William Carl Wallace

Self-focusing in air with phase-stabilized few-cycle light pulses

We investigate the nonlinear optical phenomenon of self-focusing in air with phase-stabilized few-cycle light pulses. This investigation looks at the role of the carrier-envelope phase by observing a filament in air, a nonlinear phenomenon that can be utilized for few-cycle pulse compression [Appl. Phys. B79, 673 (2004)]. We were able to measure the critical power for self-focusing in air to be 18+/-1 GW for a 6.3 fs pulse centered at 800 nm. Using this value and a basic first-order theory, we predicted that the self-focusing distance should deviate by 790 mum as the carrier-envelope phase is shifted from 0 to pi/2 rad. In contrast, the experimental results showed no deviation in the focus distance with a 3sigma upper limit of 180 mum. These counterintuitive results show the need for further study of self-focusing dynamics in the few-cycle regime.

Precise and Accurate Measurements of Strong-Field Photoionization and a Transferable Laser Intensity Calibration Standard.

Ionization of atoms and molecules in strong laser fields is a fundamental process in many fields of research, especially in the emerging field of attosecond science. So far, demonstrably accurate data have only been acquired for atomic hydrogen (H), a species that is accessible to few investigators. Here, we present measurements of the ionization yield for argon, krypton, and xenon with percent-level accuracy, calibrated using H, in a laser regime widely used in attosecond science. We derive a transferable calibration standard for laser peak intensity, accurate to 1.3%, that is based on a simple reference curve. In addition, our measurements provide a much needed benchmark for testing models of ionization in noble-gas atoms, such as the widely employed single-active electron approximation.

Experimental ionization of atomic hydrogen with few-cycle pulses

We present experimental data on strong-field ionization of atomic hydrogen by few-cycle laser pulses. We obtain quantitative agreement at the 10% level between the data and an ab initio simulation over a wide range of laser intensities and electron energies.

Carrier-envelope phase effects in above-threshold ionization of atomic hydrogen

Recent experiments in ultrafast physics have established the importance of above-threshold ionization (ATI) experiments in measuring and controlling the carrier-envelope phase (CEP) of few-cycle laser pulses. We have performed an investigation of atomic hydrogen subjected to intense CEP- stable few-cycle laser pulses. The experimental ATI spectra have been compared to predictions from an ab initio numerical solution of the time-dependent Schr¨ odinger equation in three dimensions. Good agreement between experiment

Carrier-Envelope Phase effect for Dissociation of Molecular Hydrogen

We studied the dependence on the dissociative ionization of H2 as a function of carrier-envelope phase (CEP) of few-cycle (6 fs) near-infrared (NIR) laser pulses. For low-energy channels, we present the first experimental observation of CEP dependence for combined dissociation yield (with protons emitted in both directions) and the highest degree of asymmetry reported to date (40%).

An extreme ultraviolet interferometer using high order harmonic generation

We present a new interferometer technique based on the interference of high-order harmonic generation radiation from translatable successive gas jets. The phase shifts in the apparatus are shown to originate from the Gouy phase shift of the driving laser. The technique can be used to deliver time delays between light pulses and we demonstrate the unprecedented capability of delivering pulses of extreme ultraviolet light delayed in time by as small as 100 zeptoseconds.

Photoionization yield of atomic hydrogen using intense few-cycle pulses

We present the measured photoionization yield of atomic hydrogen as a function of laser intensity for few-cycle pulses. Fits with exact ab-initio simulations produce better agreement than analytical theories and enable accurate intensity calibration.

Optimized attosecond XUV pulses with zeptosecond timing resolution

We experimentally optimize the yield of attosecond XUV pulses, with pulse duration inferred from carrier-envelope-phase resolved spectral measurements. We also demonstrate generation of dual attosecond pulses with timing resolution of 90 zeptoseconds.

Above-threshold ionization in atomic hydrogen using intense, few-cycle laser pulses

We have performed the first strong-field ionization experiment in atomic hydrogen using few-cycle laser pulses. Quantitative agreement between experimental data and advanced ab initio simulations has been achieved at the 10% level.

A Zeptosecond Phase Interferometer

In this paper we describe a new type of interferometer that offers unprecedented resolution of dynamical processes in the zeptosecond (10−21s) time domain.