Fokker-Planck approach to laser-induced damage in dielectrics with subpicosecond pulses

Abstract

Extensive numerical simulations are conducted to investigate the non-local characteristics of ultra-short pulse-laser-induced breakdowns of fused silica. The Fokker-Planck (F-P) equation is expanded to space and applied to describe the transient behaviors of electron densities considering the effects of electron avalanche, three-body recombination, and multiphoton ionization (MPI) on generation and recombination of electrons. The present study compares the predicted damage threshold fluences with experimental data for validation. When the electron density exceeds a certain threshold, recombination becomes prominent and contributes to reduce substantially the rate of increase in electron density. It is also found that once high electron density region is formed at intensities above threshold, all further laser energy is deposited within a thin skin depth because the absorbed energy is no longer used for collisional ionization and rather used to increase the average energy per electron, resulting in drastic increase in the absorption coefficient due to the Joule heating process.