C. P. J. Barty

Picosecond-milliangstrom lattice dynamics measured by ultrafast X-ray diffraction

Fundamental processes on the molecular level, such as vibrations and rotations in single molecules, liquids or crystal lattices and the breaking and formation of chemical bonds, occur on timescales of femtoseconds to picoseconds. The electronic changes associated with such processes can be monitored in a time-resolved manner by ultrafast optical spectroscopic techniques, but the accompanying structural rearrangements have proved more difficult to observe. Time-resolved X-ray diffraction has the potential to probe fast, atomic-scale motions. This is made possible by the generation of ultrashort X-ray pulses, and several X-ray studies of fast dynamics have been reported,. Here we report the direct observation of coherent acoustic phonon propagation in crystalline gallium arsenide using a non-thermal, ultrafast-laser-driven plasma — a high-brightness, laboratory-scale source of subpicosecond X-ray pulses. We are able to follow a 100-ps coherent acoustic pulse, generated through optical excitation of the crystal surface, as it propagates through the X-ray penetration depth. The time-resolved diffraction data are in excellent agreement with theoretical predictions for coherent phonon excitation in solids, demonstrating that it is possible to obtain quantitative information on atomic motions in bulk media during picosecond-scale lattice dynamics.

Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power

We analyze the scalability of diffraction-limited fiber lasers considering thermal, non-linear, damage and pump coupling limits as well as fiber mode field diameter (MFD) restrictions. We derive new general relationships based upon practical considerations. Our analysis shows that if the fibers MFD could be increased arbitrarily, 36 kW of power could be obtained with diffraction-limited quality from a fiber laser or amplifier. This power limit is determined by thermal and non-linear limits that combine to prevent further power scaling, irrespective of increases in mode size. However, limits to the scaling of the MFD may restrict fiber lasers to lower output powers.

REGENERATIVE PULSE SHAPING AND AMPLIFICATION OF ULTRABROADBAND OPTICAL PULSES

Regenerative pulse shaping is used to alleviate gain narrowing during ultrashort-pulse amplification. Amplification bandwidths of ~ 100 nm, or nearly three times wider than the traditional gain-narrowing limit, are produced with a modified Ti:sapphire regenerative amplifier. This novel regenerative amplifier has been used to amplify pulses to the 5-mJ level with a bandwidth sufficient to support ~ 10-fs pulses.

Generation of 18-fs, multiterawatt pulses by regenerative pulse shaping and chirped-pulse amplification

Transform-limited, 18-fs pulses of 4.4-TW peak power are produced in a Ti:sapphire-based chirped-pulsed amplification system at a repetition rate of 50 Hz. Regenerative pulse shaping is used to control gain narrowing during amplification, and an optimized, quintic-phase-limited dispersion compensation scheme is used to control higher-order phase distortions over a bandwidth of ~100 nm. Seed pulses are temporally stretched >100,000 times before amplification.

Detecting clandestine material with nuclear resonance fluorescence

We study the performance of a class of interrogation systems that exploit nuclear resonance fluorescence (NRF) to detect specific isotopes. In these systems the presence of a particular nuclide is inferred by observing the preferential attenuation of photons that strongly excite an electromagnetic transition in that nuclide. Estimates for the false positive/negative error rates, radiological dose, and detection sensitivity associated with discovering clandestine material embedded in cargo are presented. The relation between performance of the detection system and properties of the beam of interrogating photons is also considered. Bright gamma-ray sources with fine energy and angular resolution, such as those based on Thomson upscattering of laser light, are found to be associated with uniquely low radiological dose, scan times, and error rates. For this reason a consideration of NRF-based interrogation systems may provide impetus for efforts in light source development for applications related to national security and industry.

An overview of LLNL high-energy short-pulse technology for advanced radiography of laser fusion experiments

The technical challenges and motivations for high-energy, short-pulse generation with the National Ignition Facility (NIF) and possibly other large-scale Nd : glass lasers are reviewed. High-energy short-pulse generation (multi-kilojoule, picosecond pulses) will be possible via the adaptation of chirped pulse amplification laser techniques on NIF. Development of metre-scale, high-efficiency, high-damage-threshold final optics is a key technical challenge. In addition, deployment of high energy petawatt (HEPW) pulses on NIF is constrained by existing laser infrastructure and requires new, compact compressor designs and short-pulse, fibre-based, seed-laser systems. The key motivations for HEPW pulses on NIF is briefly outlined and includes high-energy, x-ray radiography, proton beam radiography, proton isochoric heating and tests of the fast ignitor concept for inertial confinement fusion.

Collinear type II second-harmonic-generation frequency-resolved optical gating for use with high-numerical-aperture objectives

Ultrashort-pulse lasers are now commonly used for multiphoton microscopy, and optimizing the performance of such systems requires careful characterization of the pulses at the tight focus of the microscope objective. We solve this problem by use of a collinear geometry in frequency-resolved optical gating that uses type II second-harmonic generation and that allows the full N.A. of the microscope objective to be used. We then demonstrate the technique by measuring the intensity and the phase of a 22-fs pulse focused by a 20x, 0.4-N.A. air objective.

Laser challenges for fast ignition

Abstract The laser challenges and state of the art in high-energy, solid-state petawatt lasers for fast ignition (FI) research are reviewed. A number of new laser systems are currently under construction or being planned that will facilitate proof-of-principle FI experiments. Recent technological advances in each of the major ultrafast laser subsystems are reported, including chirped-pulse generation and broadband amplification in the front end, high-energy amplification, and pulse compression with adaptive wavefront correction. Unique challenges related to operating high-energy chirped-pulse-amplification laser systems for FI, such as protection from target back reflections, are also addressed.

High-average-power femto-petawatt laser pumped by the Mercury laser facility

The Mercury laser system is a diode-pumped solid-state laser that has demonstrated over 60 J at a repetition rate of 10 Hz (600 W) of near-infrared light (1047 nm). Using a yttrium calcium oxyborate frequency converter, we have demonstrated 31.7 J/pulse at 10 Hz of second harmonic generation. The frequency converted Mercury laser system will pump a high-average-power Ti:sapphire chirped pulse amplifier system that will produce a compressed peak power > 1 PW and peak irradiance > 1023W/cm2.

Hybrid chirped-pulse amplification

Conversion efficiency in optical parametric chirped-pulse amplification is limited by spatiotemporal characteristics of the pump pulse. We have demonstrated a novel hybrid chirped-pulse amplification scheme that uses a single pump pulse and combines optical parametric amplification and laser amplification to achieve high gain, high conversion efficiency, and high prepulse contrast without utilization of electro-optic modulators. We achieved an overall conversion efficiency of 37% from the hybrid amplification system at a center wavelength of 820nm. Generation of multiterawatt pulses is possible by use of this simple method and commercial Q -switched pump lasers.