High harmonic generation from transiently aligned molecules in a hollow-core waveguide

A coherent VUV source, based on optical high harmonic generation (HHG) of intense ultrashort laser pulses in atomic and molecular gases, has been developed at the University of Virginia. This source utilizes a hollow-core waveguide to increase the VUV yield and enable better control over its macroscopic coherence properties. Several experiments were conducted that focus on using emission to better understand molecular dynamics in intense laser fields and on exploiting these dynamics to enhance the harmonic emission.;The first reported measurements of the molecular alignment-dependent harmonic yields from CO and N2 were observed in a hollow-core waveguide. From these alignment signals, a functional form for the HHG angular dependence was determined by fitting the data to simulated alignment parameters. In particular, the contrast ratio of the harmonic yield from perfectly aligned vs. anti-aligned molecules was extracted. Next, the effect of molecular alignment on the macroscopic propagation properties of the medium was examined. At higher orders, a reversal in the time-dependent variation in the HHG yield was observed. Simulations indicate that this reversal is due to the increased ionization from aligned molecules which affects the phase-matching properties of the medium. This result illustrates that both the single molecule response and the macroscopic propagation properties of the medium contribute to the alignment-dependent variation in the HHG yield. As a technical improvement, a liquid nitrogen cooled fiber was constructed to reduce the gas temperature from 300 K to 150 K, improving the degree of alignment. The final group of experiments focused on the use of counter-propagating (CP) control and signal pulses in a quasi-phase matching (QPM) scheme to enhance the harmonic output. First, long CP pulses were used to probe the macroscopic non-linear-optical properties of gas. Then, short control pulses were used to transiently align diatomic molecules inside the waveguide, spatially mapping the periodic temporal revival structure and, thus, modulating the properties of the molecular target gas throughout the fiber. Unfortunately, this modulation resulted in only a small increase in the HHG yield. Simulations indicate that this enhancement is due to the weak time-independent alignment produced by the short, CP pulse.