Nanosecond-to-femtosecond laser-induced breakdown in dielectrics

We report extensive laser-induced damage threshold measurements on dielectric materials at wavelengths of 1053 and 526 nm for pulse durations tau ranging from 140 fs to 1 ns. Qualitative differences in the morphology of damage and a departure from the diffusion-dominated tau(1/2) scaling of the-damage fluence indicate that damage occurs from ablation for tau less than or equal to 10 ps and from conventional melting, boiling, and fracture for tau>50 ps. We find a decreasing threshold fluence associated with a gradual transition from the long-pulse, thermally dominated regime to an ablative regime dominated by collisional and multiphoton ionization, and plasma formation. A theoretical model based on electron production via multiphoton ionization, Joule heating, and collisional (avalanche) ionization is in quantitative agreement with the experimental results.