Microscopic processes in dielectrics under irradiation by subpicosecond laser pulses

Transparent solids may show strong absorption when irradiated by a high-int ensity laser pulse. Such laser induced breakdown is due to the formation of a free-electron gas. We investigate theoretically the role of ionization p rocesses in a defect-free crystal, including in our model two competing pro cesses: strong-electric-field ionization and electron impact ionization. Fr ee-electron heating is described in terms of electron-phonon-photon collisi ons. Relaxation of the free electron gas occurs through electron-electron c ollisions and electron-phonon collisions. The latter are also responsible f or energy transfer from the free-electron gas to the phonon gas. We solve n umerically a system of time dependent Boltzmann equations, where each consi dered process is included by its corresponding collision integral. Our resu lts show formation, excitation, and relaxation of the electron gas in the c onduction band. We find that strong-electric-held ionization is mainly resp onsible for free-electron generation. No avalanche occurs at femtosecond la ser irradiation. The electron density and the internal energies of the subs ystems are calculated. Critical fluences obtained using various criteria fo r damage threshold are in good agreement with recent experiments.