Correlation between plasma expansion and damage threshold by femtosecond laser ablation of fused silica

We experimentally investigated the ultrafast ablation of fused silica under 100 fs laser pulses at 800 nm in vacuum. The ablation plume dynamics was monitored by plasma fast imaging and time- and space-resolved optical emission spectroscopy (OES). We used optical microscopy to determine the ablation threshold and rate in multi-shot operation at 100 Hz. We found that, unlike metal ablation, the plasma generated by femtosecond laser pulses from a fused silica target had only one 'main' component. This evidence was supported by the results of OES. The characteristic expansion velocity of this unique component was about one order of magnitude higher than the velocity of the fast plume component observed during metal ablation. We identified in optical emission spectra Si and O neutral atoms only, characterized by very different expansion velocities. In contrast to metal ablation, the 'slow' plasma component usually assigned in the literature to optical emission from nanoparticles was not detected by either plasma fast imaging or OES even for the highest laser fluence used in our experiments. The influence of laser fluence on both plasma expansion and ablation rate was investigated. We considered that the ablation mechanism in this case was dominated by thermal material removal processes.