Microscopic processes in dielectrics under irradiation by subpicosecond laser pulses

Transparent solids may show strong absorption when irradiated by a high-intensity laser pulse. Such laser induced breakdown is due to the formation of a free-electron gas. We investigate theoretically the role of ionization processes in a defect-free crystal, including in our model two competing processes: strong-electric-field ionization and electron impact ionization. Free-electron heating is described in terms of electron-phonon-photon collisions. Relaxation of the free electron gas occurs through electron-electron collisions and electron-phonon collisions. The latter are also responsible for energy transfer from the free-electron gas to the phonon gas. We solve numerically a system of time dependent Boltzmann equations, where each considered process is included by its corresponding collision integral. Our results show formation, excitation, and relaxation of the electron gas in the conduction band. We find that strong-electric-field ionization is mainly responsible for free-electron generation. No avalanche occurs at femtosecond laser irradiation. The electron density and the internal energies of the subsystems are calculated. Critical fluences obtained using various criteria for damage threshold are in good agreement with recent experiments.