Plasma-Enhanced UV Generation

We have demonstrated that a plasma spatially localized along the direction of propagation of a fundamental laser pulse increases the conversion efficiency of ultrafast laser pulses to the UV by several orders of magnitude. A detailed model of this plasma-induced enhancement has been developed and sheds light on the mechanisms responsible for the increased conversion efficiency.

In the initial UV experiments, the plasma results from ionization of a gas by an energetic pump pulse and is confined to its focal region. The propagation of a relatively weak probe pulse is therefore perturbed due to both changes in the phase mismatch between the fundamental and third harmonic probe fields as well as plasma de-focusing. Enhancements have been observed in numerous atomic and molecular gases, over a range of experimental parameters.

Enhancement of THG due to the plasma induced by a ~800 uJ, 35-fs Ti:sapphire pump pulse is illustrated in Fig. 1. The data show third harmonic spectra generated from a 85-uJ probe pulse, recorded through a scan of the pump-probe delay through time zero (pump and probe temporally coincide). Figure 1(a): atomic gas (Ar); Fig. 1(b): Molecular gas (CO₂); at 600 Torr.

Figure 2(a) shows the enhancement observed in — Neon, — Argon, — SF₆, -·- CO₂, and -- N₂O, as a function of pressure for a 300-mm focal length focusing mirror. The enhancement data collected in Ar for a 500-mm focal length focusing mirror is shown in Fig. 2(b) and shows an enhancement ratio of about 300 at a pressure of 150 Torr.

A numerical simulation of the spatially-localized plasma enhancement has been developed. In this model, a split-step propagator is employed using the full spectrum of the fundamental and harmonic pulses as well as the full spatial frequency distribution of the pulses. We have found that the enhancement depends sensitively on the plasma ionization fraction, the plasma length, relative position of the spatially-localized plasma and the focus of the fundamental pulse. Results for a sample calculation are shown in Fig. 3 f or conditions similar to the experimental conditions in Fig. 2(b).