HARMONIC-GENERATION AND PHASE-MATCHING IN THE TUNNELING LIMIT

Numerical calculations are Presented of harmonic generation and phase matching in a rare-gas-like plasma. A simple model for the atomic dyna mics is used, in which tunnelling ionization is assumed to be the domi nant process. The non-linear tunnelling current generates a series of odd harmonics of the fundamental radiation, which then propagate throu gh the ionizing plasma. The use of a simple model for the atoms makes it possible to solve the full time-dependent wave equation, and this a llows direct investigation of phase matching effects due to free elect rons. Such a treatment is necessary in the high-intensity, short-pulse limit. where conventional slowly-varying-envelope calculations are no t valid. We investigate the pressure dependence of the generation effi ciencies, and also observe blueshifting of the higher-order harmonics.