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.