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 understan d the complex physical processes involved. The numerical method of solving a general time-dependent Fokker-Planck type equation for free electron prod uction is discussed in detail. The calculation shows that the collisional a valanche ionization competes with the multiphoton ionization even for pulse length shorter than 25 fs. Sensitivity tests of all the rates in the equat ion are performed and the most critical ones are identified. From these tes ts we obtain valuable information in developing new materials that have the desired damage fluence for specific applications. To describe the relaxati on of electron plasma, a three body recombination rate is included. Thus, t he temporal behavior of the electron density due to a single pulse is treat ed, as well as the case of exposure to two laser pulses with a time delay b etween them. The model is only partially successful in reproducing the rece nt experimental data. Effect of the presence of a linear decay term and opt ical defects on the damage threshold is considered in the context of the ra te equation input. (C) 2000 American Institute of Physics. [S0021-8979(00)0 9813-3].