Damages in Transparent Materials by Ultrashort Laser Pulse

Abstract

Ultrashort lasers that generate laser pulses down to picosecond and femtosecond range have advanced over the past decade from delicate lab systems to robust industry machines. The availability of stable ultrashort laser systems opens up a wide range of exciting applications from transparent material processing such as waveguide fabrication to bio applications such as cell ablation. The fundamental aspects of ultrashort pulse material interaction are still remaining an active research topic. Here, we focus on the investigation of how ultrashort pulses, particularly picosecond laser pulses, interact with the bulk of transparent materials such as borosilicate glass, fused silica and sapphire. We investigate damage inside the bulk of borosilicate glass, fused silica, and sapphire by a single shot of IR picosecond laser pulse experimentally. In our experiments, extended bulk damage tracks with an aspect ratio of about 1:10 are generated. The damage morphology in each of the material is found to be different. We also numerically model pulse propagation and electron dynamics in borosilicate glass and fused silica in both picosecond and femtosecond regimes. The shape and size of the damage sites are shown to correspond to an electron cloud with density of about 10^20 cm^-3. The underlying mechanism of electron generation by multiphoton ionization and avalanche ionization is numerically investigated. The multiphoton ionization rate and avalanche ionization rate are determined by fitting experimental results. The relative role of multiphoton ionization and avalanche ionization are numerically studied and the percentage of electron contribution from each ionization channels are investigated.