Different tendencies of breakdown threshold on pulse duration in the subpicosecond regime in fused silica

Measurements of the single-shot laser-induced breakdown threshold in bulk fused silica were performed at a wavelength of 800 nm for pulse durations ranging from 240 fs to 2.5 ps. Objectives with different numerical apertures (NAs) were used to focus the laser pulse into the sample. It was found that the threshold started to increase with the decrease of the pulse duration when it was less than 700 fs for effective NA = 0.126, while for effective NA = 0.255 it kept decreasing. Numerical simulations based on the nonlinear propagation model revealed that the generated plasma played an important role in the breakdown process and was responsible for the different tendencies observed in this study. Moreover, it was noticeable that the trend of the threshold on pulse duration was sensitive to the assumed threshold electron density, which implied that various definitions of breakdown might lead to different tendencies of the threshold.