Model of ultrashort-pulse laser excitation of bulk and thin film dielectrics: Coupling material excitation and electric field propagation

The interaction of ultrashort laser pulses with dielectrics is associated with strong coupling between the material response and the light. The ultrashort pulses excite the materials, and the corresponding rapid changes in optical properties, induced by the generation of free carriers, determine the propagation of the light. In this paper, we describe a self-consistent model for this interaction. The material excitation is described by a model in which the electron dynamics is captured by a set of multiple rate equations coupling energy levels in a conduction band that has been discretized into energy states separated by the photon energy. The propagation of the light is calculated from Maxwell's equations using the finite-difference time-domain approach. The model is used to calculate key observables from laser ablation experiments, e.g., the threshold fluence versus pulse duration and the depth of laser-ablated structures versus fluence, for bulk and thin film samples. For both freestanding thin samples and dielectric stacks, corresponding to high-reflectivity mirrors, the results of the model are strongly influenced by interference phenomena, which significantly influence the material response and ablation. The model is compared to an existing approach using propagation of the intensity envelope of the pulse. In the absence of interference, the two methods predict almost identical ablation thresholds as well as hole depths.