Plasma formation and structural modification below the visible ablation threshold in fused silica upon femtosecond laser irradiation

We have investigated the temporal and spatial evolution of the ablation process induced in fused silica upon irradiation with single 120 fs laser pulses at 800 nm. Time-resolved microscopy images of the surface reflectivity at 400 nm reveal the existence of a transient plasma distribution with annular shape surrounding the visible ablation crater. The material in this annular zone shows an increased reflectivity after irradiation, consistent with a local refractive index increase of approximately 0.01. White light interferometry measurements indicate a shallow surface depression in this outer region, most likely due to material densification.