Christopher Barty

Picosecond-milliangstrom resolution dynamics by ultrafast x-ray diffraction

Optical pump, x-ray diffraction probe measurements have been used to study the lattice dynamics of single crystals with picosecond-milliangstrom resolution by employing a table- top, laser-driven x-ray source. The x-ray source, consisting of an approximately 30 fs, 75 mJ/pulse, 20 Hz repetition rate, terawatt laser system and a moving Cu wire target assembly, generates approximately 5 X 1010 photons (4π steradians s)-1 of Cu Kα radiation. Lattice spacing changes of as small as 1 X 10-3 Å in a few picoseconds have been detected, utilizing Bragg diffraction from GaAs single crystals. Enhancement of the diffraction intensity associated with degradation of the crystals during and after the laser irradiation has been observed, likely due to a transition from dynamic to kinematic diffraction.

ND:GLASS LASER DESIGN FOR LASER ICF FISSION ENERGY (LIFE)

Abstract We have developed preliminary conceptual laser system designs for the Laser ICF (Inertial Confinement Fusion) Fission Energy (LIFE) application. Our approach leverages experience in high-energy Nd: glass laser technology developed for the National Ignition Facility (NIF)1, along with high-energy-class diode-pumped solid-state laser (HEC-DPSSL) technology developed for the DOE’s High Average Power Laser (HAPL) Program and embodied LLNL’s Mercury laser system.2 We present laser system designs suitable for both indirect-drive, hot spot ignition and indirect-drive, fast ignition targets. Main amplifiers for both systems use laser-diode-pumped Nd:glass slabs oriented at Brewster’s angle, as in NIF, but the slabs are much thinner to allow for cooling by high-velocity helium gas as in the Mercury laser system. We also describe a plan to mass-produce pump-diode lasers to bring diode costs down to the order of

Time-gated medical imaging with ultrafast laser-plasma x rays

0.01 per Watt of peak output power, as needed to make the LIFE application economically attractive.

Ultrafast movies of atomic motion with femtosecond laser-based x-rays

Laser-generated, hard x-rays are produced in a > 1018 W/cm2 focus of an ultrashort-pulse laser system. The application of ultrashort-duration, laser-generated x-rays to diagnostic medical imaging is discussed. Time-gated detection allows removal of scattered radiation, improved image quality and possible reduction of patient exposure. Methods for improvement of x-ray yield, design of appropriate drive lasers, and applications to mammography and angiography are also discussed.

Techniques for controlling gain narrowing during ultrashort-pulse amplification

Using ultrafast x-ray diffraction from a laser-plasma x-ray source, we have observed coherent photon generation and propagation in bulk(111)-GaAs, (111)-Ge, and thin(111)-Ge- on-Si films. At higher optical pump fluences, ultrafast melting of Ge films is observed.

Ultrafast x-ray absorbtion and diffraction

Regenerative pulse shaping is used to overcome gain narrowing during ultrashort pulse amplification. We have demonstrated multiple spectral filters for broadening the amplified spectrum. We have produced amplified pulses with an energy of approximately 5 mJ and bandwidths of approximately 100 nm, or nearly 3 times wider than the gain narrowing limit of Ti:sapphire.

Grisms: novel dispersive devices for the manipulation of femtosecond pulses

Our goal is to watch the evolution of matter on the atomic length scale and on the time scale on which elementary chemical reactions take place. We present initial experiments made in collaboration between UCSD and the INRS laboratory in Canada, on time-resolved ultrafast, 3 ps temporal resolution, near-edge x-ray absorption of gas phase SF6 at 2.4 keV (4.89 A). We can see both the initial presence of the F atoms around the S and their absence after photodissociation produced by pumping with an intense optical pulse. Simulations of ultrafast EXAFS and diffraction experiments are presented. We are constructing an ultrahigh intensity laser to generate ultrafast x-ray pulses from laser-produced plasmas. This laser is especially designed to achieve high average power, short pulse duration and high intensity to produce very high temperature solid density plasmas and ultrahot electrons for ultrafast hard x-ray production at high x-ray photon flux, which should enable us to perform a variety of ultrafast x-ray absorption and diffraction experiments. Finally, we discuss several means to measure the duration of subpicosecond x-ray pulses.

Ultrashort-pulse, ultrahigh-peak-power Ti:sapphire lasers

We show that a pair of grisms (transmission gratings written onto prisms) exhibits second- and third-order dispersion opposite to that of an optical fiber. Using a fiber stretcher and a grism- pair compressor, we demonstrate chirped-pulse amplification and transform-limited compression of 135-fs pulses. This scheme can be used to produce 50-fs, microjoule pulses in a compact and simple system.

Proposed generation of subfemtosecond VUV pulses from high-order harmonics

Techniques for the production of multiterawatt, sub-20-fs, optical pulses via chirped pulse amplification are discussed. 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 approximately 100 nm. Transform-limited, 18-fs pulses of 4.4-TW peak power have been produced in a Ti:sapphire- based, chirped pulse amplification system at a repetition rate of 50 Hz. Extensions to shorter durations and peak powers approaching 100 TW are also described.

Coster-Kronig mediated inner-shell x-ray lasers pumped by 20-fs 100-TW-class lasers

We propose a method for producing sub-femtosecond VUV pulses by compressing the high order harmonic radiation emitted from a nonlinear medium driven by an intense, ultrafast laser pulse. We present single atom calculations of the harmonic amplitudes and phases as well as simulations of a VUV compressor. Single harmonics from a 27 fs driving pulse can be compressed to less than 2 fs duration, and several adjacent harmonics with similar phase structure can be combined within the same compressor to produce an attosecond pulse.