Curved light bends the rules

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

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

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

(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

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

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

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

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

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

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

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

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

(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

(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

(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

(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

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

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

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

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".