Theoretical and numerical investigations of fused silica modification using ultrafast double-pulses

Abstract

We study the mechanisms of ultrafast free-electron generation in laser-irradiated dielectrics (fused silica). The evolution of the free-electron density in the conduction band of dielectrics irradiated by ultrafast double-pulses laser is calculated. The effects of the avalanche ionization is calculated with the recently introduced multiple rate equation, which keeps track of the energy distribution of the free electrons, while maintaining the conceptual and analytic simplicity of the standard rate equation. Using temporally shaped pulse trains with picosecond separation leads to a significant improvement in the quality of ultrafast laser micro-structuring of dielectrics. The evolution of the free-electron density in fused silica irradiated by tightly focused 100 fs laser double-pulses at a center wavelength of 800 nm are numerically investigated to study the role of nonlinear photo-ionization and avalanche ionization processes in free electron generation. The role of impact ionization as compared to photoionization is analyzed.