Transient plasma dynamics and structural changes below and above the ablation threshold in glasses upon femtosecond laser irradiation

We have used femtosecond-resolved microscopy in order to investigate the temporal and spatial evolution of the ablation process in fused silica upon irradiation with single femtosecond laser pulses at 800 nm. Images of the surface reflectivity at 400 nm have been recorded at different delays after the pump pulse, covering a time span from ~100 fs up to 20 ns. The quantitative analysis of the image sequence allows to determine the ultrafast evolution of the surface reflectivity. Making use of the gaussian beam profile of the pump pulse, the reflectivity evolution can be extracted at different spatial positions (and thus different local fluences) from a single image series at constant peak laser fluence. The results obtained in fused silica are compared to those obtained in other glasses with higher linear and non-linear refractive indices and lower bandgaps.