Barry C. Walker

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.

Generation of bright isolated attosecond soft X-ray pulses driven by multicycle midinfrared lasers

Significance Attosecond pulses driven by femtosecond lasers make it possible to capture the fastest electron dynamics in molecules and materials. To date, attosecond pulses driven by widely available 800-nm lasers were limited to the extreme UV region of the spectrum, which restricted the range of materials, liquid, and molecular systems that could be explored because of the limited penetrating power. Our recent work showed that longer-wavelength midinfrared driving lasers at wavelengths from 1 to 4 µm are optimal for generating shorter-wavelength, bright, soft X-ray beams. Here we show that longer-pulse-duration midinfrared lasers are also optimal for generating shorter-pulse-duration, attosecond, soft X-rays. This is an unexpected and beautiful convergence of physics: bright, soft X-ray high harmonics naturally emerge as isolated attosecond bursts. High harmonic generation driven by femtosecond lasers makes it possible to capture the fastest dynamics in molecules and materials. However, to date the shortest subfemtosecond (attosecond, 10−18 s) pulses have been produced only in the extreme UV region of the spectrum below 100 eV, which limits the range of materials and molecular systems that can be explored. Here we experimentally demonstrate a remarkable convergence of physics: when midinfrared lasers are used to drive high harmonic generation, the conditions for optimal bright, soft X-ray generation naturally coincide with the generation of isolated attosecond pulses. The temporal window over which phase matching occurs shrinks rapidly with increasing driving laser wavelength, to the extent that bright isolated attosecond pulses are the norm for 2-µm driving lasers. Harnessing this realization, we experimentally demonstrate the generation of isolated soft X-ray attosecond pulses at photon energies up to 180 eV for the first time, to our knowledge, with a transform limit of 35 attoseconds (as), and a predicted linear chirp of 300 as. Most surprisingly, advanced theory shows that in contrast with as pulse generation in the extreme UV, long-duration, 10-cycle, driving laser pulses are required to generate isolated soft X-ray bursts efficiently, to mitigate group velocity walk-off between the laser and the X-ray fields that otherwise limit the conversion efficiency. Our work demonstrates a clear and straightforward approach for robustly generating bright isolated attosecond pulses of electromagnetic radiation throughout the soft X-ray region of the spectrum.

Generation of hard x rays by ultrafast terawatt lasers

A compact, tabletop terawatt Ti:sapphire laser drive, ultrafast hard x-ray source for time-resolved x-ray diffraction studies is described. With a copper target the energy conversion efficiency from laser photons (800 nm) to copper K x-ray radiation (1.54 A) is 0.008%. The optimal laser intensity for generating these x rays is 1018 W cm−2, lower than the highest laser intensity available (5×1018 W cm−2) from the laser system. These results are consistent with a theoretical model proposed on the basis that the x rays are produced as a result of laser driven electron ionization of core level electrons of Cu atoms near room temperature. This source also provides features such as ultrashort pulse duration, extremely small source size, variable wavelengths, high peak spectral brightness, and the potential for multiple beam line experiments. X-ray diffraction patterns from GaAs single crystals and amorphous Ni films recorded with this source are presented.

Time-resolved momentum imaging system for molecular dynamics studies using a tabletop ultrafast extreme-ultraviolet light source

We describe a momentum imaging setup for direct time-resolved studies of ionization-induced molecular dynamics. This system uses a tabletop ultrafast extreme-ultraviolet (EUV) light source based on high harmonic upconversion of a femtosecond laser. The high photon energy (around 42 eV) allows access to inner-valence states of a variety of small molecules via single photon excitation, while the sub--10-fs pulse duration makes it possible to follow the resulting dynamics in real time. To obtain a complete picture of molecular dynamics following EUV induced photofragmentation, we apply the versatile cold target recoil ion momentum spectroscopy reaction microscope technique, which makes use of coincident three-dimensional momentum imaging of fragments resulting from photoexcitation. This system is capable of pump-probe spectroscopy by using a combination of EUV and IR laser pulses with either beam as a pump or probe pulse. We report several experiments performed using this system.

0.09-terawatt pulses with a 31% efficient, kilohertz repetition-rate Ti:sapphire regenerative amplifier

We present an efficient, ultrafast regenerate amplifier that increases the energy of a laser pulse from 300 pJ to 6 mJ and produces average powers of as much as 9 W in a TEM(00) spatial mode. As an ultrafast amplifier, the system produces 4-mJ pulses with 0.09 TW of peak power.

A 50-EW/cm 2 Ti:sapphire laser system for studying relativistic light-matter interactions

A 10-Hz repetition rate, 60-TW peak power, Ti:sapphire laser system was developed for use in experiments where relativistic effects dominate the physics. The temporal, spectral, energy and spatial characteristics of the laser pulses were measured in single shot format. The pulse duration ranged from 22 fs to 25 fs and the pulse energy averaged 1.3 J. Atomic photoionization measurements quantified the peak intensity of the laser pulse in situ. The measurements indicated an intensity of at least 510 19 W/cm 2 was produced.

Fringe-free, background-free, collinear third-harmonic generation frequency-resolved optical gating measurements for multiphoton microscopy

A background-free, fringe-free form of frequency-resolved optical gating using the third-harmonic signal generated from a glass coverslip is used to characterize 100 fs pulses at the focus of a 0.65 NA objective.

Larmor radiation from the ultra-intense field ionization of atoms

The angle- and energy-resolved Larmor radiation from atomic ionization in the focus of ultra-intense laser field is calculated using a semi-classical, trajectory ensemble model of ionization. We find that including the quantum nature of the atomic ionization decreases the radiation yield by an order of magnitude compared to a classical electron calculation due to interference effects in the extended probability of the electron wavefunction and the quantum nature of tunnelling ionization. The evolution of the radiation from non-relativistic to relativistic continuum dynamics is presented as a function of the laser intensity with Larmor radiation not becoming prominent until 1019 W cm−2. For the ionization of Na10+ at a density of 1015 atoms cm−3 and a peak-focused laser intensity of 1020 W cm−2, the radiation is highly directional at an angle of 45° from the laser electric field with photon energies out to 500 eV.

Theoretical and experimental spectral phase error analysis for pulsed laser fields

Distortions in pulsed laser fields are analyzed by means of the root mean square of intensity-weighted spectral phase deviations. This method quantifies pulse errors independently of pulse duration and can be applied to both simple, transform-limited and complex, shaped pulses. A good linear relationship exists (fit correlation=0.95) between the analyzed phase deviations and temporal pulse distortion measures. In contrast, a common Taylor series analysis showed a fit correlation of only 0.5 with temporal measures. Alternative methods examined, such as the pulse FWHM, were determined to be less general measures of pulse distortion and, in modeling of spectral phase errors, were shown to have the potential of being misleading.

Electron momentum states and bremsstrahlung radiation from the ultraintense field ionization of atoms

Relativistic continuum dynamics for electrons from the ionization of atoms in an ultraintense (1017 W/cm2 to 1020 W/cm2) laser focus are analyzed using a semi-classical wavelet model. The results quantify the energy and angle resolved photoionization yields due to the developing relativistic dynamics in ultraintense fields. Using the final state momentum, the bremsstrahlung radiation yield is calculated and shows a linear relationship between the radiation cutoff and the laser intensity. At 1020 W/cm2 photons with energies out to 10MeV should be observed. The results are quantitatively comparable to the observed angle resolved photoelectron spectra of current ultraintense laser-atom experiments. The results show the azimuthal angular distributions becoming more isotropic with increasing intensity.