwater platform with an acoustic receiver, such a setup can rapidly generate oblique-angle acoustic scattering data, for imaging and identifying underwater objects. This would be a significant addition to traditional direct backscattering acoustic data.

For more information, visit: www.nrl.navy.mil

Laser pulses control single electrons in complex molecules

2009-09-01T20:50:00.001-05:00

Physicists of the Max Planck Institute of Quantum Optics (MPQ) in Garching and chemists of the Ludwig-Maximilians-Universität (LMU) in Munich, Germany, succeeded for the first time to use light for controlling single, negatively charged elementary particles in a bunch of electrons. The scientists achieved a major milestone that they aimed for within the excellence cluster "Munich Center for Advanced Photonics" (MAP). They report their results in the journal Physical Review Letters.

Electrons are extremely fast moving particles. In atoms and molecules they move on attosecond timescales. An attosecond is only a billionth of a billionth of a second. With light pulses that last only a few femtoseconds down to attoseconds it is possible to achieve control over these particles and to interact with them on the timescale of their motion. These short light pulses exhibit strong electric and magnetic fields influencing the charged particles. A femtosecond lasts 1000 times longer than an attosecond. In molecules with only a single electron, such as the deuterium molecular ion, their control with such light pulses is relatively easy. This was demonstrated in 2006 by a team of physicists including Professor Marc Vrakking and Dr. Matthias Kling from AMOLF in Amsterdam and Professor Ferenc Krausz in Garching (MPQ).

Scientists led by the junior research group leader Dr. Matthias Kling (MPQ) in collaboration with Professor Marc Vrakking (AMOLF) and Professor Regina de Vivie-Riedle (LMU) have managed to control and monitor the outer electrons from the valence shell of the complex molecule carbon monoxide (CO) utilizing the electric field waveform of laser pulses. Carbon monoxide has 14 electrons. With increasing number of electrons in the molecule the control over single electrons becomes difficult as their states lie energetically very close to each other.

In their experiments the scientists used visible (740 nm) laser pulses with 4 femtoseconds duration. The control was experimentally determined via an asymmetric distribution of C+ and of O+ fragments after the breaking of the molecular bond. The measurement of C+ and O+ fragments implies a dynamic charge shift along the molecular axis in one or the other direction, controlled via the laser pulse.

The femtosecond laser pulses initially detached an electron from a CO molecule. Subsequently the electron was driven by the laser field away from and back to the ion, where it transferred its energy in a collision. The whole process took only ca. 1.7 femtoseconds. "The collision produces an electronic wave packet which induces a directional movement of electrons along the molecular axis," says Regina de Vivie-Riedle. "The excitation and subsequent interaction with the remainder of the intense laser pulse leads to a coupling of electron and nuclear motion and gives a contribution to the observed asymmetry," explains Matthias Kling.

The scientists could also image the structure and form of the outer two electron orbitals of carbon monoxide via the ionization process. The extremely short femtosecond laser pulses allowed the scientists to explore this process in the outermost orbitals. They found the ionization of the molecules to take place with a distinct angular dependence with respect to the laser polarization direction. This observation was found to be in good agreement with theoretical calculations and also gave a contribution to the observed asymmetry. The scientists could show that the strength of this asymmetry strongly depends on the duration of the laser pulses.

With their experiments and calculations, the researchers from Garching and Munich have achieved an important milestone that they aimed for within the excellence cluster "Munich Center for Advanced Photonics" (MAP). The goals were to achieve and observe the control of single electrons within a multi-electron system.

Electrons are present in all important microscopic biological and technical processes. Their extremely fast motion on the attosecond timescale, determines biological and chemical processes and also the speed of microprocessors - technology at the heart of computing. With their experiments the researchers have made a further, important step towards the control of chemical reactions with light. The results are also related to basic research on lightwave electronics aiming at computing speeds on attosecond timescales.

More information: Znakovskaya, P. von den Hoff, S. Zherebtsov, A. Wirth, O. Herrwerth, M.J.J. Vrakking, R. de Vivie-Riedle, M.F. Kling: “Attosecond control of electron dynamics in carbon monoxide”, Physical Review Letters (4. September 2009)

New study will contribute to better understanding of nuclear ignition

2009-08-31T20:37:00.000-05:00

UCSD scientists create computer simulations like the ones above to determine how to successfully achieve controlled, miniaturized nuclear ignition of spherical fuel pellets in laboratory environments using lasers as energy drivers.

Under a recent three-year, $510,000 grant from the National Nuclear Security Administration (NNSA), Vu and his colleagues at Los Alamos National Laboratory (NM), Lodestar Research Corporation and the Laboratory for Laser Energetics at the University of Rochester in New York, are using computer simulation tools to figure out how to successfully achieve controlled, miniaturized nuclear ignition of spherical fuel pellets in laboratory environments using lasers as energy drivers.

Vu said the primary lasers for these studies are the Omega laser at the University of Rochester (NY) and the newly built National Ignition Facility at Lawrence Livermore National Laboratory (CA).

"What we would like to do is take the laser and shine the laser onto what we call a hohlraum, a cylindrically shaped black-body radiator made of high-Z materials (typically gold), in the middle of which the miniaturized fuel pellet is placed," he explained. "The material on the wall of the hohlraum absorbs the laser energy, heats up, and becomes a plasma. The plasma in turn irradiates off its newly acquired energy, and the resulting black-body radiation is what drives the miniaturized fuel pellets to nuclear ignition. It's like sunlight hitting the dashboard of a car - the energy of the sunlight is absorbed by the dashboard and is irradiated as heat, essentially electromagnetic radiation on a different wavelength spectrum from the original sunlight. It's a lot of fancy physics. But if you think about in on a fundamental level, it's pretty simple.

Source: University of California - San Diego

Transparent aluminium is 'new state of matter'

2009-08-02T16:27:00.001-05:00

(PhysOrg.com) -- Oxford scientists have created a transparent form of aluminium by bombarding the metal with the world’s most powerful soft X-ray laser. 'Transparent aluminium' previously only existed in science fiction, featuring in the movie Star Trek IV, but the real material is an exotic new state of matter with implications for planetary science and nuclear fusion.

In this week’s Nature Physics an international team, led by Oxford University scientists, report that a short pulse from the FLASH laser ‘knocked out’ a core electron from every aluminium atom in a sample without disrupting the metal’s crystalline structure. This turned the aluminium nearly invisible to extreme ultraviolet radiation.

''What we have created is a completely new state of matter nobody has seen before,’ said Professor Justin Wark of Oxford University’s Department of Physics, one of the authors of the paper. ‘Transparent aluminium is just the start. The physical properties of the matter we are creating are relevant to the conditions inside large planets, and we also hope that by studying it we can gain a greater understanding of what is going on during the creation of 'miniature stars' created by high-power laser implosions, which may one day allow the power of nuclear fusion to be harnessed here on Earth.’

The discovery was made possible with the development of a new source of radiation that is ten billion times brighter than any synchrotron in the world (such as the UK’s Diamond Light Source). The FLASH laser, based in Hamburg, Germany, produces extremely brief pulses of soft X-ray light, each of which is more powerful than the output of a power plant that provides electricity to a whole city.

The Oxford team, along with their international colleagues, focused all this power down into a spot with a diameter less than a twentieth of the width of a human hair. At such high intensities the aluminium turned transparent.

Whilst the invisible effect lasted for only an extremely brief period - an estimated 40 femtoseconds - it demonstrates that such an exotic state of matter can be created using very high power X-ray sources.

Professor Wark added: ‘What is particularly remarkable about our experiment is that we have turned ordinary aluminium into this exotic new material in a single step by using this very powerful laser. For a brief period the sample looks and behaves in every way like a new form of matter. In certain respects, the way it reacts is as though we had changed every aluminium atom into silicon: it’s almost as surprising as finding that you can turn lead into gold with light!’

The researchers believe that the new approach is an ideal way to create and study such exotic states of matter and will lead to further work relevant to areas as diverse as planetary science, astrophysics and nuclear fusion power.

A report of the research, 'Turning solid aluminium transparent by intense soft X-ray photoionization', is published in Nature Physics. The research was carried out by an international team led by Oxford University scientists Professor Justin Wark, Dr Bob Nagler, Dr Gianluca Gregori, William Murphy, Sam Vinko and Thomas Whitcher.

Military mega-lasers are too hot to handle

2009-07-11T15:53:00.001-05:00

HIGH-ENERGY laser weapons have been hailed as the future of anti-missile defence, but they may be further from being battle-ready than military chiefs hoped.

In recent tests, several prototypes have suffered serious damage to their optics at intensities well below the expected levels of tolerance. "Optical damage has been quietly alarming upper management in most major programmes," Sean Ross of the US Air Force Research Laboratory in New Mexico told a meeting of the Directed Energy Professional Society in Newton, Massachusetts, last week. There are also big problems managing the waste heat generated by high-intensity beams.

Laser weapons require mirrors and lenses to focus powerful beams onto distant moving targets, and to compensate for atmospheric perturbations that can reduce the power they deliver. The higher the intensity of the beam, the more likely it is to damage the surface of its optical components.

Optical surfaces are designed to withstand powers up to a specific damage threshold, but tiny flaws or irregularities - which can be extremely difficult to spot - reduce this threshold by making them more vulnerable to heat. Contaminants deposited on the surface can also reduce this threshold by forcing the surface to absorb energy.

These problems have begun to stall the development of laser weapons. Earlier this year in the US, engineers halted tests of the $4.3 billion megawatt-class Airborne Laser short of full power to avoid damaging "a handful of optics in the turret", according to Mike Rinn, a Boeing vice-president who manages the programme. They realised that the optics, designed years ago, would be "frail" in the presence of any contamination, which would be virtually inevitable in flight. In the next week or so, Boeing engineers will install replacement optics and test them on the ground before running the laser at full power in flight.

Finding a way of preventing laser weapons from frying themselves is proving just as troublesome. Depending on the type of laser, generating 1 watt of laser beam produces about 4 watts of waste heat that must be dissipated. The challenge is to develop a cooling system that is both small and extremely robust.

Laser-created temporal lens could lead to movies of molecular processes

2009-06-29T15:43:00.001-05:00

(PhysOrg.com) -- Finding a way to observe and record the behavior of matter at the molecular level has long been a holy grail among physicists. That ability could open the door to a wide range of applications in ultrafast electron microscopy used in a large array of scientific, medical and technological fields.

Now, a team at the University of Nebraska-Lincoln has figured out a possible way to do that. Working in collaboration with Nobel laureate Ahmed Zewail (chemistry, 1999) of the California Institute of Technology in Pasadena, they developed mathematical models to show that laser beams create ultra-high-speed "temporal lenses" that would be capable of making "movies" of molecular processes. The finding was published in the June 15-19 online edition of the Proceedings of the National Academy of Sciences.

The "lenses" in question are not made of glass like those found in standard tabletop microscopes. They're created by laser beams that would keep pulses of electrons from dispersing and instead focus the electron packets on a target. The timescales required, however, are hardly imaginable on a human scale -- measured in femtoseconds (quadrillionths of a second) and attoseconds (quintillionths of a second).

The physicists modeled two types of lenses. One was a temporal "thin" lens created using one laser beam that could compress electron pulses to less than 10 femtoseconds. The second was a "thick" lens created using two counterpropagating laser beams that showed the potential of compressing electron pulses to reach focuses of attosecond duration.

Europe's big lasers: the exawatt roadmap

2009-06-19T15:21:00.000-05:00

Europe is thinking big – big lasers, big science, big budgets. Over the next decade, a trio of planned pan-European research facilities will give scientists access to unprecedented laser powers and intensities, opening the door to exotic science that will shed light on the origins of the universe and, it is hoped, provide the foundations for a sustainable energy future.

The overall construction cost for this new generation of "super lasers" is in excess of €2 bn, with operational budgets running to several hundred million euros per year. That's a price worth paying, says Christian Kurrer, research programme officer at the European Commission.

"International infrastructures attract the best research scientists," Kurrer told delegates attending the "Emerging European Laser Facilities: Beyond Petawatt" workshop at the recent SPIE Europe conference in Prague, Czech Republic. "The infrastructures are well beyond the man-power and financial resources on a national level. This is why we need more collaborative efforts."

One of those collaborations is the High Power Laser Energy Research (HiPER) facility. Headed up by the UK Science and Technology Facilities Council (STFC), a research funding body, HiPER's mission is to carry out proof-of-principle research into energy generation from laser-driven inertial-confinement fusion. The grand challenge: to initiate and study nuclear-fusion reactions via laser heating of a millimetre-sized fuel pellet (containing a mixture of deuterium and tritium) to temperatures greater than 100 million °C.

Although construction of HiPER is not slated to begin until 2014, the process of whipping existing laser technology into shape to deliver a light source with the requisite capabilities is already under way. "Current laser capability has reached its culmination in the petawatt (10^15 W) scale," observed Mike Dunne, project director of HiPER and a senior scientist at the Rutherford Appleton Laboratory (RAL), UK. "We're looking at how to take it [the technology] to the next generation."

This is the purpose of the three-year preparatory phase on HiPER, which is running alongside initial experiments at the US Department of Energy's $4 bn (€3.1 bn) National Ignition Facility (NIF) in California. (As with HiPER, the end-game for NIF, a huge facility consisting of 192 pulsed laser beams with a total energy of 1.8 MJ, is the creation of nuclear fusion in the laboratory.)

Another major European laser facility in the works is the Extreme Light Infrastructure (ELI), a project that's being led by scientists at the Laboratoire d'Optique Appliquée (LOA) at the Ecole Polytechnique, Palaiseau, France. Scheduled to fire up in 2015, ELI will enable fundamental science to be carried out at the very highest laser powers (in the exawatt regime, 10^18 W) and intensities (10^24 W/cm2).

Like HiPER, ELI will allow academic researchers to explore fundamental science at the extremes (stuff like photon–photon scattering and other nonlinear quantum vacuum effects). Other missions outlined in the ELI project include attosecond science (e.g. the study of the ultrafast motion of electrons inside atoms over timescales of the order of 10^–18 s) and generating a secondary source of electron beamlines from the light-matter interaction. HiPER, meanwhile, will also enable scientists to study laser–plasma interactions and "laboratory astrophysics" (e.g. the creation of conditions in the lab that could yield insights into supernovae evolution).

The European X-ray Free Electron Laser (European XFEL) is dedicated to generating ultrashort, hard X-ray flashes for a range of basic and applied research, including atomic-scale metrology and time-resolved studies of chemical reactions down to the 100 fs regime. Construction began on the 3.4 km long laser facility at DESY, an established particle physics and photonics research laboratory in Hamburg, Germany, at the beginning of the year.

DESY has taken the technology and knowhow from an existing pilot facility, FLASH, which is optimized for the extreme UV and soft X-ray range. "The European XFEL is based on the knowledge that has been accumulated at FLASH," said Tschentscher. "DESY will continue to operate FLASH as a user facility for the 6–60 nm regime and the European XFEL will basically build a new machine covering the wavelength range from below 0.1 nm up to 6 nm."

Upon completion, the European XFEL is intended to provide the brightest source of hard X-ray pulses at the highest repetition rate (30,000 flashes per second). In an initial version electron bunches will be separated into three beamlines delivering coherent pulses to six different experimental stations tuned to specific wavelengths.

• This article originally appeared in the June 2009 issue of Optics & Laser Europe magazine.

Study gives clues to increasing X-rays' power

2009-06-16T15:16:00.001-05:00

In a paper to be published in an upcoming edition of Physical Review Letters, UNL Physics and Astronomy Professor Anthony Starace and his colleagues give scientists important clues into how to unleash coherent, high-powered X-rays.

"This could be a contributor to a number of innovations," Starace said.

Starace's work focuses on a process called high-harmonic generation, or HHG. X-ray radiation can be created by focusing an optical laser into atoms of gaseous elements - usually low-electron types such as hydrogen, helium, or neon. HHG is the process that creates the energetic X-rays when the laser light interacts with those atoms' electrons, causing the electrons to vibrate rapidly and emit X-rays.

But the problem with HHG has been around almost as long as the onset of the method in 1988: The X-ray light produced by the atoms is very weak. In an effort to make the X-rays more powerful, scientists have attempted using higher-powered lasers on the electrons, but success has been limited.

"Using longer wavelength lasers is another way to increase the energy output of the atoms," Starace said. "The problem is, the intensity of the radiation (the atoms) produce drops very quickly."

Instead of focusing on low-electron atoms like hydrogen and helium, Starace's group applied HHG theory to heavier (and more rare) gaseous atoms having many electrons - elements such as xenon, argon and krypton. They discovered that the process would unleash high-energy X-rays with relatively high intensity by using longer wavelength lasers (with wavelengths within certain atom-specific ranges) that happen to drive collective electron oscillations of the many-electron atoms.

"If you use these rare gases and shine a laser in on them, they'll emit X-Rays with an intensity that is much, much stronger (than with the simple atoms)," Starace said. "The atomic structure matters."

Starace said that unlocking the high-powered X-rays could lead one day, for example, to more powerful and precise X-ray machines. For instance, he said, heart doctors might conduct an exam by scanning a patient and creating a 3D hologram of his or her heart, beating in real time.

Nanoscientists, who study the control of matter on an atomic or molecular scale, also may benefit from this finding, Starace said. Someday, the high-intensity X-rays may be used to make 3D images of the microscopic structures with which nanoscientists work.

"With nanotechnology, miniaturization is the order of the day," he said. "But nanoscientists obviously could make use of a method to make the structures they're building and working with more easily visible."

Big lasers, big science, big questions

2009-05-01T19:56:00.001-05:00

Leading optical scientists agree that research and industry stakeholders need to do more if Europe is to maximize the benefits from a planned new generation of high-power laser facilities. That was one of the headline messages from the "Emerging European Laser Facilities: Beyond Petawatt" workshop at the SPIE Europe conference in Prague, Czech Republic, last week.

Marking 50 years since the invention of the laser, the workshop was intended to open debate among senior figures from planned pan-European petawatt laser facilties (1015 W and beyond). Among the "blue-ribbon" initiatives under discussion were projects like HiPER (the High Power Laser Energy Research project), ELI (the Extreme Light Infrastructure), and the European X-Ray Laser project (XFEL).

"International infrastructures attract the best research scientists," Christian Kurrer, research programme officer at the European Commission, told delegates. "The infrastructures are well beyond the man-power and financial resources on a national level. This is why we need more collaborative efforts."

With access to unprecedented laser power and scientific expertise, it is easy to see why large-scale science facilities are attractive to users. In fact, some might argue that they are too good and that they will pull in users (and resources) simply because they can guarantee results where smaller national institutions can't. "Industries want facilities for reproducible, reliable results and 100% service," was the opinion of Mike Dunne, HiPER project director.

At the same time, workshop participants agreed that there's plenty of work to do to ensure that stakeholders in research and industry are in position to maximize their interactions with "big science". "They [the laser facilities] have the scientific experts and we bring the industrial methods where the networks can really make a difference," said Federico Canova of Amplitude Technologies, a French laser manufacturer.

New European Union member states might also question the economic returns on their investment in big science, not least because the planned locations for all of these big laser facilities are in western Europe. Kurrer, however, prefers to view such challenges as opportunities. "While distribution [of projects] may never be good, the key will be to break down the borders. Europe is all about talking to each other and overcoming barriers."

Europe's new generation of high-energy laser facilities form part of an ambitious big-science roadmap coordinated by the European Strategy Forum on Research Infrastructures (ESFRI). The ESFRI roadmap covers capital and operational investments running to tens of billions of euros in strategic research areas like energy, environmental science and advanced materials.

Shaking the Fundamentals of Physics: At the Limits of the Photoelectric Effect

2009-04-23T19:50:00.002-05:00

By way of the classical photoeffect, Einstein proved in 1905 that light also has particle character. However, with extremely high light intensities, remarkable things happen in the process. Scientists of the Physikalisch-Technische Bundesanstalt (Germany) have found this out with colleagues at FLASH in Hamburg, the first free-electron laser (FEL) for soft X-rays worldwide.

The current models based on Einstein's idea are simply described in such a way: A photon knocks an external electron out of an atom, provided that the photon energy is high enough. However, with wavelengths of only 13 nanometers and high radiation intensities of several petawatt per square centimeter something else - at least with some atoms - happens: With xenon, a whole light-wave packet immediately seems to knock out a huge number of internal electrons. This effect is strongly dependent on the material and not only on the characteristics of the exciting radiation, as accepted before. The work, which is currently published in the journal Physical Review Letters, has significance for future experiments of materials research at the new large X-ray laser facilities of the world.

The scientists actually wanted to develop methods for the radiometric characterization of X-ray lasers. They irradiated different gases to derive the laser strength from the ionization effect. The aim: with the laser well characterized was, for example, the testing of EUV lithography mirrors. The EUV lithography (EUV stands for extreme ultraviolet) at wavelengths in the range of 13 nanometers is considered as the future technology for the production of ever smaller computer chips.

However, during their experiments at FLASH, the new free-electron laser (FEL) in Hamburg, which currently allows the generation of EUV radiation and soft X-rays of the highest intensity in the world, they unexpectedly discovered things which concern the fundamentals of physics.

With the classical photoelectric effect (a), a single light particle (photon) of sufficient energy interacts with a single electron of the material. The process is energetically described by the Einstein equation (1905) and demonstrates the quantum structure of light. Only at very high intensities, does the multiphoton ionization occur, a process which is described in the extreme case of highly intensive ultra-short light flashes as emitted by long-wave femtosecond lasers, again, in the wave picture of light (b).

Nevertheless, the suitable theoretical models fail in the short-wave X-ray regime as shown by the experiments in Hamburg in which, for the first time, soft X-ray irradiance levels of several petawatts per square centimeter were achieved by strong beam focusing. The comparative quantitative studies prove that the degree of light-matter interaction and, thereby, the nature of the X-ray light are decisively determined by the structure of the atom and correlations in, above all, inner electron shells.

In the extreme case (xenon), a whole wave packet of photons seems to lead to the simultaneous emission of several inner electrons (c).

More information: Extreme ultraviolet laser excites atomic giant resonance. M. Richter et al., Phys. Rev. Lett. (2009) - online publication expected: April 27, 2009.

Photoelectric effect at ultra-high intensities. A. A. Sorokin et al., Phys. Rev. Lett. 99, 213002 (2007)

Source: Physikalisch-Technische Bundesanstalt

Curved light bends the rules

2009-04-10T22:47:00.000-05:00

Everyone knows that light travels in a straight line — right? A couple of years back, however, physicists discovered something very different for certain laser pulses that have one intense peak next to a series of smaller peaks. The brightest part of these lopsided "Airy" pulses, they found, appear to follow a curved trajectory.

Researchers in the US have now found that sufficiently intense Airy pulses can ionize the surrounding air molecules and create curved filaments of plasma. What's more, Airy pulses interact with air such that the pulses are continually focused and so can travel long distances without being dispersed.

The bright white light given off by the plasma filaments could be used make remote spectroscopic measurements of the atmosphere — and the bending effect itself could be exploited in new kinds of waveguide.

The bendy behaviour of Airy pulses was first discovered in 2007 by Demetrios Christodoulides and colleagues at the University of Florida. Interference between the peaks causes the intense peak to veer off in one direction, while the other peaks move in the opposite direction. Although the total momentum of the pulse travels in a straight line, its brightest part appears to follow a curved path.

Christodoulides and his colleagues have now teamed up with Pavel Polynkin and others at the University of Arizona to create curved “filaments” of plasma using Airy pulses. The key to their success, according to Jerome Kasparian of the University of Geneva who was not part of the group, is their ability to — for the first time — create Airy pulses of extremely high intensity.

The team began with an intense infrared laser pulse that is about 35 fs in duration. The initially pancake-shaped pulse, which is symmetric around its direction of propagation, is then passed through a “phase mask” and then a lens, giving it a chevron shape with an intense peak at the vertex (see figure). This Airy pulse then travels about 1 m through air to a fluorescent screen where the light is detected.

As well as confirming that extremely intense Airy pulses appear to curve, the pulses also produced curved filaments of plasma by ionizing nearby molecules in the air.

Although physicists have long known that symmetric laser pulses can create such filaments, the process has proved very difficult to study. This is because symmetric laser pulses travel in the same direction as the white light given off by the plasmas they create, which means that any device that attempts to detect this light is dazzled or even destroyed by the pulse.

With Airy pulses, however, Polynkin, Christodoulides and colleagues discovered that the plasma light travels in straight lines tangentially to the curvature of the bright peak. The plasma light can therefore be detected — and perhaps even be used as a source of white light for spectroscopy.

Firing intense and long-range pulses into the air, for example, could allow researchers to make remote spectroscopic measurements of the atmosphere.

Polynkin also speculates that intense pulses could be fired into thunderclouds to create filaments that "guide" lightning to safe locations on the ground.

Studying the plasma light itself could even help physicists gain a better understanding of the complicated non-linear optics that define how intense laser beams travel through air. These include a “self-healing” effect whereby the beam is continually refocused by the plasma — rather than being dispersed — allowing intense pulses to travel very long distances.

The team are now studying the creation of curved filaments in water rather than air.

Soliton laser offers broad tunability

2009-04-07T22:24:00.000-05:00

A femtosecond soliton source with fast and broad spectral tunability has been developed by researchers in Argentina. The source, which comprises a Ti:sapphire laser and a highly nonlinear photonic-crystal fibre, can be tuned from 850 nm to 1000 nm with nearly constant pulse width and average power (Optics Letters 34 842).

The key to the laser's tuning performance is the use of solitons generated in the photonic-crystal fibre. At the low-power coupling regime, solitons can be tuned over a broad range of wavelengths from 850 to 1000 nm. The solitons generated in the fibre maintain almost constant pulse and spectral widths regardless of input power.

In the set-up, a photonic-crystal fibre measuring 75 cm in length is pumped with a Ti:sapphire laser that provides 37 fs pulses at a repetition rate of 94 MHz and a wavelength of 830 nm. Average power ranging from 1 to 10 mW is pumped into the fibre, controlled by an acousto-optic modulator (AOM).

World's highest-energy laser to create mini-stars

2009-03-15T22:11:00.000-05:00

To produce the temperatures and pressures needed for fusion, the facility will aim all of its 192 laser beams simultaneously on a hydrogen target. This all happens inside this 10-metre-diameter chamber, which weighs 130 tonnes. The sphere is made up of 18 aluminium sections that are each 10 centimetres thick.

The square openings are for the lasers, and the round openings are used to accommodate nearly 100 pieces of diagnostic equipment.

This is a view of the target chamber from the inside. The laser beams enter through ports in the chamber to deliver almost 500 trillion watts of power to the tip of the positioner (right), which will hold the target for each experiment. When all of its beams are fully operational, NIF will focus nearly 2 million joules of ultraviolet laser energy at that tiny target, delivering 60 times more energy than any previous laser system.

All 192 lasers that enter the National Ignition Facility chamber will be trained on this pencil-eraser-sized cylinder. This capsule will hold the pea-sized target, which for fusion experiments will be a pellet of frozen hydrogen. Laser beams will enter through openings at each end to compress and heat the hydrogen in the hopes of creating a self-sustaining fusion reaction.

As laser beams hit the interior of the gold-plated capsule, they will create intense X-rays that can squeeze the pea-sized pellet of hydrogen down to a speck about the width of a human hair and heat it to some 3 million °C. The burst of laser light will last just billions of a second, but physicists hope the intense pulse will force hydrogen atoms to combine to form helium, releasing enough energy to fuse all other neighbouring hydrogen atoms until the fuel is spent.

Before reaching the chamber, laser light must be converted from infrared light to ultraviolet light, which is more effective at heating the target.

This conversion is accomplished with plates sliced from large potassium dihydrogen phosphate (KDP) crystals.

This crystal, which weighed about 360 kilograms, started out from a seed crystal and grew to its pictured size inside a 2-metre-tall vat of solution over a period of two months. Each crystal is sliced into plates measuring 40 cm². More than 600 of these plates are needed for the National Ignition Facility. (Image: Lawrence Livermore National Security, LLC/Lawrence Livermore National Laboratory/Department of Energy)

World's largest laser gears up for ignition experiments

2009-03-10T17:00:00.002-05:00

(PhysOrg.com) -- Construction of the National Ignition Facility (NIF), the world's largest and highest-energy laser system, was essentially completed on Feb. 26, when technicians at Lawrence Livermore National Laboratory (LLNL), where the laser is located, fired the first full system shot to the center of the NIF target chamber.

The test was the first time all 192 laser beams converged simultaneously in the 10-meter-diameter chamber. NIF has met all of its project completion criteria except for official certification of project completion by the U.S. Department of Energy, due by March 31.

An average of 420 joules of ultraviolet laser energy, known as 3-omega, was achieved for each beamline, for a total energy of more than 80 kilojoules (a joule is the energy needed to lift a small apple one meter against the Earth's gravity).

The energy level will be increased during the next several months, and when all NIF lasers are fired at full energy, they will deliver 1.8 megajoules of ultraviolet energy to a BB-sized target in a 20-nanosecond shaped laser pulse, generating 500 trillion watts of peak power -- more than the peak electrical generating power of the entire United States. This is considered more than enough energy to fuse the hydrogen isotopes of deuterium and tritium in the target into helium nuclei (alpha particles) and yield considerably more energy in the process than was required to initiate the reaction.

The last of NIF's 6,206 various optical-mechanical and controls system modules, called "line replaceable units" or LRUs, was installed on Jan. 26. The first LRU, a flashlamp, was installed on Sept. 26, 2001.

Workers have aligned and tuned NIF's final optical assemblies, which focus and convert the frequency of the project's 192 laser beams as they enter the target chamber and converge on the tiny target. Experimental systems and diagnostics are also being installed. Software for the integrated computer control system, which handles shot automation, has been completed.

KBBF crystal gives direct access to DUV

2009-01-08T11:42:00.000-06:00

Researchers in China have created a tunable all-solid-state laser that emits milliwatt power levels in the deep ultraviolet (DUV). Applications requiring light around the 200 nm mark, such as photoemission spectroscopy and photolithography, could benefit from this work (Applied Physics B 93 323).

"Our source tunes from 175 to 210 nm via fourth harmonic generation from a Ti:sapphire laser," Zuyan Xu of the Chinese Academy of Sciences told optics.org. "The highest output power is 2.23 mW at 193 nm but the power is above 1 mW between 182 and 210 nm. This is the first demonstration of a milliwatt-level widely tunable all-solid-state laser below 200 nm by direct second harmonic generation."

The team's set-up can essentially be broken into three stages: the initial nanosecond-pulsed Ti:sapphire laser, optics to generate the second harmonic in the UV and additional components to generate the fourth harmonic in the DUV.

The output from the Ti:sapphire (more than 3W across the range of 690–840 nm) is focused into a set of BBO crystals to generate UV light between 340 and 415 nm. This light is then passed into the KBBF crystal to generate the DUV wavelengths.

When it comes to producing tunable DUV light, one alternative approach is sum-frequency mixing. This however uses two laser beams making the system complex and of limited practical use. To remove this complexity, Xu and colleagues use a KBBF crystal that offers a direct route to DUV light below 200 nm using just one beam.

Light Bends Glass

2008-12-20T19:47:00.000-06:00

Light gives a push rather than a pull when it exits an optical fiber, according to experiments reported in the 12 December Physical Review Letters. The observations address a 100-year-old controversy over the momentum of light in a transparent material: Is it greater or smaller than in air? In the experiments, a thin glass fiber bends as light shines out the end, apparently a recoil in response to the light gaining momentum as it passes from glass to air. But the many experimental subtleties mean that the issue is unlikely to be settled soon.

Light moves slower inside a material than it does in air or vacuum. In 1908 German mathematician Hermann Minkowski suggested that the momentum of light goes up as its speed goes down. A year later, German physicist Max Abraham claimed the exact opposite, that the momentum goes down with decreasing speed.

Abraham might appear to be correct, since the momentum of ordinary objects always goes down with decreasing speed. But Minkowski seems to be favored by quantum mechanics, which says that a photon's momentum goes up as the light's wavelength decreases--and the wavelength always shortens as light enters a material from air. Many theoretical arguments appear to point to an Abraham momentum, but most of the experimental evidence to date argues for Minkowski. The experimental difficulty is that in most cases, both formulations lead to the same predicted forces, after one accounts for the momenta of both the light and the medium. So experiments must be carefully designed to isolate the effect of the light's momentum and avoid other phenomena, such as thermal effects, that can mask the light-induced force.

In their experiment, Weilong She of Zhongshan University in Guangzhou, China, and his colleagues used a filament of silica half a micron wide and 1.5 millimeters long. As the fiber dangled vertically, the researchers shined 270-millisecond laser pulses at a wavelength of 650 nanometers down the fiber. As the light pulses exited out the bottom, a gain in momentum (à la Abraham) would cause the fiber to recoil back like a gun, whereas a loss (à la Minkowski) would pull the fiber straight down. "When I began this experiment, I was really unsure which one is correct," She recalls. The fiber bowed outward with each pulse, which the researchers say is a sign that it's recoiling as Abraham would predict.

The researchers performed a second experiment with a longer fiber and continuous--rather than pulsed--laser light and found similar results. The tip of the hanging fiber moved sideways like a pendulum by about 30 microns, which agreed with the tiny force (less than a billionth of a Newton) that they predicted. The team also verified that thermal effects, such as heat expansion, would be too small to influence the fiber's movement.

The researchers performed a second experiment with a longer fiber and continuous--rather than pulsed--laser light and found similar results. The tip of the hanging fiber moved sideways like a pendulum by about 30 microns, which agreed with the tiny force (less than a billionth of a Newton) that they predicted. The team also verified that thermal effects, such as heat expansion, would be too small to influence the fiber's movement.

Direct diode-pumped laser produces terawatt powers

2008-12-06T21:20:00.004-06:00

A team of researchers from Germany has published details of what it believes is the first direct diode-pumped laser to produce terawatt peak powers. The system relies on a ytterbium-doped calcium fluoride (Yb:CaF2) crystal to amplify femtosecond pulses to the terawatt level, a milestone of particular interest to the laser fusion community (Optics Letters 33 2770).

Alternative ways of reaching the terawatt regime are high-energy Nd:glass or short-pulse Ti:Sapphire laser systems, although both of these methods rely on mature flash-lamp technology.

The heart of the system is a ten-pass amplifier based on Yb:CaF2, which is pumped by two diode laser stacks emitting at 940 nm. The amplifier itself is seeded by either a two-stage chirped pulse Yb:glass MOPA (which the team refers to as the pre-amplifiers of POLARIS or the POLARIS front end) or a Q-switched nanosecond Yb:YAG MOPA.

The team produced 192 femtosecond pulses with a pulse energy of 197 mJ (corresponding to a peak power of 1 TW) using the POLARIS front end. It was also able to amplify nanosecond pulses from the Q-switched MOPA to the joule level.

Drift-free femtosecond timing synchronization of remote optical and microwave sources

2008-12-05T21:13:00.001-06:00

Researchers at MIT, US, are joining forces with MenloSystems to commercialize a set of large-scale synchronization techniques that maintain sub-10-femtosecond timing accuracy over 10 hours and a distance of 300 m and more. This is said to be the first demonstration of such high-precision, robust timing synchronization (Nature Photonics 2 733).

According to Franz Kaertner, principle investigator of the project, this result will benefit the design and operation of seeded free-electron lasers, which require extremely high timing accuracy and may be applicable to the synchronization of large-scale phased-array antennas for radio astronomy.

"Just a few years ago, people thought this level of precision could not be achieved for such a long period of time," he commented. "Our result will enable scientists and engineers in different fields to really think about how to solve their problems or enhance performance by introducing the capabilities that we have shown."

Femtosecond modelocked lasers simultaneously carry extremely low jitter optical and microwave signals. Owing to their ultralow jitter properties, they have been expected to clock large-scale scientific facilities requiring extremely high timing accuracy that conventional electronic clock distribution cannot provide. However, lack of long-term stable synchronization techniques has hindered the realization of this pervasive clocking idea.

"The timing signal needs to be detected with both high timing detection sensitivity and high thermal stability," explained Kim. "Conventionally, this timing detection has been performed in the electronic domain using high-speed photodetection of optical pulse trains followed by phase-detection with microwave mixers. However, excess noise and thermal drift has seriously limited the stability that could be achieved."

To overcome this problem, Kaertner and colleagues shifted the timing detection from the electronic to the optical domain. Extensive details of the methods used can be found in the paper. In summary, the group uses ultralow-noise optical pulse trains generated by modelocked lasers as the timing signals, then distributes them by means of timing-stabilized fibre links and, finally, synchronizes the delivered timing signals with the optical and microwave sources being targeted.

The MIT team is optimistic that due to the scalable nature of its techniques, further improvements in precision and distance are possible. "The next milestone is attosecond-precision ultrafast photonics, which will open up more applications and opportunities that require even higher timing precision," concluded Kim.

Ultraviolet pulses close in on 1 fs regime

2008-11-29T21:04:00.004-06:00

Deep ultraviolet (DUV) pulses with a duration of just 3.7 femtoseconds have been generated by researchers at the Max-Planck-Institute for Quantum Optics in Germany. The pulses are said to be the shortest ever observed at this wavelength range and could allow the team to influence the outcome of chemical reactions by controlling the motion of electrons in molecules. (Optics Express 16 18956)

Previously, the shortest pulses in the DUV (wavelengths shorter than 300 nm) had a duration of 8 femtoseconds. Much of the research effort focused on compressing the ultraviolet pulses after they had been generated, which is a formidable challenge as this requires precise dispersion control.

Here, Ulrich Graf and colleagues take a different approach. They upconvert 780 nm, 0.25 mJ pulses with a duration of 6 femtosecond pulses directly into the UV range by means of harmonic generation in a noble gas jet.

The resulting 3.7 femtosecond pulses are characterized using a dispersion-minimized SD-FROG approach and have energies in excess of 1.4µJ. The conversion efficiency is approximately 0.6%.

The group is now hoping to extend its approach to generate ultrashort pulses in the vacuum ultraviolet (VUV) spectral range (wavelengths less than 180 nm). "In the VUV, matter absorbs light even more and this spectral range will offer plenty of opportunities to explore and control the microcosm on an ultrafast scale," said Goulielmakis. "At the same time, we anticipate that these pulses will be substantially shorter and will approach the 1 femtosecond frontier."

Ultra-short Laser Pulse Produces Positrons

2008-11-20T13:33:00.000-06:00

More than 100 billion particles of antimatter have been created by using a short-pulse, ultraintense laser to irradiate a gold sample the size of the head of a push pin. The antimatter, also known as positrons, shoots out of the target in a cone-shaped plasma "jet."

This new ability to create a large number of positrons in a small laboratory opens the door to several avenues of antimatter research, including an understanding of the physics underlying various astrophysical phenomena such as black holes and gamma ray bursts. Antimatter research also could reveal why more matter than antimatter survived the Big Bang at the start of the universe.

In the experiment, the laser ionizes and accelerates electrons, which are driven right through the gold target. On their way, the electrons interact with the gold nuclei, which serve as a catalyst to create positrons. The electrons give off packets of pure energy, which decays into matter and antimatter, following the predictions by Einstein's famous equation that relates matter and energy. By concentrating the energy in space and time, the laser produces positrons more rapidly and in greater density than ever before in the laboratory.

Particles of antimatter are almost immediately annihilated by contact with normal matter, and converted to pure energy (gamma rays). There is considerable speculation as to why the observable universe is apparently almost entirely matter, whether other places are almost entirely antimatter, and what might be possible if antimatter could be harnessed. Normal matter and antimatter are thought to have been in balance in the very early universe, but due to an "asymmetry" the antimatter decayed or was annihilated, and today very little antimatter is seen.

Over the years, physicists have theorized about antimatter, but it wasn't confirmed to exist experimentally until 1932. High-energy cosmic rays impacting Earth's atmosphere produce minute quantities of antimatter in the resulting jets, and physicists have learned to produce modest amounts of antimatter using traditional particle accelerators. Antimatter similarly may be produced in regions like the center of the Milky Way and other galaxies, where very energetic celestial events occur.

The presence of the resulting antimatter is detectable by the gamma rays produced when positrons are destroyed when they come into contact with nearby matter. Laser production of antimatter isn't entirely new either. Livermore researchers detected antimatter about 10 years ago in experiments on the since-decommissioned Nova petawatt laser -- about 100 particles. But with a better target and a more sensitive detector, this year's experiments directly detected more than 1 million particles. From that sample, the scientists infer that around 100 billion positron particles were produced in total.

Short fibre creates ultrafast OPO

2008-11-15T13:50:00.001-06:00

A fibre optical parametric oscillator (FOPO) based on a 4.2 cm length of microstructured fibre that emits 70 fs pulses has been unveiled by researchers at the US universities of Cornell and California Merced. The system is said to deliver the shortest optical pulses reported for any FOPO and is a significant step towards making the technology commercially viable. (Optics Express 16 18050)

"The majority of OPOs are not portable and occupy a large footprint on an optical table," Jay Sharping of Merced's School of Natural Sciences told optics.org. "My motivation is to generate tunable pulsed light of sufficient output power in a portable fibre platform. This result is a step in that direction as it explores the generation of ultrafast laser light with tens of mW of average power."

Sharping and colleagues start with a commercially available microstructured fibre that has been drawn down to a reduced core size in order to modify the fibre's dispersion profile. They place the 4.2 cm length of fibre in a Fabry-Perot cavity and pump it using a ytterbium-doped fibre laser emitting at 1032 nm. The end result is 70 fs, 0.4 nJ pulses at 880 nm with an output peak power for 5kW for a pump peak power of 22 kW.

Optical oscilloscope is fit for high-speed studies

2008-11-13T20:40:00.000-06:00

Physics in the US have made an oscilloscope that can take snapshots of optical waveforms at a resolution fives times better than current devices. Based on an all-optical rather than an electronic design, the oscilloscope should be able to accurately profile modern telecommunications signals and various ultrafast chemical and physical phenomena.

Oscilloscopes are used to trace graphs of signals over time. Conventional models are based on microelectronics and, using photodetectors, can take snapshots of optical signals at as low as 30 ps resolution.

But as telecommunication data transmission gets faster and faster, and as scientists want to probe more high-speed systems, oscilloscopes based on microelectronics are being stretched to the limit. This is because they can only cope with a relatively narrow frequency spread or “bandwidth”, which holds back their resolution.

All-optical circuits, on the other hand, can process much wider bandwidths. Although optical techniques already exist — indeed, with resolutions going down to a few femtoseconds — these have only been able to take snapshots of small segments of waveforms, and take a long time to update.

A team led by Alexander Gaeta at Cornell University in New York has found a way to exploit the fine resolution of optical techniques for longer waveforms. The researchers make use of the fact that electromagnetic waves have a space–time duality, in that there is a link between their spatial and temporal wavefunctions. This means that the researchers can use a lens to convert the temporal profile of a dispersed snapshot into a detailed, spectral output via a so-called Fourier transformation.

In the Cornell team’s device, an input waveform enters an optical fibre and mixes with a pump laser pulse, which ensures the waveform matches the focal length of the lens. As the waveform travels through the fibre it stretches out or “disperses”. Then, at the end of the fibre the lens — a nano-scale silicon waveguide — converts the waveform into a spectrum that can be measured with a spectrometer (Nature 456 81).

The device can record an input waveform at a resolution of 220 fs over lengths greater than 100 ps, giving the largest length-to-resolution ratio (more than 450) of any snapshot oscilloscope technique. Moreover, the technique uses components that can easily be integrated on chips.

Watching Electrons with Lasers

2008-11-10T20:32:00.000-06:00

(PhysOrg.com) -- A team of researchers from the Stanford PULSE Institute for Ultrafast Energy Science at SLAC National Accelerator Laboratory has recently moved a step closer to visualizing the motions of electrons in molecules using a technique called high harmonic generation, or HHG.

Understanding these movements may help scientists better understand the early stages of chemical reactions. Electrons fuel chemical reactions. When chemicals react, electrons move between the molecules, building and breaking the connections, or bonds, that link atoms.

But in the world of quantum mechanics, electrons aren't easy to pin down. Physicists and chemists create mathematical descriptions called orbitals to illustrate the chance of finding an electron at a specific location of a molecule. Representations of these orbitals look like balloons attached to an atom's nucleus, the center of the atom.

SLAC researcher Markus Guehr and the PULSE team used HHG to learn about the electron orbitals of nitrogen gas molecules. In an HHG experiment, the researchers use molecules as tiny accelerator light sources. A laser beam is focused onto a stream of cooled nitrogen gas. The electric field of the laser tears an electron from a nitrogen molecule. As the laser field oscillates, the electron is accelerated back into the molecule and recombines with its orbital. Once the electron returns to the molecule, its energy is converted into light in the extreme ultraviolet range.

The spectrum of the light emanating from the molecule depends on the nature of the orbital the electron hits. By analyzing the number of photons at particular energies produced by this molecular laser, the team can characterize a specific orbital in the molecule.

But to understand how electrons move within a molecule over time, physicists need to characterize multiple orbitals.

In a report published in Science Express on October 30, the PULSE team, which also included Brian McFarland, Joseph Farrell and PULSE director Philip Bucksbaum, described the first evidence of HHG light signals from two different orbitals. Before these experiments, scientists had observed only light generated from electrons colliding with an orbital called the highest occupied molecular orbital, or HOMO. This orbital is the highest energy orbital that contains an electron. Physicists had theorized that detecting other orbitals was possible, but no one had observed multiple signals in an experiment.

The PULSE team reported detecting light from another orbital called the HOMO-1, which is one energy level lower than the HOMO. To detect light from the HOMO-1, the researchers had to align the nitrogen molecules perpendicular to the laser's electric field, to produce more efficient collisions between electrons and the orbital.

Science article: http://www.sciencemag.org/cgi/rapidpdf/1162780.pdf

Generating Monoenergetic Heavy-Ion Bunches with Laser-Induced Electrostatic Shocks

2008-11-08T08:27:00.002-06:00

(PhysOrg.com) -- “When a laser goes through a plasma,” John Cary tells PhysOrg.com, “it pushes electrons away. Then when it snaps back, it generates an electric wake behind the laser pulse, picking the electrons up and carrying them along.” Cary is a physics professor at the University of Colorado in Boulder, as well as the founder of Tech-X Corporation, a company that specializes in computational physics and simulation software. He is a member of a collaboration that wanted to see if it was possible to accelerate heavy ions with a laser.

“Accelerating electrons is easier, because they are light,” he says. “Instead, we wanted to see if there could be the possibility of doing this with protons and heavier nuclei.” The collaboration, a team from the Shanghai Institute of Optics and Fine Mechanics in China and Cary, produced a simulation outlining possibilities. The results of the simulation are reported in Physical Review Letters: “Generating Monoenergetic Heavy-Ion Bunches with Laser-Induced Electrostatic Shocks.”

Cary says that the information found in the simulation may have a variety of applications. “But the most exciting application, and the one that many people are looking to use,” he points out, “is for use in cancer therapy.”

The simulation shows that for heavier ions, it is possible to accelerate them, as well as control what is known as the Bragg Peak. “When you have a small charge to mass ratio,” Cary explains, “as an ion beam travels through matter, it deposits energy. At the end, just before it comes to rest, there is a very sharp peak of energy deposition.” This Bragg Peak is used in proton therapy to concentrate the energy on cancerous tumors to destroy them.

But there can be a problem: “If the beam is not monoenergetic, the peak smears out, potentially overlapping healthy tissue, which can then be damaged,” Cary says. “Researchers are trying to narrow this peak so that it is more precise, destroying the tumor but not the surrounding healthy cells.” This new simulation implies that this could be possible: “We found that carbon may have what is needed. The configuration seems to have nice properties, with a small energy spread and a fair amount of beam.”

More information: Generating Monoenergetic Heavy-Ion Bunches with Laser-Induced Electrostatic Shocks, Phys. Rev. Lett. 101, 164802 (2008)

Single laser traps and dissects cells

2008-11-07T08:15:00.000-06:00

(Optics.org)--Researchers have shown for the first time that a single near-infrared laser can be used to both trap and dissect living cells.

A team of researchers from the Philippines and Japan has shown that a single laser can be switched between continuous wave (cw) and femtosecond pulsed mode to trap and penetrate yeast cells respectively. The researchers say that using a single laser with two operational modes simplifies the trapping system and allows complex and intricate manipulation of cell components (Review of scientific instruments 79 103705).

"As far as we know, this is the first time that the cw-mode of an ultrafast laser is functionally used in a system," Vincent Ricardo Daria, a researcher at the University of the Philippines, told optics.org. "While optical trapping and optical surgery is not new, it is the combination of both functionalities using a single laser that makes this work unique."

Optical trapping requires a low energy laser to avoid damaging the sample, while the opposite is required for optical surgery. Until now, two separate lasers were necessary to carry out each function - one for trapping and another for surgery. Daria and colleagues have now developed a single 780 nm Ti:Sa laser that when operated in cw mode enables non-invasive trapping of cells and by switching to femtosecond pulsed mode, precise surgery can be performed.

"When the laser is in cw mode using around 10 mW of power, we can perform the trapping part of the experiment," explained Daria. "By modulating the intensity of the incident laser at a repetition rate of 80 MHz, at the same power level, we can attain incision sizes smaller than the diffraction limit of light."

The team switched between cw and pulsed mode by adjusting the output of the pump laser. "Ultrafast lasers normally come with an electronic control module to trigger fs-pulse mode," explained Daria. "CW-mode is achieved by disturbing the fs-laser cavity by temporarily shutting-off the pump laser."

In the setup, the laser is introduced into an inverted laser-scanning microscope and brought to focus inside the sample using a water immersion objective lens. A dichroic mirror is used to direct the NIR laser to the objective lens while allowing images of cells at the focus of the same objective lens to be viewed on a CCD camera.

According to Daria the potential of such a setup goes beyond trapping and surgery of yeast cells alone. The combined system could also be used for yeast cell growth analysis, tissue and cell engineering and micro-manipulation for reproductive medicine.

Ultrafast lasers give researchers a snapshot of electrons in action

2008-10-30T16:13:00.002-05:00

(PhysOrg.com) -- In the quest to slow down and ultimately understand chemistry at the level of atoms and electrons, University of Colorado at Boulder and Canadian scientists have found a new way to peer into a molecule that allows them to see how its electrons rearrange as the molecule changes shape.

Understanding how electrons rearrange during chemical reactions could lead to breakthroughs in materials research and in fields like catalysis and alternative energy, according to CU-Boulder physics professors and JILA fellows Margaret Murnane and Henry Kapteyn, who led the research efforts with scientist Albert Stolow of the Canadian National Research Council's Steacie Institute for Molecular Sciences.

To be able to chart a chemical reaction, scientists need to be able to see how bonds are formed or broken between atoms in a molecule during chemical reactions. But only extremely limited tools are available to view the rapidly changing electron cloud that surrounds a molecule as the atoms move around, Murnane said. Changes in the electron cloud can happen on timescales of less than a femtosecond, or one quadrillionth of a second, representing some of the fastest processes in the natural world.

In a paper to appear in the Oct. 30 issue of Science Express, the online version of the journal Science, the CU team describes how they shot a molecule of dinitrogen tetraoxide, or N2O4, with a short burst of laser light to induce very large oscillations within the molecule. They then used a second laser to produce an X-ray, which was used to map the electron energy levels of the molecule, and most importantly, to understand how these electron energy levels rearrange as the molecule changes its shape, according to Kapteyn.

The researchers describe their process of stretching the N2O4 molecule as being similar to pulling on a Slinky toy and then letting it go and watching it vibrate. They used the N2O4 molecule because it vibrates more slowly compared to other molecules, allowing them to observe the physical processes under way.

In many ways, molecules are like tiny masses connected by tiny springs of differing strengths, Murnane said. These springs are the chemical bonds, made up of shared electrons, which hold all matter together. In this experiment they used ultrafast laser pulses to "twang" these springs, making the nanoscale molecular Slinkies vibrate. However, unlike real springs, when researchers vibrate the molecules their properties can change, she said.

Being able to watch and understand why the electrons did what they did is very useful in fields like alternative energy, according to the researchers.

Chirped fibre delivers short pulses

2008-10-28T16:05:00.001-05:00

Reported from optics.org: A photonic crystal fibre has for the first time been engineered to transmit sub-100 fs pulses over extended distances.

Researchers from Russia and Germany have fabricated a chirped photonic crystal fibre that guides ultrashort pulses with much less distortion than previous designs. The team says that applications requiring ultrashort pulses and flexible beam delivery such as photodynamic therapy and two-photon microscopy could benefit (Nature Photonics doi:10.1038/nphoton.2008.203).

"No fibre-based technology performs as well in terms of guiding sub-100 fs pulses of considerable energy (nanojoules)," Günter Steinmeyer, a researcher at the Max Born Institute in Germany, told optics.org. "Our design effectively reduces distortion effects such as stretching of the pulses and pulse break-up, which decreases the peak power of the pulse."

The team fabricated photonic crystal fibres with core diameters of 22 and 53 microns and believes that fibre lengths of up to 1 km can be manufactured. The fibres operate around 800 nm range and exhibit transmission bandwidths of up to 120 nm.

Until now, photonic crystal fibres have been manufactured so that every cell in the design is of equal size. The team has instead introduced a radial chirp so that the cell size changes along the radius of the fibre.

"Chirping is a popular concept in ultra-fast optics and has already been successfully applied in one dimensional photonic structures such as chirped mirrors and chirped fibre Bragg gratings," commented Steinmeyer. "Chirping usually increases the dispersion of the device, but in chirped photonic fibre, dispersion effects are much weaker than an unchirped structure."

In the design, the fibre consists of a hollow core surrounded by five circular layers of glass tubes of different diameters. Each layer consists of 30 identical cells with radii ranging from 1.35 µm at the innermost layer to 2.6 µm at the outermost layer.

Steinmeyer and colleagues speculate that the weaker dispersion is due to a process called amorphization.

"Depending on the wavelength, light experiences reflection in different resonant sections of the chirped cladding, effectively localizing reflection to a particular layer of the structure," explained Steinmeyer. "We think that distributing the cell resonances over a wide wavelength range lessens their negative effect on the dispersion."

One drawback of the group's design is that it is no match for the extremely low linear guiding losses demonstrated by single-cell hollow core fibres. The group hopes to reduce these losses and fabricate fibres with even wider transmission bands by modifying the geometry of the fibre.

Powerful x-rays made from sticky tape

2008-10-23T07:22:00.001-05:00

Peeling ordinary sticky tape can generate bursts of X-rays intense enough to produce an image of the bones in your fingers.

Seth Putterman and colleagues from the University of California, Los Angeles used a motor to unwind a roll of sticky tape and recorded the electromagnetic emissions. Ripping the tape from its roll at 3 centimetres per second generated X-ray bursts of 15 kiloelectronvolts – each lasting one-billionth of a second, and containing over a million photons.

Putterman admits he is not sure exactly what is going on. "My attitude is to marvel at the phenomenon – all we are doing is peeling tape, and nature sets up a process that gives you nanosecond X-ray bursts."
Charged mystery

Exactly what drives this process is still a mystery, but it is well known that if two surfaces rub over one another, one becomes positively charged and one negatively charged.

In this case, the sticky adhesive becomes positive, and the polyethylene roll negative. This charge difference builds up until an electron jumps from the adhesive to the roll, with enough energy to produce X-rays when it hits the tape.

The strength of the X-rays means that they could be a useful source for X-ray photography.
Sticky tape fusion

Putterman has even loftier ambitions. "The energy in the X-rays is enough to generate nuclear fusion, if it is given to the molecules rather than the electrons," he says. "It's a matter of engineering design, not physics."

Tom Todd, chief engineer of UKAEA Culham Division says, "It is true that the emitted X-ray energies are broadly representative of the electron energies – and that, if you could produce copious quantities of deuterium and tritium [the heavy hydrogen atoms needed for fusion] ions at around 15 keV, in sufficiently high density, they would produce fusion reactions."

However, it is unlikely that all these conditions will be met at the same time, so any power produced from the fused nuclei would be tiny, compared to the power required to unwind the sticky tape.

"It's not unphysical, just uneconomical by a great many orders of magnitude," concludes Todd.

Journal reference: Nature, DOI: 10.1038/nature07378

Brilliantly bright light source is one step closer to reality

2008-10-10T07:10:00.001-05:00

The European X-ray Laser Project (XFEL) will harness a high energy short-wave laser light that is one billion times more brilliant than most modern x-rays to provide immensely detailed images of molecules and atoms.

Scientists believe a greater understanding of atoms and molecules could be used to develop better drugs to treat diseases or more environmentally efficient technologies for cleansing chemical effluents including carbon dioxide from the atmosphere.

Scientists will be able to carry out a range of experiments that were previously impossible before. For instance, researchers will be able to film atoms as they undergo chemical reactions, or see molecules that were once too small for conventional technology, and analyze gas plasma, the stuff of which stars are made, in microscopic detail.

To see these images, electrons are shot down a 3.3 km long tube at very high speeds and are stimulated to emit x-ray light. These can analyze molecules and atoms in unprecedented detail because the x-ray light emitted is at extremely short wavelengths, between six and one tenth of a nanometer, which enables very high resolution images to be taken of microscopic surfaces.

Countries participating in the XFEL project include Denmark, France, Germany, Greece, Hungary, Italy, Poland, Russia, Slovakia, Spain, Sweden, Switzerland, China and the UK.

Europe moves forward with laser-fusion plans

2008-10-09T14:07:00.003-05:00

I have blogged a report about this HiPER project last year, here I cited a report from physics world website.

Physicists and politicians from across Europe and beyond gathered at London's Science Museum on Monday to mark the beginning of a three-year "preparatory phase" of a new €1bn project known as the European High Power Laser Energy Research Facility (HiPER). So why do we need another fusion energy project? physicsworld.com looks for the answers.

What is HiPER?

HiPER is designed to show that laser-driven fusion can provide the world with energy in the future. The idea is to direct a series of extremely powerful laser beams onto a small capsule of deuterium-tritium fuel, heating up the outer surface of the capsule and forcing it to expand outwards, which, by Newton's third law, causes the centre of the capsule to implode.

Another ultra high-power laser heats this high-density core to around one hundred million degrees Kelvin. This energizes the deuterium and tritium nuclei sufficiently so that they overcome their mutual repulsion and fuse, releasing excess energy in the form of neutrons, which can be used to produce electricity.

Aren't physicists already studying inertial confinement?

They are, but of a different sort. Scientists know that inertial-confinement works, since it is this that generates the fusion reactions inside hydrogen bombs. These bombs use an initial fission explosion to rapidly compress a deuterium-tritium mixture, with shock waves created inside the mixture heating it to the point of ignition. This "central ignition" process is being reproduced in a controlled way at billion-dollar military facilities — the National Ignition Facility (NIF) at the Lawrence Livermore Laboratory in the US and the Mégajoule laboratory in France — where a single set of lasers both compresses and heats the fuel. HiPER, on the other hand, will use a separate laser pulse to do the heating, a process known as "fast ignition" because the second laser must heat the fuel within 10-11s of the implosion.

What are the advantages of fast ignition?

It is more efficient than central ignition. Setting up shock waves requires the fuel to be compressed to enormous densities, which needs very high laser energy per unit mass of fuel. Since fast ignition requires only intermediate densities, it can in principle be used to ignite a larger mass of fuel for a given input energy. And more mass equals more output energy, which means higher efficiencies. In fact, proponents of fast ignition reckon that it is some two to three times more efficient than central ignition. In addition, fast ignition does not require the same degree of precision in the uniformity of the compressing laser pulses and the shape of the fuel pellet.

So where does HiPER fit in?

HiPER is being designed to show that fast ignition, once proven in principle, can then be used as an energy source. This means demonstrating that the fusion process can be repeated at high frequencies. Inertial confinement is a pulsed technique — similar in principle to the repeated cycles of chemical combustion in the engine of a car — and at NIF the laser system fires perhaps once a month, whereas a commercial power plant would need to fire about five times a second to provide the 2 gigawatts typical of a large power station. HiPER will trial the fully-robotic process needed to achieve such a frequency.

What happens next?

The six countries that have officially backed HiPER - the UK, France, Spain, the Czech Republic, Italy and Greece - marked the formal start of a three-year "preparatory phase" for the project on Monday. This phase, which will involve detailed studies of short-pulsed lasers and fuel pellets, as well as decisions on costs, location etc., is being funded with €13m of cash and €50m of work in kind.

Some two to three years down the line another €100m will be needed to develop prototypes, and a few years after that the remainder of the roughly ?1bn construction costs will be needed to actually build the thing. Operating costs over the facility's roughly 20-year life time will also be about €1bn. If all goes well, the facility should start up by around the end of the next decade. As to where it will be built, this depends ultimately on who is prepared to commit the cash, but the UK, which is coordinating the project, is certainly in the running.

When might a commercial fusion plant start operating?

The billion-dollar question. The quest to derive energy from nuclear fusion has been plagued by wildly optimistic expectations in the past, and critics have quipped that fusion is always 40 years from commercialization. Fusion advocates, however, are confident that it could happen by about 2050. Indeed, Dunne thinks this estimates holds good for both magnetic and inertial confinement. He concedes that magnetic fusion is "a generation ahead" of its laser equivalent, but believes that fast ignition could potentially close the gap quickly.

David Meyerhofer of Rochester University believes that fusion reactors could ultimately replace all large power plants and be used to extract hydrogen from water for transport. "Thus," he says "it is possible that fusion could eventually produce more than 50% of the world's energy needs." However, he adds that this estimate is "very speculative".

Lasers slim down radiotherapy equipment

2008-09-22T07:18:00.001-05:00

Hospitals could benefit from a new technique that uses ultra-short laser pulses to simplify radiotherapy equipment.

via www.optics.org

Researchers from Italy, France and Germany have shown that a tabletop laser can be used to accelerate a beam of electrons suitable for use in radiotherapy. The group, led by Antonio Giulietti of the Institute for Physical Chemistry Processes in Pisa, believes that such laser-based particle acceleration could considerably reduce the size and simplify the operation of radiotherapy facilities (Physical Review Letters 101 105002).

In radiotherapy beams of photons, electrons, protons, neutrons or ions are used to destroy tumours by ionizing the atoms within the tumours' DNA. Usually this involves irradiating the patient from a number of different directions in order to pinpoint the tumour, and in the case of deep tumours, using higher-energy particles. This inevitably leads to some damage of the healthy tissue surrounding the tumour.

Damage limitation

Damage can be limited using a technique known as intraoperatory radiotherapy (IORT), which involves irradiating the patient just once with electrons. This occurs in the operating theatre right after the tumour has been surgically removed. The idea is to destroy tumour cells that the surgery has missed. Because they do not have to penetrate deeply, the electrons can be fewer in number and have a lower energy, which means that the accelerators employed can be smaller.

However, as Giulietti points out, IORT, like ordinary radiotherapy, still uses radiofrequency electric fields to accelerate the electrons, which requires a machine more than two metres high and over half a tonne in weight. The machine must be shielded from the operating theatre and any maintenance requires the shut down of the theatre. "This therefore limits the energy of the electrons that can be used in the technique," he adds.

Giulietti and colleagues have shown that these problems can be overcome by using a laser rather than radiofrequency electric fields to accelerate the electrons.

At the SLIC laboratory in Saclay, France, the researchers fired ultra-short laser pulses onto a jet of gas, creating a plasma with a fluctuating electron density. The electric field generated by these fluctuations accelerated the free electrons within the plasma such that they had energies and spatial characteristics suitable for use in IORT. By then passing these electrons through a 2 mm–thick piece of tantalum (and therefore decelerating them rapidly) the researchers were able to create gamma–ray photons that could also be used in radiotherapy.

Just a small metal box

Because the laser beam can travel for several tens of metres without any appreciable loss, the laser itself can be located outside the operating theatre. According to Giulietti, the only thing that would need to be in the theatre is a metallic box perhaps 50 by 20 by 20 cm across that would convert the laser beam into the electron beam, and which would contain a roughly 10 cm–long device to generate the gas jet and focusing optics of a similar size.

Giulietti points out that scaling up the facility would allow IORT to be carried out at higher energies than is currently possible, which would render the technique more effective against certain kinds of tumours. He adds that more work is needed to design a laser–based system suitable for actual hospital use, in particular ensuring the stability of both the laser output and the acceleration process within the plasma.

The brightest, sharpest, fastest X-ray holograms yet

2008-08-05T21:18:00.004-05:00

A group of scientists have produced two of the brightest, sharpest x-ray holograms of microscopic objects ever made. Working at both the Advanced Light Source (ALS) at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, and at FLASH, the free-electron laser in Hamburg, Germany, this group is boasting a method that is thousands of times more efficient than previous x-ray holographic methods.

Inspired by an ancient technique known as the pinhole camera, the x-ray hologram (made at ALS beamline 9.0.1) was of Leonardo da Vinci’s “Vitruvian Man.” This lithographic reproduction of less than two micrometers (millionths of a meter) square, was etched with an electron-beam nanowriter. The hologram required a five-second exposure and had a resolution of 50 nanometers (billionths of a meter).

The other hologram, made at FLASH, was of a single bacterium, Spiroplasma milliferum, made at 150-nanometer resolution and computer-refined to 75 nanometers, but requiring an exposure to the beam of just 15 femtoseconds (quadrillionths of a second).

The values for these two holograms are among the best ever reported for micron-sized objects. With already established technologies, resolutions obtained by these methods could be pushed to only a few nanometers, or, using computer refinement, even better.

Holography was invented over 60 years ago by the physicist Dennis Gabor, but its use has long been limited by technology. Whereas a pinhole camera employs ray optics, in which the photons travel like a stream of particles, holography depends on the wave-like properties of light.

The principle is straightforward: a beam of light illuminates an object, which scatters the light onto a detector such as a photographic plate, while a second, identical beam of light shines directly on the detector. The scattered light waves from the object beam form interference patterns with the unscattered light waves from the reference beam.

This interference pattern serves to reconstruct an image of the object. One easy way to do so, if the detector is a photo transparency, is for the observer to look through the transparency in the direction of the (now absent) object; if only the reference beam is shining on the detector, the interference pattern serves to “unscatter” (diffract) the wavefront and reconstruct the object’s image.

Lasers, which produce coherent light, were the first invention that made holography practical; it is now possible to make small holograms using just a laser pointer. FLASH is a powerful free-electron laser (FEL); a new generation of FELs of much shorter wavelength will be capable of producing coherent light pulses so short they’ll be able to freeze atomic motion in the midst of chemical reactions.

Soft x-rays like those from ALS beamline 9.0.1 can also be made coherent, or laser-like, using a pair of pinholes. (The beam is conditioned by these pinholes, but they are not directly involved in imaging, except to make the beam laser-like.) To make a hologram, the beam issuing from the synchrotron scatters from the target object and is collected on a CCD detector. Meanwhile, the same beam simultaneously passes through the multiple-“pinhole” URA, mounted on the same plate as the target object, and produces a bright reference beam.

The scattered image of the object and the many overlapping reference beams from the URA combine to make an interference pattern which contains all the information, including the relative depth of individual features, needed to mathematically reconstruct a three-dimensional image of the object.

The hologram of the Spiroplasma bacterium was made in precisely the same way, with much brighter x-ray beams and a much shorter pulse of light. So bright was the flash of light that the sample was vaporized, but not before both the scattered object beam and the reference beams from the URA had been recorded.

Together, the two experiments demonstrate that holographic x-ray images with nanometer-scale resolution can be made of objects measured in microns, in times as brief as femtoseconds. Moreover, sample preparation time is fast and easily repeated for high throughput during repetitive experiments.

Citation: "Massively parallel x-ray holography," by Stefano Marchesini, Sébastien Boutet, Anne E. Sakdinawat, Michael J. Bogan, Sǎsa Bajt, Anton Barty, Henry N. Chapman, Matthias Frank, Stefan P. Hau-Riege, Abraham Szöke, Congwu Cui, David Shapiro, Malcolm Howells, John Spence, Joshua Shaevitz, Joanna Lee, Janos Hajdu, and Marvin M. Siebert, appears in advanced online publication of Nature Photonics and is available online to subscribers at http://dx.doi.org/10.1038/nphoton.2008.154 .

High-dispersion mirrors shrink femtosecond laser

2008-07-28T16:52:00.000-05:00

Ti:sapphire femtosecond lasers currently use optical systems based on prisms or diffraction gratings to stretch and recompress pulses before and after amplification. These optical systems are complex, rather lossy and alignment sensitive. Researchers from Ferenc Krausz's group at the Ludwig-Maximilians University and Max-Planck Institute of Quantum Optics, both in Garching, Germany, believe high-dispersion mirrors (HDMs) are the solution (Optics Express 16 10220).

Vladimir Pervak and his colleagues believe low-loss, HDMs can take over the role of prisms and possibly gratings in conventional chirped-pulse amplifier (CPA) systems with the added benefit of providing high-order dispersion control.

The group has demonstrated the usability of HDMs in high-energy femtosecond oscillators, such as a chirped pulse Ti:Sapphire oscillator and an Yb:YAG disk oscillator. In both cases a group delay dispersion (GDD) of the order of 2 × 104 fs2 was introduced, accompanied with an overall transmission loss as low as ∼ 2%.

The penetration depth of spectral components into the HDM structure. The electrical field components at 830 nm penatrate much deeper into the multilayer structure than the components at 770 nm. This means that the 830 nm components become delayed relatively to the 770 nm components.

The group had to make mirrors with very high dispersion in order to replace prisms and gratings. To make the mirrors, the researchers used magnetron sputtering to deposit alternate layers of tantalum pentoxide (Ta2O5) and silicon dioxide (SiO2). These materials have high (2.12 @ 800 nm) and low (1.47 @ 800 nm) refractive index, respectively. The resultant HDMs have layer thicknesses ranging between 25 nm and 400 nm, and a total physical thickness of approximately 10 µm. The total group delay (GD) in the HDM structure is a result of two combined effects: penetration effect (used in a conventional dispersive mirror); and an interferometer effect. "For our HDMs, the maximal GD that can be obtained by the pure penetration effect is 100 fs," said Pervak. "But our HDM provides a total GD of 150 fs. Therefore, 50 fs of the delay can be attributed to the interferometer effect."

He admits that making HDMs is challenging and that this has been the limiting factor to their use in this application. "But the advantages they offer means that it is worth the effort," he said. "When compared with using prisms, HDMs offer a much higher output efficiency; have no wavelength bandwidth limit; enable a more compact system; and give a clean pulse with no satellite pulses."

However, extraordinary sensitivity of the HDM design to manufacturing errors suggests that it may be difficult to manufacture a HDM with well-established technologies, such as electron-beam (ion-assisted) evaporation and ion-beam sputtering.

Ultrafast technology shifts to wafer scale

2008-07-13T16:31:00.000-05:00

Could compact femtosecond laser be used to clock the multicore computer processors of the future? Optics & Laser Europe magazine recently reports ultrafast pioneer Ursula Keller of ETH Zurich, Switzerland, to find out about her latest idea.

Ultrafast VECSELs(vertical external-cavity surface-emitting lasers) have three main cavity elements: the gain structure, an output coupler and a semiconductor saturable absorber mirror. The essence of Keller's idea is to integrate the gain and the saturable absorber into a single structure. With the only other cavity element being an output coupler, Keller thinks that MIXSELs(modelocked integrated external-cavity surface-emitting laser) could become a true wafer-scale technology.

The full file link: http://images.iop.org/objects/optics/analysis/13/7/2/pdf.pdf

Ultrashort pulses create ultrabroad source

2008-07-12T16:13:00.001-05:00

By sending laser pulses with a duration of just 5 femtoseconds through a helium cell held at high pressure, researchers have created a coherent supercontinuum with near-uniform spectral intensity spanning the range 270 to 1000 nm. The result relies on a process known as self-channeling and gives the team a new tool with which to explore electron motion inside atoms (Optics Letters 33 1407).

"Our ultimate goal is to generate coherent continuum light that spans several optical octaves," researcher Eleftherios Goulielmakis from the Max-Planck Institute for Quantum Optics in Garching, Germany, told optics.org. "In attosecond physics, we aim to steer the electron motion on atomic scales of space and time. To do this, we require fields that can be precisely controlled and shaped with sub-cycle (attosecond) accuracy and that are intense enough to enable nonlinear interactions with matter."

High and near-uniform efficiency are the prerequisites for generating light fields on a sub-cycle scale. While supercontinuum generation has been at the forefront of ultrafast research for several years, with groups using nonlinear propagation in photonic crystal fibres and solids, this prerequisite combination has remained elusive.

"Using few-cycle pulses dramatically improves the situation," explained Goulielmakis. "Once the duration of the pulse approaches the oscillating period (around 2.5 fs) of the light wave, phenomena like ionization-induced blue shift and shockwave effects result in a dramatic enhancement of the generation of light in the blue wing of the spectrum. We have been able to generate light that extends into the UV part of the spectrum at nearly uniform intensity."

The team focused 5 fs pulses with a central wavelength of 750 nm into a gas cell filled with helium. Self-channeling sets in at a pressure of around 25 bar, which results in a 5 cm long channel and a substantial reduction of the beam divergence in the far field. A intensity-calibrated fibre spectrometer monitored the emerging supercontinuum.

With this impressive result under its belt, the team now has several new experiments in the pipeline. "We plan to extend the supercontinuum source into the VUV by means of quasi-monocyle (~ 1.5 cycles of the field) laser pulses recently realized in out laboratories," said Goulielmakis.

A second follow-on experiment will see Goulielmakis and colleagues split the supercontinuum into narrower bands. The plan is to control properties such as the duration, phase and amplitude of these narrower bands separately before recombining them to synthesize intense light waveforms with a desired shape. "We plan to use these waveforms to control the generation of intense attosecond soft x-ray pulses from atoms," said Goulielmakis.

Other partners in the team come from the Technical University of Vienna, Austria; the Lomonosov Moscow State University, Russia; and the Ludwig-Maximilians University, also in Garching, Germany.

Source: optics.org

FROG reaches the attosecond scale

2008-07-09T07:24:00.003-05:00

Researchers in Germany have developed a new FROG retrieval method for characterizing the electric field of attosecond pulses. The technique works by analysing attosecond streaking measurements and could be used to probe numerous physical phenomena that occur on the attosecond timescale (Applied Physics B 92 25).

"Conventional FROG (frequency-resolved optical gating) algorithms lose their accuracy when we try to characterize shorter pulses," Justin Gagnon of the Max Planck Institute for Quantum Optics in Garching told optics.org. "But ours retains its accuracy and robustness no matter how short (or broadband) the XUV pulse."

No electronic device is fast enough to record the electric field of ultrashort attosecond (10-18 s) pulses. Researchers therefore rely on less direct techniques, such as attosecond streaking spectroscopy, to characterize these fields.

Attosecond streaking spectroscopy measures streaked electron kinetic energy spectra by photoionizing an atom with an attosecond XUV pulse in the presence of an infrared laser field. By varying the delay between the XUV and IR pulses, a sequence of streaked spectra, known as a streaking spectrogram, is obtained.

This spectrogram contains complete phase and amplitude information about the XUV and IR field that can be extracted using FROG retrieval. "Extracting phase information from the spectrogram is an example of a 2D phase retrieval problem, known to possess a solution," explained Gagnon. "By applying alternating constraints between the frequency and time domains, the FROG retrieval algorithm can identify the XUV and IR pulses that reproduce the measured spectrum."

The y-axis is electron energy, and the x-axis is the delay between the XUV and IR fields. This spectrogram shows how the electron spectrum is modified by the laser field, as a function of the delay. (See the right picture)

The new technique is able to characterize pulses that last just 80 as, compared with the 130 as achieved by previous researchers. Gagnon and co-workers have also optimized FROG retrieval and have established the range of experimental parameters for which this technique can be used.

Attosecond pulses are essential tools that will allow us to probe physical phenomena occurring on the attosecond timescale, said Gagnon. These include electron dynamics in atoms, molecules and metal surfaces, Auger decay and autoionization. Attosecond streaking spectroscopy can be used to glean information about electron wave packets in such processes because a streaking spectrogram contains information about the time-related behaviour of these wave packets – and therefore time-related information about the attosecond process itself.

Chirped fibre laser drills faster

2008-07-06T09:45:00.001-05:00

A chirped-pulse amplification (CPA) fibre laser is helping researchers in Germany drill holes in metal faster than conventional laser sources. The high-energy femtosecond pulses emitted by the source are allowing the team to investigate laser-metal interactions at repetition rates approaching 1 MHz for the first time (Optics Express 16 8958).

"Both heat accumulation and particle shielding are investigated for the first time at high repetition rates," Antonio Ancona, a visiting scientist at the Friedrich-Schiller University in Jena, told optics.org. "We have found that our fibre CPA system decreases the process time, but still produces a high-quality hole, which is important for industrial applications."

The precision, quality and reliability of holes produced using laser systems that deliver pulses of around 1 ns or longer is limited. This is because the ablation of metals is often accompanied by the formation of large heat-affected zones and a throw-out of the molten material.

Although femtosecond lasers offer improved performance, one of the main drawbacks is a low processing speed, which results from the lower repetition rate of these sources. On the other hand, at high repetition rates, heat accumulation effects might lead to melting and increased heat-affected zones. A further restriction on the highest useful repetition rate could be the interaction of the generated plasma or ablated particles with subsequent laser pulses as this will distort or shield the laser radiation.

The group's approach was to develop a CPA fibre laser system and investigate the effects of particle shielding and heat accumulation on the process time and hole quality for repetition rates ranging from 50–975 kHz. The 1030 nm source emit 800 fs pulses with energies ranging from 10–70 µJ and a corresponding average power of up to 68 W. The laser unit consists of a passively modelocked Yb:KGW oscillator, a dielectric grating stretcher-compressor unit, an acousto-optic modulator for pulse selection and two ytterbium-doped photonic crystal fibres, both in single-pass configuration, as amplification stages.

Two drilling techniques were studied: percussion – in which consecutive pulses are superimposed at the same focal spot, and trepanning – which involves moving the laser beam on a circular path relative to the target. The researchers found that the percussion approach offers faster drilling, but laser trepanning produces higher quality holes. "However, we found that working at such high repetition rates and pulse energies allows us to considerably reduce the processing time even when using the laser trepanning technique," commented Ancona.

The next steps for the Jena group are to investigate the influence of pulse duration from femtoseconds to tens of picoseconds in laser ablation processes at high average power and high repetition rates. "We are developing new processing strategies using the CPA laser system to further improve the processing speed and produce the highest quality holes," concluded Ancona. "We will also be scaling the output laser parameters to higher powers."

Antonio Ancona holds a permanent position at the CNR-INFM Regional Laboratory "LIT3" of Bari, Italy. The CNR-INFM of Bari is supported by the Italian Ministry of Research and University (MIUR) under the project 297 "FIBLAS". The University of Jena was supported by the German Federal Ministry of Education and Research (BMBF) under the PROMPTUS project.

Light Pulse Speed Record Set

2008-07-02T15:56:00.002-05:00

Researchers have set a new record in ultrafast metrology, producing the first light pulses lasting only 80 attoseconds (a billionth of a billionth of a second).

The 80-attosecond achievement marks the first time scientists have achieved light pulse speeds below 100 attoseconds and was accomplished by a team of physicists led by professor Ferenc Krausz at the Max Planck Institute for Quantum Optics (MPQ) in Garching and professor Ulf Kleineberg at Ludwig Maximilians University Munich, working in cooperation with colleagues at the Advanced Light Source at Berkeley Lab in California.

To generate attosecond pulses, the Garching physicists use the strong electric field of flashes in the near-infrared spectrum. In the hypershort laser flashes this field performs hardly more than a single strong oscillation with a period of about 2.5 femtoseconds (a femtosecond is 1000 attoseconds). That is: the light wave now comprises just two high wave peaks and a deep wave valley between them. The force exerted by the electric light field on the electrons is strongest at the summits and the lowest point of the valley; strong enough to liberate electrons which are ejected from rare-gas atoms in the experiment at Garching. This leaves ion rumps.

The vacuum chamber for attosecond metrology: Attosecond pulses of extreme ultraviolet light (depicted as a blue beam) are focused by a mirror (right) on a jet of neon atoms effusing from a thin valve. At the same time an infrared beam is striking the atoms. Both beams in combination allow real-time observation of the motion of electrons in the neon atoms and measurement of the duration of the attosecond pulse.

With the oscillation of the light field the force changes direction and very soon hurls the electrons back to the ion rumps. The recolliding free electrons induce extremely fast electron oscillations which last just attoseconds and emit light flashes of the same duration. These flashes are then in the region of extreme ultraviolet light (XUV, a wavelength of approximately 10 to 20 nm).

Controlled production of this single strong light oscillation within a hypershort flash has now allowed the Garching research team for the first time to release electrons exactly three times during a single laser pulse. On returning to the ion they then emit exactly three attosecond pulses. Each femtosecond laser flash generates three attosecond pulses. One of these pulses has a particularly high intensity, providing more than 100 million photons in a period of just 80 attoseconds.

This pulse is filtered out with special x-ray mirrors from Kleineberg, resulting in a single isolated x-ray pulse lasting 80 attoseconds.

A paper on their work, "Single-cycle Nonlinear Optics," appeared in the June 20 edition of Science.

Source: Photonics.com

Laser Microscalpel Created

2008-06-28T15:44:00.003-05:00

Femtosecond lasers have just become more accurate and versatile, thanks to Adela Ben-Yakar, mechanical engineering assistant professor at The University of Texas at Austin.

By nature, Femtosecond lasers produce extremely brief, high-energy light pulses that can sear a targeted cell so quickly and accurately that the lasers’ heat has no time to escape and damage nearby healthy cells.

However, the very same laser systems, typically used in LASIK and other eye surgeries, have been too bulky - until now. Ben-Yakar’s laboratory has developed a femtosecond laser microscope system that includes a tiny, flexible probe that focuses light pulses to a spot size smaller than a human cell.

Ben-Yakar dubbed her creation the Microscalpel.

The Microscalpel can destroy a single cell while leaving nearby cells intact, which could improve the precision of surgeries for cancer, epilepsy and other diseases.

"You can remove a cell with high precision in 3-D without damaging the cells above and below it," Ben-Yakar says. "And you can see, with the same precision, what you are doing to guide your microsurgery."

As a result, the medical community envisions the lasers' use for more accurate destruction of many types of unhealthy material. These include small tumors of the vocal cords, cancer cells left behind after the removal of solid tumors, individual cancer cells scattered throughout brain or other tissue and plaque in the arteries.

Within a few years, Ben-Yakar expects to shrink the probe's 15-millimeter diameter by three-fold, so it would match endoscopes used today for laparoscopic surgery. The probe tip she has developed also could be made disposable -- for use operating on people who have infectious diseases or destroying deadly viruses and other biomaterials.

To develop the miniature laser-surgery system, Ben-Yakar worked with co-author
Olav Solgaard at Stanford University's Electrical Engineering Department to incorporate a miniaturized scanning mirror. Ben-Yakar and her graduate student Chris Hoy, another co-author, also used a novel fiber optic cable that can withstand intense light pulses traveling from an infrared, femtosecond laser.

To make the intensity more manageable, they stretched the light pulses into longer, weaker pulses for traveling through the fiber. Then they used the fiber's unique properties to reconstruct the light into more intense, short light pulses before entering the tissue.

For the study, Ben-Yakar directed laser light at breast cancer cells in three-dimensional biostructures that mimic the optical properties of breast tissue. She has since studied laboratory-grown, layered cell structures that mimic skin tissue and other tissues.

Ben-Yakar is also investigating the use of nanoparticles to focus the light energy on targeted cells. In research published last year, she demonstrated that gold nanoparticles can function as nano-scale magnifying lenses, increasing the laser light reaching cells by at least an order of magnitude, or ten-fold.

"If we can consistently deliver nanoparticles to cancer cells or other tissue that we want to target, we would be able to remove hundreds of unwanted cells at once using a single femtosecond laser pulse," Ben-Yakar says. "But we would still be keeping the healthy cells alive while photo-damaging just the cells we want, basically creating nanoscale holes in a tissue."

Ben-Yakar's experimental system is described in the June 23 issue of Optics Express.

Grants from the National Science Foundation and the National Institute of Health funded the research.

Thin-disk laser yields energetic femtosecond pulses

2008-06-20T17:23:00.000-05:00

Ursula Keller's group at ETH Zurich in Switzerland has built the first Yb:YAG thin-disk laser to deliver femtosecond pulses with energies above 10 µJ. The team believes that the design could yield a compact source of high-power ultrashort pulses for applications such as high-resolution imaging and precision micro- and nanomachining.

"Our motivation was to increase the pulse energy of femtosecond oscillators," team member Thomas Südmeyer told optics.org. "In this way, many experiments and applications that previously relied on complex and expensive amplifier systems are now within the reach of simple and cost-efficient diode-pumped solid-state lasers."

Generating high-energy femtosecond pulses is a crucial requirement for many scientific and industrial applications. The normal approach in these applications is to exploit a laser amplifier system, but the repetition rate is typically limited to the kilohertz regime – which in turn affects the signal-to-noise ratio and also restricts the throughput and precision of materials processing applications.

Other techniques are capable of delivering microjoule pulses at megahertz repetition rates, but these require complex amplifier systems with a seed laser and multiple amplification stages. In contrast, the new laser developed by Südmeyer and colleagues produces femtosecond pulses directly using a high-power oscillator, eliminating the need for any external amplification.

The laser delivers up to 45 W of average power at a repetition rate of 4 MHz. This yields 11.3 µJ pulses with a duration of 800 fs and a peak power of 12.5 MW, which Südmeyer says is sufficient for driving high-field experiments.

According to Südmeyer, the latest results are the culmination of several years' work on thin-disk lasers, which enable high average powers to be achieved with good beam quality. The Swiss team use a multiple-pass cavity to extend the length of the resonator to 37 m, while stable femtosecond pulses are produced by passive modelocking using a semiconductor saturable absorber mirror (SESAM). "This results in a power-scalable solution for the generation of pulses with durations in the femtosecond regime," said Südmeyer.

Until now, however, the pulse energies that could be produced from this set-up were limited to a few microjoules. "Our initial effort to increase the pulse energy was limited by some excess nonlinearity, which initially was not identified," commented Südmeyer. "We found that the source of the additional instabilities was the nonlinearity of the air atmosphere inside the laser cavity."

The solution, says Südmeyer, is to flood the laser cavity with helium, which has a negligible nonlinearity compared with air. The repetition rate was reduced to 4 MHz in order to produce pulses with energies greater that 10 µJ.

The team now plans to increase the average power to beyond 500 W and the pulse energy towards 100 µJ. "We will also investigate in collaboration with Professor Huber from the University in Hamburg new thin disk materials, which will allow us to achieve shorter pulse durations," said Südmeyer.

The researchers reported their work in Optics Express.

Attosecond angular streaking

2008-06-16T16:23:00.000-05:00

Petrissa Eckle, Mathias Smolarski, Philip Schlup, Jens Biegert, André Staudte, Markus Schöffler, Harm G. Muller, Reinhard Dörner & Ursula Keller(Department Physik, ETH Zurich, Wolfgang-Pauli-Str. 16, 8093 Zurich, Switzerland)

Ultrashort measurement-time resolution is traditionally obtained in pump–probe experiments, for which two ultrashort light pulses are required; the time resolution is then determined by the pulse duration. But although pulses of subfemtosecond duration are available, so far the energy of these pulses is too low to fully implement the traditional pump–probe technique. Here, we demonstrate 'attosecond angular streaking', an alternative approach to achieving attosecond time resolution. The method uses the rotating electric-field vector of an intense circularly polarized pulse to deflect photo-ionized electrons in the radial spatial direction; the instant of ionization is then mapped to the final angle of the momentum vector in the polarization plane. We resolved subcycle dynamics in tunnelling ionization by the streaking field alone and demonstrate a temporal localization accuracy of 24 as r.m.s. and an estimated resolution of approximately 200 as. The demonstrated accuracy should enable the study of one of the fundamental aspects of quantum physics: the process of tunnelling of an electron through an energetically forbidden region.

Nature Physics
Published online: 30 May 2008 | doi:10.1038/nphys982

Brightest X-ray Vision at the Nano-scale

2008-06-12T07:12:00.001-05:00

X-ray beams from an energy-recovery linac (linear accelerator) could be both a thousand times brighter and a thousand times faster--with pulses as brief as one ten-thousandth of a billionth of a second--than current state-of-the-art synchrotron X-ray sources.

"We're closer than ever to building a kind of universal toolkit for all the science and engineering disciplines," says Joel D. Brock, a Cornell University professor of applied and engineering physics.

"To date, the best-existing X-ray diffraction machines like CHESS (the Cornell High Energy Synchrotron Source) have given us ‘snapshots' of life--still pictures, for instance, of a particular virus. ERL will give us 3-D movies as the virus moves, grabs on to a cell and propagates disease. We will have X-ray vision at the nano-scale," Brock predicts, suggesting some questions to be answered:

-- Can excited-state studies of photosynthesis yield less expensive, more efficient solar energy?

-- If deep-earth pressures and temperatures turn ordinary carbon into diamond, what will those forces do to carbon nanotubes?

-- What really happens in the split second when a stem cell "decides" to become heart muscle?

But an equally pertinent question for Brock and other advocates of the next-generation of X-ray sources is this: How much longer can biomedical researchers, chemists, materials and environmental scientists, engineers, nanotechnologists and biophysicists maintain their competitive advantages without an instrument like ERL?

How ERLs Work

Moving beyond traditional X-ray crystallography systems--where the arrangement of atoms in crystalline material is revealed by analyzing the way X-ray beams are scattered from electrons in the crystal--the energy-recovery linac offers significant advantages. For one, materials subjected to ultrabright X-ray pulses need not be in crystalline form. And the tightly focused beam allows studies at much smaller scales.

As envisioned and invented by experimental physicists at Cornell, energy-recovery linear accelerators produce high-energy, pulsed X-ray beams by injecting electrons into the electromagnetic fields of a series of superconducting microwave cavities in a linear accelerator. Then, in a return loop, the electron beam is turned into X-rays by passing through undulators, which force the beam to oscillate to the right and left of its mean path with horseshoe magnets of alternating orientations. The pulsed X-rays are now ready for studies in multiple stations at the facility.

While the ERL X-ray beam loses about 0.04 percent of its energy during oscillation, 99.98 percent of its remaining energy is recaptured into the electromagnetic fields when the electrons are re-injected into the linac for deceleration--providing energy to accelerate subsequent bunches of electrons.

Compared to a traditional storage-ring X-ray source, such as CHESS, which recycles electrons billions of times but suffers from a compromised beam size, ERLs send each bunch of electrons through the undulators only once. Again and again, ERLs recover and reuse energy that accelerates electron bunches, while maintaining very small beam size--the key to the brilliance needed to study intimate details at the nano-scale.

The superconducting microwave cavities, which are cooled to -456 degrees Fahrenheit to produce hardly any heat during continuous operation, are among the novel components that proved their worth during the prototype-testing stage of the ERL project. Another component was the photocathode gun that produces electrons--in extremely intense short-duration bunches--for acceleration in the superconducting microwave cavities.

What Comes Next?

Development of ERL technologies, as well as prototype production and testing, was made possible by about $18 million in support from the National Science Foundation (NSF) and $12 million from New York State (for civil engineering feasibility studies, plus technology and infrastructure development). Cornell University has invested some $10 million in the project, with additional investment planned. ERL technology-development studies were conducted in conjunction with physicists at Jefferson Laboratory (the Thomas Jefferson National Accelerator Facility) in Newport News, Virginia.

Because ERL technology was developed with public money, it is now available to any institution that hopes to build a next-generation X-ray source--including Cornell University, which will propose assistance from federal and state sources.

Construction of an ERL X-ray facility--with national and international availability to researchers in all fields of science and engineering--is estimated to cost between $300 million to $400 million. Just as an ERL recovers energy, building an ERL in Ithaca, New York, Cornell officials observe, would save money by repurposing parts of CHESS and the Wilson Synchrotron Laboratory that were built at Cornell with public resources.

ERL for All

Cornell's Joel Brock wants an ERL, wherever it is built, because his particular line of research needs better X-rays.

"I'm trying to understand the growth of thin films of electronic materials, and it certainly would help to watch--in atomic detail--as we form exotic new materials for advanced optoelectronic applications," he says.

"But the beauty of ERL beams is that they can be used, simultaneously, for every form of science, from archaeology to zoology. In one station on the beam line on any given day you might have an environmental scientist working next to an art historian and a biophysicist--from Minneapolis or Beijing or Amsterdam. ERL really can become a universal toolkit."

Source: NSF, by Tracy Vosburgh

Laser sets heart beating to a new rhythm

2008-06-10T16:32:00.002-05:00

Researchers in Japan have shown that a train of femtosecond laser pulses can cause heart muscle cells to contract and synchronize to the laser exposure. This optical pacemaker effect could provide crucial insights into abnormal heart rhythms and be combined with anti-fibrillation drugs to understand these effects at the cellular level. (Optics Express 16 8604)

"Calcium regulates the contraction of cardiomyocytes (heart muscle cells)," Nicholas Smith from Osaka University told optics.org. "We knew that if we could artificially perturb the calcium levels in the cell, we could control the beating and change its frequency. We used periodic femtosecond laser irradiation to synchronize the cell beat frequency and effectively create a laser pacemaker for the cells."

The team used a Ti:sapphire laser operating at 780 nm and emitting 80 fs pulses at a repetition rate of 82 MHz to stimulate the contraction. The optimal conditions were found to be average powers of between 15 and 30 mW and 8 ms exposures applied periodically at 1–2 Hz using a mechanical shutter.

The laser synchronization works by causing a transient leak from the cell's calcium stores. When the cells are cultured (in this case neonatal rat cardiomyocytes), the team introduces a fluorescent tag called Fluo-4, which it monitors using fluorescence microscopy to see the cells contracting.

Smith and his colleagues carried out over 200 experiments on single as well as groups of cardiomyocytes using different laser powers and periodicities. The synchronization of the cell contraction to the laser periodicity occurred in approximately 25% of all trials for average laser powers of between 15 and 30 mW.

"The laser contractions are as strong as the spontaneous contractions without the laser," said Smith. "We did not expect that whole cell groups would so easily synchronize with the laser periodicity."

When the laser is switched off, the researchers say that the target cells continue contracting for around 64 seconds or 20 laser cycles. But when the laser is switched back on, the cell cannot contract because of the high intracellular calcium levels. "Short-term damage can be negligible for a range of laser powers but long-term degradation of cell health can and will occur," commented Smith.

There are now several avenues of research that the team is hoping to pursue. "We want to understand why not all target cells synchronize with laser and why it is so easy to use the laser as a pacemaker for a large group of cells," said Smith. "We could also use the technique to introduce new contraction periodicities into individual cells within heart tissue, or within a whole heart. We could then study how asynchronous contractions might propagate through the heart."

Source: Optics.org

Extreme UV light made easy

2008-06-05T07:17:00.002-05:00

A new system to generate coherent extreme-ultraviolet (EUV) light has been developed by researchers in Korea. The device, based on a nanostructure made of bow-tie shaped gold "antennas" on a sapphire substrate, is smaller and cheaper than existing systems and might allow an EUV source the size of a laptop computer to be made. Potential applications for the source include high-resolution biological imaging, advanced lithography of nanoscale patterns and perhaps even "X-ray clocks".

EUV light has a wavelength of between around 5 and 50 nm (100–10 times shorter than that of visible light). It can thus be used to etch patterns at tiny length scales and is ideal for spectroscopic applications because the wavelength is the same as that of many atomic transitions.

However, EUV radiation is currently produced in a very complicated process involving the use of amplified light pulses from an oscillator (a source of laser light) to ionize noble gas atoms. The electrons freed during this process are accelerated in the light field and their surplus energy is freed as attosecond (10^–18 s) pulses of light of different wavelengths. The shortest wavelengths of light can then be "filtered out" to produce a single EUV pulse.

Scientists would ideally like to produce EUV light directly from the oscillator without the need for expensive and bulky amplifiers. In this way, EUV-light generation could be simplified and the size of the source significantly reduced to tabletop dimensions. In contrast, current devices usually measure around 2–3 m across. Now, Seung-Woo Kim of KAIST in Daejeon and colleagues have shown that this might be possible.

The researchers report that a bow-tie nanostructure of gold – measuring around 20 nm across – can enhance the intensity of femtosecond laser light pulses by two orders of magnitude. This is high enough to generate EUV light with a wavelength of less than 50 nm directly from a small pulse with an energy of 10^11 W/cm^2 injected into argon gas (Nature 453 757). The energy needed is about 100 times less than in traditional approaches.

Surface plasmons

The technique works thanks to "surface plasmons" (surface excitations that involve billions of electrons) in the "gap" of the bow-tie gold nanostructures (see figure). When illuminated with the correct frequency of laser light, the surface plasmons can begin to resonate in unison, greatly increasing the local light field intensity. This phenomenon, known as resonant plasmon field enhancement, is already exploited in imaging techniques, such as surface-enhanced Raman scattering, which is sensitive enough to detect individual molecules on a metal surface.

Immediate applications include high-resolution imaging of biological objects, advanced lithography of nanoscale patterns and making X-ray clocks. These exploit a frequency-stabilized femtosecond laser and are being investigated worldwide to replace the current caesium atomic clocks for better time precision.

"This new method of short-wavelength light generation will open doors in imaging, lithography and spectroscopy on the nanoscale," commented Mark Stockman of the Georgia State University in a related article (Nature 453 731). The spatially coherent, laser-like light could have applications in many areas: spectroscopy; screening for defects in materials; and, if extended to X-ray or gamma-wavelengths, detecting minute amounts of fissile materials for public security and defence.

The team now plans to improve the conversion efficiency of the generated light by modifying the design of their nanostructure – for example, by making 3D cones with sharper tips. These will not only enable higher local field enhancement but also better interaction of the femtosecond light pulses with injected gas atoms. The team will also test the spatial and temporal coherence of the generated EUV light.

Source: Physicsworld.com; Phtonics.com; Optics.org

Moving EUVL From Lab to Fab

2008-06-03T07:06:00.000-05:00

MAUI, Hawaii, June 3, 2008 -- More than 100 leading lithographers will meet in Maui next week to begin developing a plan to speed the introduction of extreme ultraviolet lithography (EUVL) into high-volume semiconductor manufacturing.

The effort will take place June 10-12 at the 2008 International Workshop on EUV Lithography, being held at the Wailea Beach Marriott. Keynote speaker will be retired IBM senior scientist Eberhard Spiller, PhD, whom many consider the "father of EUVL" because of his pioneering work on EUVL mirrors. Spiller is owner of Spiller X-Ray Optics in Livermore, Calif., and will speak on "Imaging in the EUV Region."

Optical lithography, which has dominated chip manufacturing for more than three decades, involves the direction of light onto a mask -- a sort of stencil of an integrated circuit pattern -- which projects the image of the pattern onto a semiconductor wafer covered with light-sensitive photoresist. The process has allowed more and more features to be crammed onto a computer chip, but current techniques have pushed the method about as far as it can go. Creating faster circuits with smaller and smaller features requires using shorter and shorter wavelengths of light -- such as those in the EUV range -- but technical difficulties and its cost continue to keep EUVL from becoming commercially viable for high-volume manufacturing.

In focused sessions at the International Workshop on EUV Lithography, key researchers from North America, Europe and Asia will present findings aimed at solving the most critical EUVL challenges in source power, mask defects and resist performance.

Special emphasis will be placed on the power scaling potential of discharge-produced plasma (DPP) and laser-produced plasma (LPP) as potential EUV power sources. LPP-based sources using high-power lasers are strong candidates for delivering 180 W of EUV power, enough to meet manufacturing requirements.

Single-module high-power laser supplier Gigaphoton will describe its high-power pulsed CO2 lasers, while researchers from MIT's Lincoln Lab in Lexington, Mass., will document the performance of Yb:YAG lasers as an alternate high-power laser technology. Additional candidates for high-power source technology also will be reviewed.

Six papers on enhancing the performance of current EUVL resists and highlighting new approaches for developing new resists will be presented. Other papers will cover EUVL source, optics, optics design, contamination, reticle protection, mask and mask metrology. Also, many leading researchers will give invited talks to highlight potential solutions to EUVL challenges.

Interspersed among the presentations will be three expert panels on EUVL source, mask, and general research and development issues. Panelists will identify areas where additional R&D is needed to solve the challenges to bringing EUVL into the factory.

Preceding the workshop, several courses will be offered June 9-10 on the fundamentals and underlying physics of EUVL. These courses recognize that EUVL is a multidisciplinary science, and are designed for technologists whose expertise lies outside lithography.

Organizing the workshop is EUV Litho Inc., an organization dedicated to promoting and accelerating introduction of EUVL into high-volume manufacturing through workshops and education. SPIE is co-sponsoring the event and will publish its proceedings.

Registration details and additional information are available at: www.euvlitho.com

3D microscopy images cells with nanoscale resolution

2008-06-01T07:02:00.001-05:00

The highest resolution 3D images of the inside of single cells have been generated by scientists at the Max Planck Institute in Germany. The group says that its scanning fluorescent microscope obtains a spatial resolution far below the wavelength of light, allowing it to image the interior of cells with unprecedented detail (Nature Methods DOI:10.1038).

"We have produced the smallest focal spots that have been attained so far in a scanning fluorescent microscope, and thus attained the best 3D resolution inside a transparent object such as a cell," Stefan Hell, from the Department of NanoBiophotonics in Göttingen, told optics.org. "Contrary to previous systems, the focal spot, which measures 40–45 nm in diameter, is both spherical and of subdiffraction dimensions enabling isotropic 3D resolution on the nanoscale."

The resolution of a standard confocal system is limited to over 200 nm in the focal plane and over 500 nm along the optic axis. "We have achieved a resolution of 40–45 nm in all directions and this can be improved even further since our scheme allows further confinement of fluorescent spot (i.e. the effective PSF of the microscope)," commented Hell.

3D microscopy offers the only method of non-invasively visualizing the interior of cells. According to Hell, the level of 3D resolution is approaching that of an electron microscope but with the advantage of being able to use fluorescent tags to identify specific proteins.

The group's setup is a lens-based fluorescence microscope that merges the fundamentals from two existing microscopy techniques. "We have developed an effective microscopy scheme called isoSTED, which combines elements of stimulated emission depletion (STED) and 4Pi microscopy to compress the fluorescent spot to subdiffraction dimensions," explained Hell.

In STED microscopy, very short laser pulses excite a fluorescent tag attached to the sample under observation. This excitation pulse is immediately followed by a depletion pulse, tuned to an emission line of the fluorescent tag. The depletion pulse causes stimulated emission, which moves electrons from the excited state from which fluorescence occurs to a lower energy state. This decreases the effective spot size of the excited region resulting in higher resolution imaging.

The idea behind 4Pi microscopy is to illuminate the sample with a pair of synchronized laser pulses in such a way that the pulses interfere constructively. This is equivalent to increasing the total aperture of the system.

Source: optics.org

Rochester's Omega Laser Receives 50-Fold Power Increase to Become 'Petawatt' Laser

2008-05-22T19:39:00.000-05:00

The University of Rochester will mark another important step in the effort toward attaining sustainable fusion, the ultimate source of clean energy, Friday, May 16.

University President Joel Seligman, along with special guests, who include U.S. Senator Charles Schumer, U.S. Representative Thomas Reynolds, and Undersecretary and National Nuclear Security Administration Administrator Thomas D'Agostino, will dedicate the new Omega EP (Extended Performance) laser facility at the Robert L. Sproull Center for Ultra High Intensity Laser Research at the Laboratory for Laser Energetics (LLE).

The Omega EP comprises a new set of four ultra-high-intensity laser beams that will unleash more than a petawatt—a million billion watts—of power onto a target just a millimeter across. Working in conjunction with LLE's original 60-beam Omega laser, the Omega EP will open the door to a new concept called "fast ignition," which may be able to dramatically increase the energy derived from fusion experiments and provide a possible new avenue toward clean fusion power. If successful, fast ignition could lead to the highest energy densities ever achieved in a laboratory.

"I look forward to the profound scientific contributions the Omega EP extension will bring to the University and to the world," says Seligman. "It is a vital component of our nation's scientific capital and leadership, a key to strategic work on an independent energy future, and a vital part of the local economy, including $44 million in local expenditures just last year."

"Over the years, the University of Rochester's Laboratory for Laser Energetics has consistently brought Upstate New York's high-tech sector to the forefront of energy innovation," says Schumer. "It is a vital national resource as well as an economic boon to Rochester and to the entire Finger Lakes region. I was proud to secure over $61 million to support their efforts last year and will continue to look for ways in which the federal government can further collaborate with this dynamic laboratory in the future."

"Employing more than 500 Western New Yorkers, the Laboratory for Laser Energetics of the University of Rochester is essential to the growth of our community and ensures Rochester is on the cutting edge of technology," says Reynolds. "The new Omega EP laser is truly remarkable and serves as a clear demonstration of how our region remains a leader in world-class innovation. The Omega EP's success is a testament to the scientists, engineers, technicians, and students who made the project possible."

The original Omega laser fires multi-trillion watt bursts of energy—more powerful than the entire electrical generating capacity of the United States—making it among the three most powerful lasers in the world. Yet Omega will become approximately 50 times more powerful still with the inclusion of Omega EP. Such incredible intensities are necessary because creating electricity from fusion means heating the target fuel to a high temperature and confining it long enough so that more energy is released than is supplied to sustain the reaction. To release energy at a level required for electricity production, the fusion fuel must be heated to about 100 million degrees, more than six times hotter than the interior of the Sun.

Fusion, nuclear fission and solar energy, which includes biofuels, are widely seen as the only energy sources capable of satisfying the growing need for power for the next century without the harmful environmental impacts of fossil fuels. In a fusion power plant, one gallon of seawater would provide the equivalent energy of 300 gallons of gasoline; fuel from 50 cups of water contains the energy equivalent of two tons of coal. A fusion power plant would produce no climate-changing gases, as well as considerably less environmentally harmful radioactive byproducts than nuclear power plants currently do. And there would be no danger of a runaway reaction or core meltdown in a fusion power plant.

Beyond clean energy production, Omega and Omega EP will facilitate research impossible to attempt almost anywhere else on Earth. The way matter behaves in stars can be replicated on a small scale inside Omega's target chamber. Laser and materials technologies, electro-optics, and plasma physics will also be able to be studied under conditions never before possible.

Source: University of Rochester

Femtosecond laser delivers breakthrough performance

2008-05-20T19:32:00.000-05:00

A Ti:Sa laser that emits sub-50 fs pulses at repetition rates of 10 GHz offers unique advantages for applications in spectroscopy.

Scientists in Germany and the US have built a passively modelocked Ti:Sa laser that ahieves an unbeatable combination of high bandwidth, high average power and repetition rates of up to 10 GHz. The laser, which delivers pulses of down to 42 fs, is intended to increase the signal-to-noise ratio of spectroscopic measurements that require a laser frequency comb, and could also be used in the development of optical clocks.

"The combination of high repetition rate and large fractional bandwidth is a measure for the merit of a frequency comb for spectroscopy purposes," said Albrecht Bartels of Gigaoptics, a German company that already markets femtosecond lasers operating at 1 GHz and 5 GHz. "The fractional bandwidth is typical for other Ti:Sa lasers, but very large compared with other 10 GHz sources — which usually deliver picosecond pulses."

The crucial advantage of such high repetition rates is that for the first time it allows the individual modes of the femtosecond laser — in other words, the "teeth" of the frequency comb — to be separated with a simple grating spectrometer.

"The spacing between the frequency comb modes depends on the repetition rate," explained Bartels. "Most applications of femtosecond laser frequency combs only require a single or a few specific modes out of the many available. Now we are able to isolate these modes and individually direct them to an experiment, while unwanted modes that only create additional noise are excluded."

According to Bartels, the laser supports around 500 modes, each separated by precisely 10 GHz. And because the spacing between the modes is larger than at lower repetition rates, the output power from the Ti:Sa laser is spread between fewer modes.

As a result, each mode delivers power levels of more than 1 mW, which is more than enough for most spectroscopic applications. "Some applications require only nanowatts per mode, but more power means more signal-to-noise ratio and thus quicker measurements."

The new laser design, which was unveiled in a post-deadline paper at the Conference for Lasers and Electro-Optics (CLEO) in May, was developed by Bartels in collaboration with researchers at the University of Konstanz in Germany and the US National Institute of Standards and Technology (NIST) in Boulder, Colorado. Bartels told optics.org that a commercial version of the laser is due to be launched within the next six months.

According to Bartels, the key parameter for achieving high repetition rates is the peak intracavity intensity, which is increased by tightly focusing the pump laser to a 10 µm spot within the Ti:Sa laser. The ring cavity design also supports higher repetition rates for a given number of cavity mirrors, in this case a minimum of four.

The use of a Ti:Sa crystal also ensures a broad gain bandwidth, as well as efficient pump light absorption and high gain over a short length — which is essential to achieve the 10 GHz repetition rate. The output wavelength of 783 nm is also useful for many applications.

"Most importantly, it matches the resonances of useful atomic systems, such as rubidium and caesium, which will allow the 10 GHz frequency comb to be locked to such atomic references for precision spectroscopy purposes," said Bartels.

The next stage, says Bartels, is for his co-workers at NIST to demonstrate the use of the laser in applications such as direct frequency comb spectroscopy, waveform generation, and astronomical spectrograph calibration. Further work on the mechanical packaging of the laser will also be needed before a commercial device can be released.

New technique measures ultrashort laser pulses at focus

2008-05-16T17:39:00.003-05:00

Lasers that emit ultrashort pulses of light are used for numerous applications including micromachining, microscopy, laser eye surgery, spectroscopy and controlling chemical reactions. But the quality of the results is limited by distortions caused by lenses and other optical components that are part of the experimental instrumentation.

To better understand the distortions, researchers at the Georgia Institute of Technology developed the first device to directly measure complex ultrashort light pulses in space and time at and near the focus. Measuring the pulse at the focus is important because that’s where the beam is most intense and where researchers typically utilize it. Knowing how the light is distorted allows researchers to correct for the aberrations by changing a lens or using a pulse shaper or compressor to manipulate the pulse into the desired form.

The device was described in a presentation at the Conference on Lasers and Electro-Optics on May 8. This research was funded by the National Science Foundation and published in the August 2007 issue of the journal Optics Express.

It is difficult to measure ultrashort pulses because they typically last between a few femtoseconds and a picosecond, which are 10-15 and 10-12 of a second, and faster than the response time of the fastest electronics. To achieve the highest possible intensity of the laser, the pulse must be as small as possible in space and as short as possible in time. However, focused pulses nearly always have distortions in time that vary significantly from point to point in space due to lens aberrations in focusing optics. To address those issues, the new device, called SEA TADPOLE (Spatial Encoded Arrangement for Temporal Analysis by Dispersing a Pair of Light E-fields), allows researchers to measure complicated ultrashort pulses simultaneously in space and time as they go through the focus.

The research team – which also included former graduate students Pablo Gabolde and Selcuk Akturk – used the concept of interferometry to measure a pulse in space and time. Two pulses, one reference and one unknown, were sent through optical fibers. The fibers were mounted on a scanning stage so that the pulses could be measured at many locations around the focus.

The pulses were crossed and an interference pattern was recorded for each color of the pulse at each location with a digital camera. The patterns were used to determine the shape of the unknown pulse in space and time and to create movies showing how the intensity and color of the pulse changed in space and time as it focused.

he researchers tested the device by measuring ultrashort pulses focused by various lenses, since each lens can cause different complex distortions. To validate the measurements, Bowlan performed simulations of pulses propagating through the experimental lenses. Results showed that a common plano-convex lens displayed chromatic and spherical aberrations, whereas more expensive aspheric and doublet lenses exhibited mostly chromatic aberrations.

Spherical aberrations occur when the light that strikes the edges of the lens gets focused to a different point than the light that strikes the center, creating a larger, inhomogeneous focused spot size. Chromatic aberrations occur because the many colors in the laser travel at different speeds and do not stay together in space and time as the pulse passes through glass components in the experimental setup, such as lenses. As a result, each color arrives at the focus at a different time, creating a rainbow of colors in the electric field images.

Aberrations can drastically increase the pulse length, which decreases the laser intensity. A lower intensity forces researchers to increase the power of the laser, increasing the possibility of damaging the sample. Aberrations can also yield odd pulse and beam shapes at the focus, which complicate the interpretation of the experiment or application.

Faster than a Speeding Bubble

2008-05-03T17:46:00.003-05:00

X-ray scattering images (above) and corresponding 3D depictions (below) of nucleation events, or "bubbles," forming in the semiconductor Indium Antimonide in the first instances after being hit with a laser pulse.

What do melting chocolate and bubbles in a champagne glass have in common? Besides being treats one might sample at a sophisticated soiree, they are both handy examples of first-order phase transitions in which a material transforms from one phase to another—that is, atoms changing from an orderly arrangement into a more chaotic arrangement.

Now, in an experiment led by Aaron Lindenberg, an international collaboration of scientists has uncovered new clues about the first instants of that process. The results are published in the April 4 edition of Physical Review Letters.

"We did not at all expect to see what we saw," said Lindenberg, "although in the aftermath we can go back and realize perhaps we should have. What's amazing about the process is that it spans such a huge range of time scales."

The process of melting, or in the case of champagne, of bubbling, has long been of interest to scientists. Phase transitions take place in the tiniest fraction of a second. In the case of Indium Antimonide (InSb), a semiconductor used by scientists to study such processes, the first steps in melting take a few hundred femtoseconds, a quadrillionth of a second. But no one knew what happened after that.

In the current study, the group used a laser to excite the sample and then measured the structure of the disordered liquid using X-rays, a technique called "pump-probe." Critical to the experiment is timing the initial laser used to pump the sample with energy, and the X-ray beam used to probe the results, to within mere femtoseconds. The resulting diffuse pattern of scattered X-rays from the disordered sample is used to map out where the atoms are at a given instant. Subsequent repeats of the pumping and probing at different relative delays between the laser and X-ray beam enables the researchers to reconstruct how the material evolves over time.

Lindenberg and colleagues found that the structure of the disordered liquid was far different from what one would have expected. Tiny atomic-scale bubbles, called nucleation events, form first and seed the process, a unique transient state of matter in which large fluctuations dominate the response of the material.

The group captured the process on a timescale 100 times shorter than any other previous X-ray study. The results give scientists a deeper understanding of how disordered materials behave on short timescales, and could lead to improved materials processing techniques, such as electronics manufacturing.

The current study also represents the last scientific paper to come from SLAC's Sub-Picosecond Pulse Source (SPPS) collaboration, led by Jerry Hastings, which was undertaken to study very fast atomic scale processes using ultra short pulses of X-rays. The work at SPPS presages the science to come from SLAC's Linac Coherent Light Source (LCLS), now under construction, which will create coherent X-ray laser pulses that are even shorter.

"SPPS was a remarkable success," said SSRL Director Jo Stohr. "It was great to see prominent X-ray scientists from all over the world coming to SLAC to participate in this unique experiment. It is an indication of what is yet to come with LCLS."

Source: by Brad Plummer, SLAC Today

Laser experiments offer insight into evolution of 'gas giants'

2008-05-03T17:42:00.001-05:00

By shooting the high-energy Omega laser onto precompressed samples of planetary fluids, scientists are gaining a better understanding of the evolution and internal structure of Jupiter, Saturn and extrasolar giant planets.
The properties of dense helium (He) — which happens to be a principal constituent of giant gas planets like Jupiter — at thermodynamic conditions between those of condensed matter and high-temperature plasmas are theoretically challenging and unexplored experimentally.

Laboratory scientists collaborating with researchers at the Laboratory for Laser Energetics, CEA France and UC Berkeley were able to determine the equation of state (EOS) for fluid He at pressures above 100 GPa (one million times more pressure than the Earth’s atmosphere — one GPa (gigapascal) equals 10,000 atmospheres).

The only previous high temperature and pressure He EOS data available for constraining planetary models was performed at LLNL by Bill Nellis and his team using a two-stage gas gun. However, those earlier experiments used cryogenic techniques at ambient pressure so their densities were significantly lower than those achieved with the precompressed samples. Also, the final pressures, 16 GPa for a single shock, were significantly lower than the new laser shock data.

Theoretical research points out that material deep within a planet’s interior could exhibit unusual characteristics, such as high-temperature superconductivity, superfluidity and Wigner crystallization.

“The state of materials in the center of a giant planet are difficult to observe and challenging to create or predict,” said Gilbert Collins of the Physical Sciences Directorate. “Defining the equation of state of helium at these pressures is a first step to deepen our understanding of these massive objects.”

Jupiter is thought to contain matter to near 100 Mbar (100 million atmospheres of pressure.)

The LLNL team of Jon Eggert, Peter Celliers, Damien Hicks and Collins, together with several university collaborators from UC Berkeley, the Carnegie Geophysical Institute, CEA, Princeton, Washington State and the University of Michigan, plan to conduct experiments at the National Ignition Facility. There they will be able to recreate and characterize the core states of solar and extrasolar giants, as well as terrestrial planets, such as the recently discovered “superEarths,” to better understand the evolution of such planets throughout the universe.

Using the Omega laser at the Laboratory for Laser Energetics at the University of Rochester, the team launched strong shocks in He that was already compressed to an initial high state of pressure and density in a diamond anvil cell. Precompression allows researchers to tune the sample’s initial density and the final states that can be achieved with strong shocks.

Quartz was used as a reference material, allowing shock velocities to be determined just before and after the shock crossed the quartz-He interface. This technique reduced the measurement uncertainty as compared to previous studies.

“By applying a strong shock to a precompressed sample,” Collins said, “we can re-create the deep interior states of solar and extrasolar giant planets.”

The diamond anvil’s thickness determines the initial precompressed pressure. To prevent the sample from being heated before the shock, a preheat barrier was used to absorb the high-energy X-rays. An ultrafast diagnostic called VISAR (Velocity Interferometer System for Any Reflector), which works like a speedometer for shocks, recorded the shock velocity of the sample and reference material. From these data, the team determined the density and pressure of the shocked precompressed helium.

A pre-compressed helium sample is shown prior to shot in diamond anvil cell. The square is quartz reference, the circle is a gasket containing high-pressure fluid helium. After the shot, all that remains is a 2 mm hole in the target.

By applying laser-driven shocks to statically compressed samples, equation of state data for fluid He have been obtained with sufficient accuracy in the 100 GPa pressure range to test theoretical predictions.

They also discovered that near 100 GPa, the shock-compressed He transformed to an electronically conductive state and the shock front reflects the 532-nanometer probe laser beam of the VISAR.

The research also has other applications in the national security arena because the extreme conditions in a planet’s deep interior also occur during a nuclear weapon detonation. Plans are under way to significantly extend these research results with experiments at the National Ignition Facility.

The research appeared in the March 28 edition of Physical Review Letters.

World's shortest single photon pulse created

2008-04-29T08:00:00.000-05:00

The world’s shortest light pulse containing just one photon has been produced by Oxford University scientists.

The Oxford team can create individual photons that are 65 femtoseconds in duration: that’s approximately fifty times shorter than any single photon previously produced.

And every photon this source produces is identical to the previous one. Such photons could be a major breakthrough in quantum computing: the harnessing of quantum effects to perform calculations that would take conventional computers thousands of years to resolve.

‘Creating single photons even under controlled conditions is extremely challenging,’ said Peter Mosley of Oxford’s Department of Physics. ‘Even the purest laser light beam consists of many photons all bunched together. Our approach enables us to generate individual photon replicas, identical packets of light of very short duration that are ideal for quantum computing.’

Peter Mosley, a member of Oxford’s Ultrafast Group, is a co-author of a report of the research in Physical Review Letters.

Laser triggered lightning

2008-04-14T16:57:00.002-05:00

Reported from Photonics.com: Scientists have used ultrashort laser pulses to trigger electrical activity in thunderclouds, a first step toward creating man-made lightning.

In a modern-day take on Benjamin Franklin's experiment during a storm more than 200 years ago with a kite, a key and a silk ribbon to prove electricity exists in the atmosphere, the French, Swiss and German scientists aimed high-power pulses of laser light into two passing thunderstorms at the top of South Baldy Peak in New Mexico. The laser pulses created plasma filaments that could conduct electricity. No air-to-ground lightning was triggered because the plasma filaments were too short-lived, but the laser pulses generated discharges in the thunderclouds themselves, the scientists said.

Triggering lightning strikes is an important tool for basic and applied research because it enables researchers to study the mechanisms underlying lightning strikes. Triggered lightning strikes will also allow engineers to evaluate and test the lightning sensitivity of airplanes and critical infrastructure such as power lines.

The idea of using lasers to trigger lightning strikes was first suggested more than 30 years ago, but until recently lasers were not powerful enough to generate the long plasma channels needed. The current generation of more powerful pulsed lasers, like the one developed by Kasparian's team, may change that because they can form a large number of plasma filaments -- ionized channels of molecules in the air that act like conducting wires extending into the thundercloud.

Kasparian and his colleagues involved in the Teramobile project, an international program initiated by the National Center for Scientific Research (CNRS) in France and the German Research Foundation (DFG), built a powerful mobile femtosecond-terawatt laser capable of generating long plasma channels by firing ultrashort laser pulses. They chose to test their laser at the Langmuir Laboratory in New Mexico, which is equipped to measure atmospheric electrical discharges. Sitting at the top of 10,500-ft South Baldy Peak, this laboratory is in an ideal location because its altitude places it close to the high thunderclouds.

During the tests, the research team quantified the electrical activity in the clouds after discharging laser pulses. Statistical analysis showed that their laser pulses indeed enhanced the electrical activity in the thundercloud where it was aimed—in effect they generated small local discharges located at the position of the plasma channels.

The limitation of the experiment, though, was that they could not generate plasma channels that lived long enough to conduct lightning all the way to the ground. The plasma channels dissipated before the lightning could travel more than a few meters along them. The team is currently looking to increase the power of the laser pulses by a factor of 10 and use bursts of pulses to generate the plasmas much more efficiently.

The paper, "Electric Events Synchronized With Laser Filaments in Thunderclouds," appears in the April 14 issue of Optics Express, the Optical Society of America's (OSA) open-access journal.

Petawatt Power Peak Reached

2008-04-09T21:22:00.003-05:00

The Texas Petawatt laser produced a petawatt of peak power on March 31, making it the highest powered laser in the world, said Todd Ditmire, a physicist at the University of Texas at Austin.

There has only been one petawatt laser in the US history, the Nova laser at Lawrence Livermore Laboratory (LLNL, operated by the University of California for the energy department). Nova, which took up a football field in space, is now defunct. In the past eight or so years, there has been a worldwide push to achieve petawatts (10 to the 15th power). Terawatts (10 to the 12th power) were produced by short pulse lasers in the late 1980s using chirped pulse amplification, the method Ditmire is using.

Other US petawatt projects include the OmegaEP laser at the University of Rochester, The Ohio State University petawatt, and the Z-Beamlet project at the Sandia National Labs Z-Petawatt Laser Facility. Projects are also underway in the UK, France, Germany, Japan, China, and other countries.

The challenge for researchers is to produce a lot of energy in a little time, and a petwatt can be the result if enough energy can be produced in a short enough pulse. The Hercules laser at the University of Michigan, for example, is only 0.3 petawatts, but it focuses to an incredibly tiny spot. For sheer power -- energy divided by pulse duration -- the Texas petawatt laser now leads the way in the US.

The laser produces a very short duration, very low-energy pulse, and this pulse is stretched in time to a very long pulse, is amplified to huge energy, then finally is compressed to a high-energy, super-short-duration pulse. One of the critical aspects of the system is the diffraction gratings used to compress the pulse; these were made by Jerry Britten's group at LLNL, and they are some of the most difficult-to-manufacture optics in the world.

Related Link: Texas High Intensity Laser

via: photonics.com

Petawatt laser approaches diffraction limit

2008-03-05T21:21:00.004-06:00

A French team has combined adaptive optics (AO) with an elaborate alignment system to effectively correct wavefront aberrations in a high peak-power laser, achieving focal spots close to the diffraction limit. "The optimization procedure produces a considerable improvement in focal spot quality with a Strehl ratio of 0.7 for full-energy kilojoule shots," Ji-Ping Zou of the LULI laboratory told optics.org. "The procedure, once integrated into our control system, is straightforward and there are no operational penalties." (Applied Optics 47 704.)

Spatial phase and focal spot measurements using a low-energy pulsed probe before the fourth shot of a kJ shot sequence (5 shots, one shot every hour): a) and b): before and after the closed-loop convergence. c) Focal spot measurement during the fourth shot.

The LULI (Laboratoire pour l'Ulilisation des Lasers Intenses) laser delivers kilojoule pulses in the nanosecond range at 1053 nm, and is capable of reaching the petawatt regime through chirped pulse amplification.

The first category of aberrations is minimized by precise beam realignment between two successive shots, combined with a closed-loop AO system employing a bimorph deformable mirror with 32 actuators. An additional semi-automatic realignment of beam pointing and centring between shots controls the second category, while the AO system tackles the third group. The right is the schematic of the four amplification stages of the LULI2000. A bimorph deformable mirror is implemented between the second and the third stages. A wavefront sensor is positioned at the chain output.

The result has been reproducible focal spots close to the diffraction limit for full-energy kilojoule shots fired at one shot per hour. Zou's group has achieved a focal spot with a Strehl ratio - a measure of the fractional drop in light intensity as a function of wavefront error - of 0.7. The focal intensity can therefore reach 2.2 x 10¹⁸ W/cm² in the kilojoule per nanosecond range, and intensities as high as 10²¹ W/cm² are foreseen by Zou. Shot-to-shot reproducibility of the focal spot is said to be excellent, which is very important for laser-matter interaction experiments.

via Optics.org

Protons bring fusion into view

2008-02-29T17:18:00.004-06:00

Researchers in the US have now developed an imaging technique that could help bring fusion power to fruition. Richard Petrasso and colleagues at the Massachusettes Institute of Technology and Wolfgang Theobold and colleagues at the University of Rochester have used "proton radiography" to map the electromagnetic structure of the extremely hot, dense plasmas in which fusion reactions take place. The technique has revealed hitherto unseen magnetic and electric fields, and could help researchers to get fusion plasmas to ignite—the key to electricity generation.

The MIT-Rochester technique applies to inertial-confinement fusion (ICF), which is one of two possible routes to a fusion reactor. The idea behind ICF is to bombard fuel capsules (typically containing deutrium and tritium) with high-powered laser pulses so that they implode, generating a small volume of hot, dense plasma in which the deutrium and tritium nuclei can overcome their electrical replusion and produce a helium nucleus plus a free neutron. Since these reaction products are lighter than the original nuclei, copious energy is released via Einstein's mass-energy equivalence.

In the new work, the MIT and Rochester researchers used 36 beams at the high-powered OMEGA laser facility at Rochester to symmetrically implode ICF fuel capsules (Science 319 1223). The same beams also struck a different capsule 1 cm away which was filled with deuterium and helium-3 gas. Protons released from this "backlighter" capsule all have the same (known) energy, so by measuring the deflection of the positively charged protons that had transited some plasma the team was able to map the electromagentic fields present in ICF implosions for the first time.

Diagram of the experiment used to image the plasma. Protons from the backlighter capsule (left) travel through the target capsule before their position and energy is determined by a detector. (Courtesy: Science)

Electron filmed for first time ever

2008-02-25T17:36:00.001-06:00

Now it is possible to see a movie of an electron. The movie shows how an electron rides on a light wave after just having been pulled away from an atom. This is the first time an electron has ever been filmed, and the results are presented in the latest issue of Physical Review Letters.

Previously it has been impossible to photograph electrons since their extremely high velocities have produced blurry pictures. In order to capture these rapid events, extremely short flashes of light are necessary, but such flashes were not previously available. With the use of a newly developed technology for generating short pulses from intense laser light, so-called attosecond pulses, scientists at the Lund University Faculty of Engineering in Sweden have managed to capture the electron motion for the first time.

View video: avi or mov.

More information: http://www.atto.fysik.lth.se/

The most intense laser pulse in the universe

2008-02-19T17:45:00.004-06:00

The people on HERCULES laser at the University of Michigan has claimed to have created the most intense laser pulse in the universe.

The record-setting beam measures 20 billion trillion watts per square centimeter. It contains 300 terawatts of power, about 300 times the capacity of the entire US electricity grid. The laser beam’s power is concentrated to a 1.3-µm speck about 100th the diameter of a human hair. To achieve this beam, the research team added another amplifier to HERCULES (high-energy repetitive CUOS laser system) laser system, which previously operated at 50 terawatts.

Femtosecond laser creates subsurface structures

2008-02-13T12:00:00.003-06:00

Reported by optic.org

German researchers have used an ultrashort pulsed laser to create subsurface nanostructures in a sapphire crystal. The team believes that the techniques could be used to fabricate microfluidic devices as well as 3D photonic structures. (Optics Express 16 1517.)

SEM images of the entrance of the modified and etched channel directly after etching (left) and cross section of hollow nanoplanes in 500 μm depth of the same track. Laser beam propagated from top to bottom, three parallel scans with an offset of 3 μm, focused with NA=0.55, f=500 kHz, P=450 mW.

Femtosecond laser produced the colored metals

2008-02-01T14:50:00.000-06:00

A tabletop femtosecond laser has been used to change the surface properties of metals to reflect a specific color or combination of colors. Silver, platinum, gold, and other metals have been turned colors such as blue, gray, black, and purple.

Today Photonics.com and Physorg.com reported Unversity of Rochester's Professor Guo's recent research achievement.

The intense blast forces the surface of the metal to form nanostructures -- pits, globules and strands that response incoming light in different ways depending on the way the laser pulse sculpted the structures. Since the structures are smaller than the wavelength of light, the way they reflect light is highly dependent upon their specific size and shape, Guo said. Varying the laser intensity, pulse length, and number of pulses, allows Guo to control the configuration of the nanostructures, and hence control what color the metal reflects.

Ultrafast X-ray study of dense-liquid-jet flow dynamics using structure-tracking velocimetry

2008-01-28T07:29:00.000-06:00

Yujie Wang1, Xin Liu2, Kyoung-Su Im1, Wah-Keat Lee1, Jin Wang1, Kamel Fezzaa1, David L. S. Hung3 & James R. Winkelman3

1. X-Ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
2. Mayo Clinic, Rochester, Minnesota 55905, USA
3. Visteon Corporation, Van Buren Township, Michigan 48111, USA

Nature Physics. doi:10.1038/nphys840

High-speed liquid jets and sprays are complex multiphase flow phenomena with many important industrial applications. Great efforts have been devoted to understand their dynamics since the pioneering work of Rayleigh on low-speed jets. Attempts to use conventional laser optical techniques to provide information about the internal structure of high-speed jets have been unsuccessful owing to the multiple scattering by droplets and interfaces, and the high density of the jet near the nozzle exit. Focused-X-ray-beam absorption measurements could provide only average quantitative density distributions using repeated imaging. Here, we report a novel approach on the basis of ultrafast synchrotron-X-ray full-field phase-contrast imaging. As illustrated in our case study, this technique reveals, for the first time, instantaneous velocity and internal structure of optically dense sprays with a combined unprecedented spatial and time resolution. This technique has tremendous potential for the study of transient phenomenon dynamics.

The X-ray beam is generated from the electron storage ring. The fill pattern shown is the hybrid-singlet mode: a single electron bunch (150 ps long and carrying 15 mA of current) is separated from a longer train of electrons (472 ns long, 94 mA) by a 1.59 s gap on both sides. The fast shutter absorbs more than 99% of the beam power, and lets the beam through for a few milliseconds at 1 Hz. The sample image is formed on a fast scintillator crystal (LYSO:Ce) and read on a CCD (charge-coupled device) camera via a microscope objective and a mirror at 45^° angle. The inset shows the APS undulator-A energy spectrum at 31 mm gap on a logarithmic scale. The fundamental sharp peak at 13.3 keV is 100 times brighter than the harmonics.

The concepts of entrance pupil and exit pupil

2008-01-13T16:58:00.000-06:00

The entrance pupil of a system is the image of the aperture stop as seen from an axial point on the object through those elements preceding the stop. In contrast, the exit pupil is the image of the aperture stop as seen from an axial point on the image plane through the interposed lenses, if there are any.

The definition of pupil can be found from any optics textbook, however it's still hard to imagine what difference between the exit and entrance pupils. After seeing the left picture, you may impress the pupil on the memory. It's a normal camera lens, the entrance pupil is the image of aperture from the front side, and the exit pupil is the image of the same aperture from the back side .

Hot attosecond pulse at 2007

2008-01-09T16:47:00.000-06:00

Physics News Update listed Ten Top Physics Stories for 2007. There are 2 news about ultrafast laser:

Electron tunneling in real time can be observed with the use of attosecond pulses (http://www.aip.org/pnu/2007/split/818-2.html);
The shortest light pulse, a 130-attosecond burst of extreme ultraviolet light (http://www.aip.org/pnu/2007/split/823-1.html); I reported this on my blog before.

The other stories are:

Light, slowed in one Bose Einstein condensate (BEC), is passed on to another BEC (http://www.aip.org/pnu/2007/split/812-1.html);
Laser cooling of coin-sized object, at least in one dimension (http://www.aip.org/pnu/2007/split/818-1.html);
The best test ever of Newton’s second law, using a tabletop torsion pendulum (http://www.aip.org/pnu/2007/split/819-1.html);
First Gravity Probe B first results, the measurement of the geodetic effect---the warping of spacetime in the vicinity of and caused by Earth-to a precision of 1%, with better precision yet to come(http://www.aip.org/pnu/2007/split/820-2.html);
The MiniBooNE experiment at Fermilab solves a neutrino mystery, apparently dismissing the possibility of a fourth species of neutrino (http://www.aip.org/pnu/2007/split/820-1.html);
The Tevatron, in its quest to observe the Higgs boson, updated the top quark mass and observed several new types of collision events, such as those in which only a single top quark is made, and those in which a W and Z boson or two Z bosons are made simultaneously (http://www.aip.org/pnu/2007/split/821-1.html);
Based on data recorded at the Auger Observatory, astronomers conclude that the highest energy cosmic rays come from active galactic nuclei (http://www.aip.org/pnu/2007/split/846-1.html);
And the observation of Cooper pairs in insulators (http://www.aip.org/pnu/2007/split/849-1.html).

A compact synchrotron radiation source driven by a laser-plasma wakefield accelerator

2007-12-11T07:13:00.000-06:00

Scientists tried to send the electrons generated from laser-plasma wakefield accelerator to the undulator to produce the light. This new experiment was reported in the recent Nature Physics.

The laser pulse is focused by an off-axis parabolic mirror into a supersonic helium gas jet where it accelerates electrons (blue line) to several tens of mega-electron volt energy. The electron beam profile may be monitored by a removable scintillating screen. The electrons propagate through an undulator, producing synchrotron radiation, and into a magnetic electron spectrometer. Radiation is collected by a lens and analysed in an optical spectrometer. The spectrometer is protected against direct laser and plasma exposure by a thin aluminium foil in front of the undulator.

Abstract: Ultrashort light pulses are powerful tools for time-resolved studies of molecular and atomic dynamics1. They arise in the visible and infrared range from femtosecond lasers2, and at shorter wavelengths, in the ultraviolet and X-ray range, from synchrotron sources3 and free-electron lasers4. Recent progress in laser wakefield accelerators has resulted in electron beams with energies from tens of mega-electron volts to more than 1 GeV within a few centimetres, with pulse durations predicted to be several femtoseconds9. The enormous progress in improving beam quality and stability makes them serious candidates for driving the next generation of ultracompact light sources. Here, we demonstrate the first successful combination of a laser-plasma wakefield accelerator, producing 55–75 MeV electron bunches, with an undulator to generate visible synchrotron radiation. By demonstrating the wavelength scaling with energy, and narrow-bandwidth spectra, we show the potential for ultracompact and versatile laser-based radiation sources from the infrared to X-ray energies.

STED microscopy sees details on the nanoscale

2007-12-08T18:49:00.000-06:00

Stimulated emission depletion (STED) microscopy has demonstrated that, contrary to a longstanding notion, diffraction-unlimited spatial resolution is viable with conventional lenses and visible light. Currently providing 15–70 nm resolution, it is entering the life sciences at a fast pace, while still undergoing technical improvements. Scientists from Max Planck Institute summarized its principles and recent outcomes. The whole summary should be found from optics.org.

A simple stage-scanning STED setup. Inset: overlay of the excitation focus (green) and the STED efficiency (red) for three different STED laser powers. Credit: Max Planck Institute for Biophysical Chemistry.

Laser light alone can open, close world's fastest optical shutter without heating or cooling

2007-12-06T19:06:00.000-06:00

A new study reports that a laser can be used to switch a film of vanadium dioxide back and forth between reflective and transparent states without heating or cooling it. It is one of the first cases that scientists have found where light can directly produce such a physical transition without changing the material’s temperature.

The study, "Coherent Structural Dynamics and Electronic Correlations during an Ultrafast Insulator-to-Metal Phase Transition in VO2", which was published in the Sept. 18 issue of Physical Review Letters, was conducted by a team of physicists from Vanderbilt University and the University of Konstanz in Germany headed by Richard Haglund of Vanderbilt and Alfred Leitenstorfer from Konstanz.

World’s largest laser picks up the pace

2007-12-02T08:27:00.000-06:00

With their target completion date just a year and a half away, scientists and technicians at the National Ignition Facility (NIF) are quickening their pace to install and test the rest of NIF’s 192 lasers and prepare for a new round of preliminary experiments in 2008.

This is a report from Lawrence Livermore National Laboratory (LLNL) official web site:

Ninety-six NIF beamlines have been fired together for the first time, with “excellent” control system and laser stability, according to NIF & Photon Science Principal Associate Director Ed Moses. Last month the facility’s injection laser systems, which initiate the laser pulses, were fired for 144 beamlines.

“A total infrared energy of more than 2.5 megajoules has now been fired,” Moses said. “This is more than 40 times what the Nova laser (NIF’s predecessor) typically operated at the time it was the world's largest laser.”

The first of the facility’s two 96-beam laser bays was commissioned at the end of July. Each of the 96 beams fired an infrared output energy of about 22,000 joules, more than enough to meet NIF’s operational and performance requirements. Since then six more eight-beam “bundles” are being commissioned in the second laser bay, and three of these bundles have been operationally qualified.

Overall commissioning of the NIF beamlines is scheduled for 2009.

The laser shots last about 25 billionths of a second, a tiny fraction of the time it takes to blink an eye. Firing the beams requires operation of 2,300 high-quality optics and instrumentation modules and nearly 400 computers running a million lines of control system code.

The tests measure the quality of each beam’s spatial profile and temporal pulse shape. Even though each shot is exceedingly short in time, its energy output and frequency is designed to vary significantly throughout its duration depending on the type of experiments being conducted.

Meanwhile, data gathered from experiments conducted at NIF in 2003-2004 have enabled sophisticated computer simulations that confirm NIF’s ability to reach the energy levels and beam quality required to produce the world’s first demonstration of inertial confinement fusion.

The “NIF Early Light” experiments included four shots using four laser beams at high energy on a full-scale target for the first time. Simulations of the experiments on LLNL’s world-class supercomputers matched the actual experimental data to an unprecedented degree. The experiments and simulations indicate that NIF’s laser beams will propagate effectively in plasma-filled targets designed to achieve fusion ignition and thermonuclear burn.

NIF experiments next year will focus 96 beams on a gold hohlraum (the eraser-sized capsule containing the fusion target) filled with a light gas mixture. Dubbed “Eos” for the Greek goddess of dawn, the experiments will use the first set of beams from the completed laser bay, traveling to the center of the ten-meter diameter target chamber. They are designed to help validate key aspects of the full-scale ignition campaign that begins in 2010.

Photonic crystal fiber produces ultrafast pulses

2007-11-29T06:17:00.000-06:00

Reported from optic.org:

A new design of hollow-core photonic crystal fiber (HC PCF) has been developed by an international team led by Fetah Benabid of Bath University in the UK. One immediate result has been a method to produce attosecond laser pulses more efficiently than previous techniques.

The fiber's unique properties have led directly to a second breakthrough, the efficient generation of a broad spectrum of ultrafast pulses from a hydrogen-filled PCF through stimulated Raman scattering.

The conventional technique to create attosecond pulses is high-harmonic generation (HHG), which produces central wavelengths in the XUV or soft X-ray region through the firing of a very intense laser pump pulse into a gas. Benabid's fiber was able to produce ultrashort pulses more simply using through stimulated Raman scattering. Benabid's fiber is claimed to require a pump pulse with power levels six orders of magnitude lower and five orders of magnitude longer than those previously needed for HHG.

Surface heating of wire plasmas using laser-irradiated cone geometries

2007-10-29T07:30:00.000-05:00

It's reported on the recent issue of Nature Physics.

Petawatt lasers can generate extreme states of matter, making them unique tools for high-energy-density physics. Pressures in the gigabar regime can potentially be generated with cone-wire targets when the coupling efficiency is high and temperatures reach 2–4 keV. The only other method of obtaining such gigantic pressures is to use the megajoule laser facilities being constructed (National Ignition Facility and Laser MégaJoule). The energy can be transported over surprisingly long distances but, until now, the guiding mechanism has remained unclear. Here, we present the first definitive experimental proof that the heating is maximized close to the wire surface, by comparison of interferometric measurements with hydrodynamic simulations. New hybrid particle-in-cell simulations show the complex field structures for the first time, including a reversal of the magnetic field on the inside of the wire. This increases the return current in a spatially separated thin layer below the wire surface, resulting in the enhanced level of ohmic heating. There are a significant number of applications in high-energy-density science, ranging from equation-of-state studies to bright, hard X-ray sources, that will benefit from this new understanding of energy transport.

LSP modelling of the azimuthal magnetic field structure at the cone tip, 600 fs after the main interaction. A reversed field can be seen on the inside of the wire surface corresponding to the ohmic return current, which is shown on the right picture.

Generation of intense continuum EUVradiation by many-cycle laser fields

2007-10-29T07:18:00.000-05:00

The scientists at Institute of Electronic Structure & Laser in Greece and Max-Planck-Institut für Quantenoptik in Germany reported their research results in recent issue of Nature Physics.

Continuing efforts in ultrashort pulse engineering have recently led to the breakthroughs of the generation of attosecond (10^-18 s) pulse trains and isolated pulses. Although trains of multiple pulses can be generated through the interaction of many-optical-cycle pulses with gases—a process that has led to intense extreme-ultraviolet emission—the generation of isolated high-intensity pulses, which requires few-cycle driving pulses, remains a challenge. Here, we report a vital step towards the generation of such pulses, the production of broad continuum extreme-ultraviolet emission using a high-intensity, many-cycle, infrared pulsed laser, through the interferometric modulation of the ellipticity of 50-fs-long driving pulses. The increasing availability of high-power many-cycle lasers and their potential use in the construction of intense attosecond radiation—with either gas or solid-surface targets—offer exciting opportunities for multiphoton extreme-ultraviolet-pump–extreme-ultraviolet-probe studies of laser–matter and laser–plasma interactions.

The Dual Michelson interferometer device is shown in the left picture, BS: beam splitters. M: flat mirrors. TS1,2,3: piezoelectric translation stages. A: intensity attenuator. First and second MI: first and second Michelson interferometers.

Beam Homogenizer

2007-10-17T20:01:00.000-05:00

A beam homogenizer is a device that smooths out the irregularities in a laser beam profile and creates a more uniform one. Most beam homogenizers use a multifaceted mirror with square facets. The mirror reflects light at different angles to create a beam with uniform power across the whole beam profile (a "top hat" profile).

The best results have been achieved with fly eye homogenizers which are composed of individually polished cylindrical lenses. The incoming laser beam is divided by an array of cylindrical lenses f_1 into several beamlets with size d. These beamlets match with the cylindrical lenses of a second array f_2. This second array and a condenser lens f_3 overlap all these beamlets in the focal plane of f_3. The homogenizer size D is proportional to the focal length of the collecting lens, the diameter and focal length of the micro-lens, and can be calculated using Equation:

D=(f_3/f_2)d

Interested Links:
Beam-shaping optics expand excimer-laser applications
How to Design a Gaussian to Top-Hat Beam Shaper

Solar laser or solar energy laser?

2007-10-05T15:39:00.000-05:00

Based on the news from Optics.org, Japanese team revives solar lasers in quest for clean fuels.

The idea of using solar energy to power lasers is not new. Current designs work by using a system of mirrors to concentrate sunlight into an Nd:YAG crystal, but these lasers are not widely used because they require huge mirrors to collect the light – and even then achieve only low efficiency.

To address these issues, Takashi Yabe and colleagues at the Tokyo Institute of Technology experimented with using a Fresnel lens instead of mirrors as light collectors. They also found that doping the Nd:YAG crystal with small amounts of chromium significantly increases the power output of the laser.

The laser demonstrated by the team produces a power output of 24 W at 1064 nm. The design, which incorporates a 1.3 m2 Fresnel lens, offers an unprecedented slope efficiency of 12% above a threshold solar input of 500 W.

Compressor Alignment Procedure

2007-08-28T21:20:00.000-05:00

I concentrated on the pulse compressor these days. Several motors were used to control two big 1200 line/mm gratings. The motors were controlled by Aerotech UNIDEX 511 motion controller. The computer interface program was written in LabView. It's easy to change the grating reflected angles and distances by rotating a hand wheel. The alignment procedure is:

1). Steering the mirrors to center the beam in the grating G1.
2). Set G1 to 0 degree to check the retro reflection.
3). Set G1 to Littrow angle (28.76 in our case) by adjusting the rock.
4). Change G1 to 13.358 for deviation.
5). Set G2 angle 26.716 for 0 degree back reflection checking.
6). Set G2 angle 55.48 for Littrow checking.
7). Set G1 and G2 both 13.358 degree.

related link: UNIDEX 511