Numerical analysis of electronic transport characteristics in dielectrics irradiated by ultrashort pulsed laser using the nonlocal Fokker-Planck equation

摘要

The ultimate goal of this article lies in investigating theoretically the electronic transport characteristics in fused silica (SiO2) irradiated by ultrashort pulsed lasers. The nonlocal type of multivariate Fokker-Planck equation is modeled on the basis of the Boltzmann transport formalism to describe the ultrashort pulsed laser-induced damage phenomena in the energy-position space, together with avalanche ionization, three-body recombination, and multiphoton ionization. From the results, it is observed that the recombination becomes prominent and contributes to reduce substantially the rate of increase in electron number density when the electron density exceeds a certain threshold. With very intense laser irradiation, a strong absorption of laser energy takes place and an initially transparent solid is converted to a metallic state, a phenomenon well known as laser-induced breakdown. It is also found that full ionization is provided at intensities above threshold, and all further laser energy is deposited within a thin skin depth. The absorption length is of the order of a wavelength at very high laser fluence, and it becomes thinner as laser fluence is larger. This is because the absorbed energy is no longer consumed for multiphoton ionization, but rather leads to a drastic increase in the absorption coefficient because of Joule heating.