Attosecond angular streaking

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

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

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

Brightest X-ray Vision at the Nano-scale

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

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

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

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

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

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

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

How ERLs Work

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

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

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

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

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

What Comes Next?

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

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

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

ERL for All

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

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

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

Source: NSF, by Tracy Vosburgh

Laser sets heart beating to a new rhythm

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

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

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

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

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

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

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

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

Source: Optics.org

Extreme UV light made easy

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

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

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

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

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

Surface plasmons

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

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

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

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

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

Moving EUVL From Lab to Fab

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

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

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

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

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

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

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

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

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

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

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

3D microscopy images cells with nanoscale resolution

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

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

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

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

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

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

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

Source: optics.org

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

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

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

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

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

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

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

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

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

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

Source: University of Rochester

Femtosecond laser delivers breakthrough performance

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

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

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

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

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

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

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

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

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

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

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

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

New technique measures ultrashort laser pulses at focus

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

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

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

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

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

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

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

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

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

Faster than a Speeding Bubble

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

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

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

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

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

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

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

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

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

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

Source: by Brad Plummer, SLAC Today

Laser experiments offer insight into evolution of 'gas giants'

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

World's shortest single photon pulse created

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

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

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

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

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

Laser triggered lightning

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

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

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

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

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

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

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

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

Petawatt Power Peak Reached

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

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

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

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

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

Related Link: Texas High Intensity Laser

via: photonics.com

Petawatt laser approaches diffraction limit

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

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

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

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

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

via Optics.org

Protons bring fusion into view

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

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

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

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

Electron filmed for first time ever

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

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

View video: avi or mov.

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

The most intense laser pulse in the universe

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

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

Femtosecond laser creates subsurface structures

Reported by optic.org

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

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

Femtosecond laser produced the colored metals

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

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

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

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

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

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

Nature Physics. doi:10.1038/nphys840

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

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

The concepts of entrance pupil and exit pupil

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

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

Hot attosecond pulse at 2007

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

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

The other stories are:

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

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

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

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

Abstract: Ultrashort light pulses are powerful tools for time-resolved studies of molecular and atomic dynamics1. They arise in the visible and infrared range from femtosecond lasers2, and at shorter wavelengths, in the ultraviolet and X-ray range, from synchrotron sources3 and free-electron lasers4. Recent progress in laser wakefield accelerators has resulted in electron beams with energies from tens of mega-electron volts to more than 1 GeV within a few centimetres, with pulse durations predicted to be several femtoseconds9. The enormous progress in improving beam quality and stability makes them serious candidates for driving the next generation of ultracompact light sources. Here, we demonstrate the first successful combination of a laser-plasma wakefield accelerator, producing 55–75 MeV electron bunches, with an undulator to generate visible synchrotron radiation. By demonstrating the wavelength scaling with energy, and narrow-bandwidth spectra, we show the potential for ultracompact and versatile laser-based radiation sources from the infrared to X-ray energies.

STED microscopy sees details on the nanoscale

Stimulated emission depletion (STED) microscopy has demonstrated that, contrary to a longstanding notion, diffraction-unlimited spatial resolution is viable with conventional lenses and visible light. Currently providing 15–70 nm resolution, it is entering the life sciences at a fast pace, while still undergoing technical improvements. Scientists from Max Planck Institute summarized its principles and recent outcomes. The whole summary should be found from optics.org.

A simple stage-scanning STED setup. Inset: overlay of the excitation focus (green) and the STED efficiency (red) for three different STED laser powers. Credit: Max Planck Institute for Biophysical Chemistry.

Laser light alone can open, close world's fastest optical shutter without heating or cooling

A new study reports that a laser can be used to switch a film of vanadium dioxide back and forth between reflective and transparent states without heating or cooling it. It is one of the first cases that scientists have found where light can directly produce such a physical transition without changing the material’s temperature.

The study, "Coherent Structural Dynamics and Electronic Correlations during an Ultrafast Insulator-to-Metal Phase Transition in VO2", which was published in the Sept. 18 issue of Physical Review Letters, was conducted by a team of physicists from Vanderbilt University and the University of Konstanz in Germany headed by Richard Haglund of Vanderbilt and Alfred Leitenstorfer from Konstanz.

World’s largest laser picks up the pace

With their target completion date just a year and a half away, scientists and technicians at the National Ignition Facility (NIF) are quickening their pace to install and test the rest of NIF’s 192 lasers and prepare for a new round of preliminary experiments in 2008.

This is a report from Lawrence Livermore National Laboratory (LLNL) official web site:

Ninety-six NIF beamlines have been fired together for the first time, with “excellent” control system and laser stability, according to NIF & Photon Science Principal Associate Director Ed Moses. Last month the facility’s injection laser systems, which initiate the laser pulses, were fired for 144 beamlines.

“A total infrared energy of more than 2.5 megajoules has now been fired,” Moses said. “This is more than 40 times what the Nova laser (NIF’s predecessor) typically operated at the time it was the world's largest laser.”

The first of the facility’s two 96-beam laser bays was commissioned at the end of July. Each of the 96 beams fired an infrared output energy of about 22,000 joules, more than enough to meet NIF’s operational and performance requirements. Since then six more eight-beam “bundles” are being commissioned in the second laser bay, and three of these bundles have been operationally qualified.

Overall commissioning of the NIF beamlines is scheduled for 2009.

The laser shots last about 25 billionths of a second, a tiny fraction of the time it takes to blink an eye. Firing the beams requires operation of 2,300 high-quality optics and instrumentation modules and nearly 400 computers running a million lines of control system code.

The tests measure the quality of each beam’s spatial profile and temporal pulse shape. Even though each shot is exceedingly short in time, its energy output and frequency is designed to vary significantly throughout its duration depending on the type of experiments being conducted.

Meanwhile, data gathered from experiments conducted at NIF in 2003-2004 have enabled sophisticated computer simulations that confirm NIF’s ability to reach the energy levels and beam quality required to produce the world’s first demonstration of inertial confinement fusion.

The “NIF Early Light” experiments included four shots using four laser beams at high energy on a full-scale target for the first time. Simulations of the experiments on LLNL’s world-class supercomputers matched the actual experimental data to an unprecedented degree. The experiments and simulations indicate that NIF’s laser beams will propagate effectively in plasma-filled targets designed to achieve fusion ignition and thermonuclear burn.

NIF experiments next year will focus 96 beams on a gold hohlraum (the eraser-sized capsule containing the fusion target) filled with a light gas mixture. Dubbed “Eos” for the Greek goddess of dawn, the experiments will use the first set of beams from the completed laser bay, traveling to the center of the ten-meter diameter target chamber. They are designed to help validate key aspects of the full-scale ignition campaign that begins in 2010.

Photonic crystal fiber produces ultrafast pulses

Reported from optic.org:

A new design of hollow-core photonic crystal fiber (HC PCF) has been developed by an international team led by Fetah Benabid of Bath University in the UK. One immediate result has been a method to produce attosecond laser pulses more efficiently than previous techniques.

The fiber's unique properties have led directly to a second breakthrough, the efficient generation of a broad spectrum of ultrafast pulses from a hydrogen-filled PCF through stimulated Raman scattering.

The conventional technique to create attosecond pulses is high-harmonic generation (HHG), which produces central wavelengths in the XUV or soft X-ray region through the firing of a very intense laser pump pulse into a gas. Benabid's fiber was able to produce ultrashort pulses more simply using through stimulated Raman scattering. Benabid's fiber is claimed to require a pump pulse with power levels six orders of magnitude lower and five orders of magnitude longer than those previously needed for HHG.

Surface heating of wire plasmas using laser-irradiated cone geometries

It's reported on the recent issue of Nature Physics.

Petawatt lasers can generate extreme states of matter, making them unique tools for high-energy-density physics. Pressures in the gigabar regime can potentially be generated with cone-wire targets when the coupling efficiency is high and temperatures reach 2–4 keV. The only other method of obtaining such gigantic pressures is to use the megajoule laser facilities being constructed (National Ignition Facility and Laser MégaJoule). The energy can be transported over surprisingly long distances but, until now, the guiding mechanism has remained unclear. Here, we present the first definitive experimental proof that the heating is maximized close to the wire surface, by comparison of interferometric measurements with hydrodynamic simulations. New hybrid particle-in-cell simulations show the complex field structures for the first time, including a reversal of the magnetic field on the inside of the wire. This increases the return current in a spatially separated thin layer below the wire surface, resulting in the enhanced level of ohmic heating. There are a significant number of applications in high-energy-density science, ranging from equation-of-state studies to bright, hard X-ray sources, that will benefit from this new understanding of energy transport.

LSP modelling of the azimuthal magnetic field structure at the cone tip, 600 fs after the main interaction. A reversed field can be seen on the inside of the wire surface corresponding to the ohmic return current, which is shown on the right picture.

Generation of intense continuum EUVradiation by many-cycle laser fields

The scientists at Institute of Electronic Structure & Laser in Greece and Max-Planck-Institut für Quantenoptik in Germany reported their research results in recent issue of Nature Physics.

Continuing efforts in ultrashort pulse engineering have recently led to the breakthroughs of the generation of attosecond (10^-18 s) pulse trains and isolated pulses. Although trains of multiple pulses can be generated through the interaction of many-optical-cycle pulses with gases—a process that has led to intense extreme-ultraviolet emission—the generation of isolated high-intensity pulses, which requires few-cycle driving pulses, remains a challenge. Here, we report a vital step towards the generation of such pulses, the production of broad continuum extreme-ultraviolet emission using a high-intensity, many-cycle, infrared pulsed laser, through the interferometric modulation of the ellipticity of 50-fs-long driving pulses. The increasing availability of high-power many-cycle lasers and their potential use in the construction of intense attosecond radiation—with either gas or solid-surface targets—offer exciting opportunities for multiphoton extreme-ultraviolet-pump–extreme-ultraviolet-probe studies of laser–matter and laser–plasma interactions.

The Dual Michelson interferometer device is shown in the left picture, BS: beam splitters. M: flat mirrors. TS1,2,3: piezoelectric translation stages. A: intensity attenuator. First and second MI: first and second Michelson interferometers.

Beam Homogenizer

A beam homogenizer is a device that smooths out the irregularities in a laser beam profile and creates a more uniform one. Most beam homogenizers use a multifaceted mirror with square facets. The mirror reflects light at different angles to create a beam with uniform power across the whole beam profile (a "top hat" profile).

The best results have been achieved with fly eye homogenizers which are composed of individually polished cylindrical lenses. The incoming laser beam is divided by an array of cylindrical lenses f_1 into several beamlets with size d. These beamlets match with the cylindrical lenses of a second array f_2. This second array and a condenser lens f_3 overlap all these beamlets in the focal plane of f_3. The homogenizer size D is proportional to the focal length of the collecting lens, the diameter and focal length of the micro-lens, and can be calculated using Equation:

D=(f_3/f_2)d

Interested Links:
Beam-shaping optics expand excimer-laser applications
How to Design a Gaussian to Top-Hat Beam Shaper

Solar laser or solar energy laser?

Based on the news from Optics.org, Japanese team revives solar lasers in quest for clean fuels.

The idea of using solar energy to power lasers is not new. Current designs work by using a system of mirrors to concentrate sunlight into an Nd:YAG crystal, but these lasers are not widely used because they require huge mirrors to collect the light – and even then achieve only low efficiency.

To address these issues, Takashi Yabe and colleagues at the Tokyo Institute of Technology experimented with using a Fresnel lens instead of mirrors as light collectors. They also found that doping the Nd:YAG crystal with small amounts of chromium significantly increases the power output of the laser.

The laser demonstrated by the team produces a power output of 24 W at 1064 nm. The design, which incorporates a 1.3 m2 Fresnel lens, offers an unprecedented slope efficiency of 12% above a threshold solar input of 500 W.

Compressor Alignment Procedure

I concentrated on the pulse compressor these days. Several motors were used to control two big 1200 line/mm gratings. The motors were controlled by Aerotech UNIDEX 511 motion controller. The computer interface program was written in LabView. It's easy to change the grating reflected angles and distances by rotating a hand wheel. The alignment procedure is:

1). Steering the mirrors to center the beam in the grating G1.
2). Set G1 to 0 degree to check the retro reflection.
3). Set G1 to Littrow angle (28.76 in our case) by adjusting the rock.
4). Change G1 to 13.358 for deviation.
5). Set G2 angle 26.716 for 0 degree back reflection checking.
6). Set G2 angle 55.48 for Littrow checking.
7). Set G1 and G2 both 13.358 degree.

related link: UNIDEX 511

Far-field distribution of the seed beam after 3rd ampfilfier

After checking the mirrors and cleaning the dirty ones, we measured the beam profile again. By changing the distance between convex lens and CCD camera, we can measure the far-field distribution of the seed beam. We believed the beam was still good after passing the third amplifier and many reflected mirrors.

Measuring the seed beam profile after 3rd amplifier

We measured the seed beam profile after third amplifier, the CCD camera was put behind the convex mirror of the telescope.

Because the camera was closed to the mirror, we measured the intensity distribution. From the picture shown in the right, we found there were some dust or damaged parts. We should check the optical path and optical components to find what caused this ugly beam profile.

YAG #4 beam alignment

Before running the third amplifier, it's necessary to check pump laser beam qualities of YAG #3 and YAG #4. We found the some beam delivering mirrors for YAG #4 had been burned. Two mirrors were replaced by the new ones, the other two mirrors were just carefully rotated to avoid beam hit the small burned dots.

It's so excited that the spectrometer can be controlled by the desktop.

Beam profile before the third amplifier

After cleaning the delivering mirrors one by one, we measured the beam profile before inputing the third amplifier. This time, the beam looked similar to the Gaussian shape perfectly.

Femtosecond time-delay X-ray holography

Researchers have used the ultrafast X-ray pulses from a free-electron laser to image a nanoscale object in just a femtosecond. The technique, which is a new form of X-ray holography, has been pioneered by Henry Chapman from Lawrence Livermore National Laboratory and colleagues in the US, Switzerland and Germany. Being able to study materials so fast brings us one step closer to the holy grail of observing, at the same time, how all the atoms in a molecule move (Nature 448 676).

The incident FEL pulse from the left passes through a hole in a multilayer-coated detector mirror. The 'dusty mirror' consists of particles on a 20-nm-thick silicon nitride membrane backed by a multilayer-coated plane mirror. This returns the direct beam back through the hole in the detector mirror, which reflects the diffracted light onto a CCD detector. The prompt diffraction (blue, the reference wave) and delayed diffraction (red, the object wave) interfere to generate the hologram on the CCD detector.

Realigning the laser pulse stretcher

It's very hard to increase the output energy from the second amplifier, even optimizing the cavity more carefully. The problem might come from the front end, we traced the beam and found it's very weak after the stretcher. So it took time to realign the laser path and made the dim laser dot became very bright.

The first amplifier output energy without the saturater was around 5 mV (normally ~4 mV), it's about more than 2 mV with the saturater (normally ~1 mV). The output from second amplifier reached about 400 mV. The beam was delivered into the spatial filter and measured the profile using the CCD camera. The diffraction pattern indicated there was a dust or damage on some mirror surface.

Spectrum from laser osillator

A loptop was used to replace the old desktop. In the beginning the spectrometer S2000-USB could not be recognized by the laptop. Somebody suggested to try another USB port, then the signal was appeared on the screen. This indicated the laptop remembered the original port connecting the hardware.

The snapshot of the spectra is shown on the right, the blue color line is reference spectrum, the red one is the real-time spectrum.

The Control Computer Out of Order

I planed to run the whole system today, unfortunately I found the computer controlling the Ocean Optics S2000 spectrometer was down in the morning. I tried to restart the computer many times, it's no response. Because this spectrometer is used to monitor the laser spectrum, which can show if mode-locking status. The control computer must be repaired as soon as possible. The connection adapter is ADC1000-USB(S/N ADUD5565), the software is OOIBASE32.

The computer power supply was totally dead, I tried another computer. However it's not easy to run OOIBEASE32 in the new computer. I tried a laptop which the software has been install, but it cannot recognize the hardware.