Physicists cool semiconductor by laser light

Researchers at the Niels Bohr Institute have combined two worlds – quantum physics and nano physics, and this has led to the discovery of a new method for laser cooling semiconductor membranes. Semiconductors are vital components in solar cells, LEDs and many other electronics, and the efficient cooling of components is important for future quantum computers and ultrasensitive sensors. The new cooling method works quite paradoxically by heating the material. Using lasers, researchers cooled membrane fluctuations to minus 269 degrees C. The results are published in the scientific journal, Nature Physics.

The experiments themselves are carried out in this vacuum chamber. When the laser light hits the membrane, some of the light is reflected and some is absorbed and leads to a small heating of the membrane. The reflected light is reflected back again via a mirror in the experiment so that the light flies back and forth in this space and forms optical resonator (cavity). Changing the distance between the membrane and the mirror leads to a complex and fascinating interplay between the movement of the membrane, the properties of the semiconductor and the optical resonances and you can control the system so as to cool the temperature of the membrane fluctuations.

From gas to solid

Laser cooling of atoms has been practiced for several years in experiments in the quantum optical laboratories of the Quantop research group at the Niels Bohr Institute. Here researchers have cooled gas clouds of cesium atoms down to near absolute zero, minus 273 degrees C, using focused lasers and have created entanglement between two atomic systems. The atomic spin becomes entangled and the two gas clouds have a kind of link, which is due to quantum mechanics. Using quantum optical techniques, they have measured the quantum fluctuations of the atomic spin.

"For some time we have wanted to examine how far you can extend the limits of quantum mechanics – does it also apply to macroscopic materials? It would mean entirely new possibilities for what is called optomechanics, which is the interaction between optical radiation, i.e. light, and a mechanical motion," explains Professor Eugene Polzik, head of the Center of Excellence Quantop at the Niels Bohr Institute at the University of Copenhagen.

But they had to find the right material to work with.

In 2009, Peter Lodahl (who is today a professor and head of the Quantum Photonic research group at the Niels Bohr Institute) gave a lecture at the Niels Bohr Institute, where he showed a special photonic crystal membrane that was made of the semiconducting material gallium arsenide (GaAs). Eugene Polzik immediately thought that this nanomembrane had many advantageous electronic and optical properties and he suggested to Peter Lodahl's group that they use this kind of membrane for experiments with optomechanics. But this required quite specific dimensions and after a year of trying they managed to make a suitable one.

"We managed to produce a nanomembrane that is only 160 nanometers thick and with an area of more than 1 square millimetre. The size is enormous, which no one thought it was possible to produce," explains Assistant Professor Søren Stobbe, who also works at the Niels Bohr Institute.

Basis for new research

Now a foundation had been created for being able to reconcile quantum mechanics with macroscopic materials to explore the optomechanical effects.

Koji Usami explains that in the experiment they shine the laser light onto the nanomembrane in a vacuum chamber. When the laser light hits the semiconductor membrane, some of the light is reflected and the light is reflected back again via a mirror in the experiment so that the light flies back and forth in this space and forms an optical resonator. Some of the light is absorbed by the membrane and releases free electrons. The electrons decay and thereby heat the membrane and this gives a thermal expansion. In this way the distance between the membrane and the mirror is constantly changed in the form of a fluctuation.

"Changing the distance between the membrane and the mirror leads to a complex and fascinating interplay between the movement of the membrane, the properties of the semiconductor and the optical resonances and you can control the system so as to cool the temperature of the membrane fluctuations. This is a new optomechanical mechanism, which is central to the new discovery. The paradox is that even though the membrane as a whole is getting a little bit warmer, the membrane is cooled at a certain oscillation and the cooling can be controlled with laser light. So it is cooling by warming! We managed to cool the membrane fluctuations to minus 269 degrees C", Koji Usami explains.

"The potential of optomechanics could, for example, pave the way for cooling components in quantum computers. Efficient cooling of mechanical fluctuations of semiconducting nanomembranes by means of light could also lead to the development of new sensors for electric current and mechanical forces. Such cooling in some cases could replace expensive cryogenic cooling, which is used today and could result in extremely sensitive sensors that are only limited by quantum fluctuations," says Professor Eugene Polzik.

Plasmonics produces extreme UV light

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

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

(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

(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

(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

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

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

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

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

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

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

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

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.
--------------------------------------------------------------
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
--------------------------------------------------------------
"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

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

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

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

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

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

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).