Enhanced Harmonic Generation with Ultra-short Laser Pulses

Table top short-pulsed UV lasers are coveted for their usefulness in time-resolved studies of molecular dynamics. However, current wave-mixing and harmonic generation techniques using crystals fail to preserve the short pulse unless the crystal is made thin, thus limiting the conversion efficiency. The alternative is to focus the fundamental beam into a gas, but the phase shift accumulated by a Gaussian beam as it propagates through the focus leads to destructive interference of the generated harmonics. This work presents two methods to alleviate this issue through the use of a semi-infinite geometry for third harmonic generation. The first method involves placing a metal septum at the waist such that the laser drills a small pinhole, which in turn disrupts the beam after the waist. The second method uses a very thin septum as a separator for two gases: one with a large third order susceptibility (before the focus), and the other with a small susceptibility (after the focus). Both methods inhibit harmonic generation immediately after the beam waist. Experiments with third harmonic generation lead to increased conversion efficiency and better mode quality, and had the appropriate perturbative behavior. Simulations supported the experimental results and were used to explore limitations on generation. The techniques were extended to fifth harmonic generation. Although an improved spectrum was observed, increased conversion efficiency was not observed in the experiment. Moreover, simulations indicated that fifth harmonic light production is due to wave-mixing, not generation. Finally, simulations with Bessel-like beams are explored as an alternative method for future experiments