Amy L. Lytle

Phase-matching techniques for coherent soft X-ray generation

Coherent beams at soft X-ray (SXR) wavelengths can be generated using extreme nonlinear optics by focusing an intense laser into a gas. In this paper, we discuss phase-matching and quasi-phase-matching techniques that use gas-filled modulated waveguides to enhance the frequency conversion process. This leads to the generation of SXR beams that are both spatially and temporally coherent.

Long-term carrier-envelope phase stability from a grating-based, chirped pulse amplifier

We demonstrate a carrier-envelope phase (CEP) stabilized, chirped pulse laser amplifier that exhibits greatly improved intrinsic long-term CEP stability compared with that of other amplifiers. This system employs a grating-based stretcher and compressor and a cryogenically cooled laser amplifier. Single-shot carrier envelope phase noise measurements are also presented that avoid underestimation of this parameter caused by fringe averaging and represent a rigorously accurate upper limit on CEP noise.

Quasi-phase matching and characterization of high-order harmonic generation in hollow waveguides using counterpropagating light

We review recent experimental and theoretical work on the use of counterpropagating light to enhance high-order harmonic generation through all-optical quasi-phase matching. Also presented is a new technique for measuring the coherence of high harmonics in the nonlinear medium. This information is crucial for understanding the process of harmonic generation over extended distances, as well as for effective enhancement using quasi-phase matching techniques.

Optimizing quasi-phase matching of high harmonic generation using counterpropagating pulse trains

We formulate a theory of quasi-phase matching of high harmonic generation using weak counterpropagating pulse trains. We predict the optimal laser intensities and pulse shapes for the counterpropagating field and find that the conversion efficiency is better than the efficiency obtained by simply suppressing harmonic emission from out-of-phase regions.

Quasi-phase matching of high-order harmonic generation at high photon energies using counterpropagating pulses

We extend all-optical quasi-phase matching of high-order harmonic generation into spectral regions where conventional phase matching is not possible. The high laser intensities required to generate harmonics at energy >130 eV, coupled with the resulting high level of ionization, preclude conventional phase matching in all nonlinear media. Selective enhancement factors between 40 and 150 in the flux of harmonics at photon energies around 140 eV are demonstrated using a train of two counterpropagating pulses.

Quantum-path control in high-order harmonic generation at high photon energies

We show through experiment and calculations how all-optical quasi-phase-matching of high-order harmonic generation can be used to selectively enhance emission from distinct quantum trajectories at high photon energies. Electrons rescattered in a strong field can traverse short and long quantum trajectories that exhibit differing coherence lengths as a result of variations in intensity of the driving laser along the direction of propagation. By varying the separation of the pulses in a counterpropagating pulse train, we selectively enhance either the long or the short quantum trajectory, and observe distinct spectral signatures in each case. This demonstrates a new type of coupling between the coherence of high-order harmonic beams and the attosecond time-scale quantum dynamics inherent in the process.

Use of a simple cavity geometry for low and high repetition rate modelocked Ti:sapphire lasers

We demonstrate a general procedure for varying the repetition rate of a modelocked Ti:sapphire laser using an asymmetric focusing geometry. Using this procedure, we have made an extended length cavity with a repetition rate of 45 MHz, and a reduced length cavity with a repetition rate of 275 MHz, each of which generates sub-20 fs pulses. This procedure allows the repetition rate of the laser to be more precisely tailored for a variety of applications without compromise in performance.

High efficiency, modular, optical pulse shaping technique for tunable terahertz generation from InAs

We demonstrate a high energy throughput, modular optical laser pulse shaping technique for generating tunable, narrowband, terahertz radiation from the surface of InAs. We achieve a frequency selectivity (Δf/f) of 0.10 at 1.18 THz and demonstrate an energy throughput of up to 98% using two etalons to create a sequence of optical pulses. In contrast with previously reported schemes, our technique does not rely on interferometry or involve diffractive optical elements, making it robust and relatively inexpensive to implement. This technique can be expanded with additional etalons in order to achieve greater frequency selectivity without sacrificing efficiency.

Influence of counterpropagating light on phase matching in second-harmonic generation

Here we show an explicit measurement of the microscopic disruption to the phase-matching conditions for second-harmonic generation caused by a counterpropagating light field. This microscopic phase disruption has been assumed as the mechanism behind successful implementations of all-optical quasi-phase matching of high-order harmonic generation but has never been previously observed. Numerical simulations reproduce the features of the observed disruption and indicate pathways for implementing in situ probing and quasi-phase matching of second-harmonic generation with counterpropagating fields.

Time evolution of the Coulomb screening effects on terahertz generation at the surface of InAs

We study the effects of electron density and temperature, both experimentally and numerically, on the Coulomb screening process responsible for saturation of the terahertz generation process at the surface of InAs. We use a pair of ultrafast pulses with adjustable time delay to generate terahertz radiation and compare the results to a 1D drift-diffusion equation model modified to include radial diffusion and cooling of the electrons through scattering. We demonstrate the necessity of these modifications by implementing the original drift-diffusion equation model, as reported by Liu et al. [Phys. Rev. B73, 155330 (2006)1098-0121PRBMDO10.1103/PhysRevB.73.155330], and show that it underestimates saturation by an order of magnitude. We find excellent agreement between our experimental and numerical results, confirming the validity of our improved model and demonstrating its potential use in a number of applications, such as terahertz pulse shaping with an ultrafast pulse train.