Time-dependent ionization models designed for intense and short laser pulse propagation in dielectric materials.

When an intense and short laser pulse propagates in a dielectric material, significant production of conduction electrons through multiphoton absorption (MPA) may occur. In addition to the laser intensity, the MPA process depends mainly on the laser frequency spectrum which may evolve significantly during the course of laser propagation in the material. Simple models for MPA accounting for possible time-dependent evolution of the laser frequency spectrum (as harmonic generation, chirping or broadening) are addressed. The first model is based on Bloch-Volkov states whereas the second approach relies on the density matrix formalism which has been adapted for the present study. Both models are well adapted for their introduction in a propagation code and are shown to correctly account for the MPA process whatever the characteristics of the laser frequency spectrum. The reliability of these approaches has been studied in two cases of practical interest. First, in the case where a second harmonic is present within the fundamental pulse, calculations show that the ionization rate may be significantly enhanced. Second, in the case of a chirped pulse, models are shown to correctly account for possible change in the multiphoton order during the course of interaction.