A Prediction Model for Ablation Fluence Threshold in Femtosecond Laser Processing of Fused Silica

The manufacture of micro–nano structures in transparent dielectrics is becoming increasingly important due to the applications in medical and biological sciences. The femtosecond pulsed laser, with its selectivity, high precision, and three-dimensional direct writing nature, is an ideal tool for this processing technology. In this paper, an improved model for the prediction of ablation crater shape and fluence threshold in femtosecond laser processing of fused silica is presented, in which self-trapping excitons and electrons' relaxation are involved to depict ionization process, Thornber's and Keldysh's models are employed to estimate ionization rate precisely, and a novel ablation criterion is proposed to judge ablation. Moreover, the relationship between the ablation fluence threshold and laser pulse duration is investigated with three different extrapolation methods. The results indicate that no matter which extrapolation method is employed, the ablation fluence thresholds predicted by the presented model agree with the published data.