Modeling of laser-induced breakdown in dielectrics with subpicosecond pulses

Theoretical study of ultrafast laser induced damage by short pulses (tau < 1 ps) is carried out on large-band-gap dielectric in an effort to understand the complex physical processes involved. The numerical method of solving a general time-dependent Fokker-Planck type equation for free electron production is discussed in detail. The calculation shows that the collisional avalanche ionization competes with the multiphoton ionization even for pulse length shorter than 25 fs. Sensitivity tests of all the rates in the equation are performed and the most critical ones are identified. From these tests we obtain valuable information in developing new materials that have the desired damage fluence for specific applications. To describe the relaxation of electron plasma, a three body recombination rate is included. Thus, the temporal behavior of the electron density due to a single pulse is treated, as well as the case of exposure to two laser pulses with a time delay between them. The model is only partially successful in reproducing the recent experimental data. Effect of the presence of a linear decay term and optical defects on the damage threshold is considered in the context of the rate equation input. (C) 2000 American Institute of Physics. [S0021-8979(00)09813-3].