When band gap materials are irradiated with an ultrashort laser pulse, the free-carrier density increases tremendously. The resulting abrupt transformation from an insulator to a conducting material, where a large portion of the electrons is excited to the conduction band, induces huge changes in the optical parameters. The transient nature of these parameters is often modeled based on the Drude model, where there is much uncertainty concerning the Drude collision frequency. As this frequency has a great impact on the modeling results, this work discusses several approaches to its treatment. Additionally, we compare simulation results to experimental data discussing shortcomings of the Drude model in combination with a collision frequency based on Debye screening for dilute plasmas.
Ultrashort-pulse laser excitation of dielectrics is an intricate problem due to the strong coupling between the rapidly changing material properties and the light. In the present paper, details of a model based on a multiple-rate-equation description of the conduction band are provided. The model is verified by comparison with recent experimental measurements of the transient optical properties in combination with ablation-depth determinations. The excitation process from the first creation of conduction-band electrons at low intensities to the formation of a highly-excited plasma and associated material fragmentation is explained by the model. For quartz samples, the optical properties are strongly influenced by self-trapped excitons, and the associated additions to the model are described.