Optical breakdown and sub-optical-cycle dynamics of ultrafast laser induced damage in fused silica

We report on the experimental and theoretical studies of ultrafast laser-induced optical breakdown on the surface of fused silica to elucidate the mechanism of damage formation and sub-optical-cycle dynamics in material processing using single and a burst of two femtosecond laser pulses. Ionization pathways, including photo-ionization (PI) and avalanche ionization (AI), are investigated by using single-beam and double-beam laser damage threshold measurements, which are used to analyze electron dynamics and extract the avalanche coefficient. The relationship between damage size and laser fluence is interpreted as a result of a combination of PI and AI. Electrical field rather than laser intensity is the fundamental influential factor in PI, and AI is found to play a significant role in creating the free electron density needed for optical breakdown. These findings are verified by a double-pulse delay-scan experiment where two cross-polarized pulses are used to induce damage with delay within a few optical cycles. Variation of the damage diameter is observed within one optical cycle, which is explained by the periodic change of polarization in the combined electric field. This finding shows the potential of controlling laser induced damage by tuning the temporal overlap of a burst of ultrashort laser pulses.