Fokker-Planck Approach to Photon-Electron Interactions in Dielectrics During Ultrafast-Pulse Laser Ablation

This article investigates numerically the electronic transport characteristics in fused silica (SiO₂) irradiated by ultra-short pulsed lasers. The non-local type of Fokker-Planck equation which accounts for avalanche ionization, three-body recombination, and multiphoton ionization is used to describe the ultra-short pulsed laser-induced damage phenomena in the energy-position space. It is observed that the recombination plays an important role in determining the ablation depth, and also 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. Full ionization is achieved at intensities above threshold and all further laser energy is deposited within a thin skin depth. The absorption length is on the order of a wavelength at very high laser fluence, and it becomes thinner as laser fluence is larger. It is because the absorbed energy is no longer consumed for multiphoton ionization, but rather leads to the drastic increase in the absorption coefficient because of the Joule heating.