We report on the experimental and theoretical study of direct laser writing on dielectrics with reduced feature size by using temporally and spatially separated pulses. Features on the front surface of fused silica within the overlapping area of two laser pulses are obtained by tuning the delay time between the two pulses. The observed dependence of feature position on delays longer than the free-carrier lifetime indicates an ionization pathway initiated by self-trapped excitons. The sensitivity of feature size with the increase of laser intensity is studied by simulating the free-carrier density distribution for different temporal and spatial separation of two laser pulses. It is found that the sensitivity is influenced by the spatial and temporal separation of the two pulses.
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.
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