Modeling optical breakdown in dielectrics during ultrafast laser processing

Laser ablation is widely used in micromachining, manufacturing, thin-film f ormation, and bioengineering applications. During laser ablation the remova l of material and quality of the features depend strongly on the optical br eakdown region induced by the laser irradiance. The recent advent of amplif ied ultrafast lasers with pulse durations of less than 1 ps has generated c onsiderable interest because of the ability of the lasers to process virtua lly all materials with high precision and minimal thermal damage. With ultr ashort pulse widths, however, traditional breakdown models no longer accura tely capture the laser-material interaction that leads to breakdown. A femt osecond breakdown model for dielectric solids and liquids is presented that characterizes the pulse behavior and predicts the time- and position-depen dent breakdown region. The model includes the pulse propagation and small s patial extent of ultrashort laser pulses. Model results are presented and c ompared with classical breakdown models for I-ns, I-ps, and 150-fs pulses. The results show that the revised model is able to model breakdown accurate ly in the focal region for pulse durations of less than 10 ps. The model ca n also be of use in estimating the time- and position-resolved electron den sity in the interaction volume, the breakdown threshold of the material, sh ielding effects, and temperature distributions during ultrafast processing. (C) 2001 Optical Society of America.