A plasma model with quantum treatments for femtosecond laser ablation of glasses

In this article, a plasma model with quantum treatments is proposed to predict ablation threshold, depth, and crater shape in femtosecond laser ablation of glasses at peak intensities on the order of 10¹³ - 10¹⁴ W/cm². Impact ionization and photoionization are the two major competing mechanisms considered for plasma generation using the flux-doubling model. Using a modified free electron plasma model, the proposed model considers the time and space dependent optical properties of ionized glass. The quantum treatment based on the Fermi-Dirac distribution is employed to investigate the free electron heating. The free electron relaxation time is calculated by using a quantum model derived from the Boltzmann transport equation. The predicted threshold fluence and ablation depth for barium aluminum borosilicate and fused silica are in excellent agreement with published experimental data. The model greatly improves the prediction precision of ablation depth and can predict the crater shape in femtosecond ablation of glasses. Some interesting phenomena observed experimentally, such as the bottom of the ablation crater by a femtosecond Gaussian beam could be rather flat under special ablation conditions, are well explained by the proposed model.