Simulation of femtosecond laser ablation sapphire based on free electron density

Femtosecond laser has become an effective method for processing super hard materials, due to the multiphoton ionization and avalanche ionization induced by its high peak power. A FEM model with Fokker-plank equation and Drude equation was established to study the space-time evolution of free electron density caused by the ionization. The study focuses on investigating the temporal and spatial evolution law of free electron density induced by femtosecond laser pulse and its influence on absorption coefficient, reflectivity and ablation depth. It was shown that there is a dramatic increase of sapphire surface absorption coefficient and reflectivity within picosecond, and femtosecond laser pulse energy is rapidly absorbed in a very small depth about 140 nm. When the single pulse energy exceeds a certain threshold (8 J/cm²), the predicated ablation depth is basically not affected by the incident laser pulse energy and remains unchanged, which are in agreement with the experimental data. The FEM model is applicable with advantages of relative simplicity and acceptable precision.