Craig W. Siders

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.

Blue-shifted third-harmonic generation and correlated self-guiding during ultrafast barrier suppression ionization of subatmospheric density noble gases

The generation of frequency upshifted, pressure-tunable third-harmonic pulses during ultrafast ionization of subatmospheric noble gases at peak intensity 1016 W/cm2 is reported. Pressure tuning of the power spectrum centroid over the range 10−5 ≲ Δω/ω ≲ 10−1, corresponding to a pressure range of 0.1 ≲ p ≲ 700 Torr, is demonstrated. We also demonstrate that the blue-shifted portions of both the 3ω and the ω pulses are preferentially self-guided in a limited pressure range (10 ≲ p ≲ 500 Torr), which is key to separating the pressure-tunable component of the harmonic from nontunable harmonic generated by means of atomic nonlinearities.

Plasma guiding and wakefield generation for second-generation experiments

A design study has been carried out for a second-generation experiment on laser guiding and wakefield excitation in a channel. From simple scaling laws for the wakefield amplitude, dephasing length, the relativistic group velocity factor /spl gamma//sub g/, and energy gain with and without guiding, we find that the parameter regime for a compact single stage GeV accelerator favors laser systems producing short pulses (10 fs/spl les//spl tau//spl les/100 fs), each containing an energy on the order of 100 mJ to a few Js. Taking the dephasing length as the maximum acceleration distance, plasma channels with lengths of 1-10 cm and densities of 10/sup 17/-10/sup 19/ cm/sup -3/ need to be produced; whereas the design study has been primarily concerned with diffraction and channel guiding, dephasing and depletion limits, and linear wakefield theory, aspects of the effect of the plasma wave on the evolution of the laser pulse are discussed. We find that transverse and longitudinal pulse distortions could indeed affect the generated plasma wave phase velocity and amplitude, and hence may limit the achievable energy gains over the one-dimensional (1-D) linear estimates. Some issues for experiments on prototype small accelerators (100 MeV-1 GeV, cm scale) are also discussed.

Efficient high-energy pulse-train generation using a 2 n -pulse Michelson interferometer

We demonstrate a novel, Michelson-based, ultrafast multiplexer with a throughput approaching 100% for a polarization-multiplexed train and 50% for a linearly polarized train, which is compatible with a high-energy pulse train and shaped-pulse generation. The interpulse spacings in the resultant 2(n)-pulse train can be adjusted continuously from multinanoseconds through zero. Using this interferometer, we also demonstrate generation of a 16-pulse train of terahertz pulses.

Ionization-induced frequency shifts in intense femtosecond laser pulses

Electromagnetic plasma computer simulations are used to analyze the frequency shifts caused by the ionization of atmospheric-density noble gases during interaction with intense femtosecond laser pulses; the results are presented and compared with experimental data. The simulations trace the temporal evolution of plasma growth during the femtosecond ionizing pulse and calculate the resulting self-induced blue shift of the ionizing pulse spectrum. Variations with pulse intensity, gas pressure, and gas species are calculated. The relative contributions of strong-field ionization and electron-impact ionization on the frequency shifts are discussed. The simulations provide qualitative explanations of most of the features observed experimentally in the blue-shifted spectra. The technique of spectral blue shifting intense femtosecond laser pulses provides a new diagnostic tool for studying strong-field ionization and laser-induced breakdown in dense plasmas.

Femtosecond growth dynamics of an underdense ionization front measured by spectral blueshifting

Time-resolved spectral blue shifts of 100-fs laser pulses caused by ionization of atmospheric density N/sub 2/ and noble gases subjected to high (10/sup 14/ W/cm/sup 2/ to 10/sup 16/ W/cm/sup 2/) light intensities are examined. Included are data for two experiments: self-shifting of the ionizing laser pulses for varying peak intensities, pressures (1-5 atm), and gas species; and time-resolved blue shifts of a weak copropagating probe pulse for the same range of ionization conditions. The self-shift data reveal a universal, reproducible pattern in the shape of the blueshifted spectra: as laser intensity, gas pressure, or atomic number increase, the self-blueshifted spectra develop from a near replica of the incident pulse spectrum into a complex structure consisting of two spectral peaks. The time-resolved data reveal different temporal dependence for each of these two features. A quantitative model for a simplified cylindrical focal geometry is presented. >

Coherent ultrafast MI-FROG spectroscopy of optical field ionization in molecular H/sub 2/, N/sub 2/, and O/sub 2/

Quantitative phase-sensitive measurements of ultrafast optical-field ionization rates in molecular H/sub 2/, N/sub 2/, and O/sub 2/ are obtained using a temporally gated frequency-domain interferometric pulse measurement technique: multipulse interferometric frequency-resolved optical gating (MI-FROG). By measuring the pump-induced frequency change on a weak copropagating probe pulse, the optical field ionization dynamics can be completely time-resolved with sub-pulsewidth time resolution. A one-dimensional nonrelativistic electromagnetic fluid code model is used to compute the ionization dynamics and optical field propagation through the plasma. Using the Ammosov-Delone-Krainov (ADK) tunnel ionization rate model originally developed for atoms, the relatively simple model proposed here has been shown to compare favorably with the MI-FROG measured ionization rates in noble gases in the intermediate intensity regime (10/sup 14/ W/cm/sup 2/) (Siders et al, Phys. Rev. Lett.). We attempt to unify our studies in noble gases and molecules by performing experiments on N/sub 2/ and O/sub 2/, which have nearly identical ionization potentials to Ar and Xe, respectively. For the molecules studied here, we show that an ADK-like description of molecular ionization rates calculated from the model agree with the experimentally measured rates using the MI-FROG technique for H/sub 2/ and N/sub 2/. In the case of O/sub 2/, however, the experimentally measured ionization rate is approximately two orders of magnitude lower than that expected from the standard ADK formula. This is in agreement with the previously observed suppressed O/sub 2/ ionization rate in ion mass spectroscopy studies (Guo, 2000). We attribute the suppressed ionization rate in O/sub 2/ to a multielectron screening effect and show that a modified version of the ADK formula, taking into account the electron screening as proposed by Guo, well approximates the MI-FROG O/sub 2/ ionization rate data.

Generation and characterization of terahertz pulse trains from biased, large-aperture photoconductors.

The saturation properties of terahertz emission from biased, large-aperture photoconductors excited by trains of amplified femtosecond optical pulses are presented. A direct comparison is made of the multiple-pulse saturation properties of terahertz emission from semi-insulating GaAs and low-temperature-grown GaAs emitters with different carrier lifetimes. When the carrier lifetime is less than or comparable with the interpulse spacing, a significant enhancement of the narrow-band terahertz output is observed. The enhancement is not observed for emitters with long carrier lifetimes, consistent with the results of a previously derived saturation theory [Opt. Lett. 18, 1340 (1993)].

Multipulse interferometric frequency-resolved optical gating: real-time phase-sensitive imaging of ultrafast dynamics

We demonstrate a powerful new tool for real-time single-shot imaging of ultrafast phase shifts based on multipulse interferometric frequency-resolved optical gating that can directly measure and display ultrafast-time-scale phase shifts without computation. In addition, this technique can, with the application of interferogram analysis and iterative phase-retrievial techniques, recover the intensity and phase of three pulses in a single shot and exhibits a linear sensitivity to the pulse field in the wings.