Free-electron generation in laser-irradiated dielectrics

We study the mechanisms of ultrafast free-electron generation in laser-irradiated dielectrics. The evolution of the free-electron density in the conduction band of laser-irradiated dielectrics is calculated with the recently introduced multiple rate equation. This system of rate equations unifies key points of detailed kinetic approaches and simple rate equations to a widely applicable description, valid on a broad range of time scales. It keeps track of the nonstationary electron energy distribution at the initial stage of ionization and provides the transition to the asymptotic avalanche regime at longer time scales. The analytic solution for the asymptotic regime yields the avalanche parameter entering the standard rate equation and the condition of its applicability. We present results on the establishment of an ionization avalanche, comparing our model with other theoretical approaches. The role of impact ionization as compared to multiphoton ionization is analyzed. A self-similarity of the fraction of impact-ionized electrons, depending only on the product of intensity and pulse duration, is revealed.