Ultrafast expansion kinetics of femtosecond laser-induced optical breakdown in bulk fused silica

Abstract In this study, we employ femtosecond pump-probe shadowgraphy and complex Nomarski interferometry to investigate the ultrafast dynamics of laser induced plasma and shock waves (SWs) in bulk fused silica subjected to high laser intensities. It was found that the development of the plasma channel takes on a columnar shape with minimal variations during the early expansion stage between 130-200 ps. However, after 200 ps, the cylindrical SW started disintegrating from the plasma channel and gradually expanded outward
along the radial direction. The spatial distribution of transient electron densities along the plasma axis reached approx. 1.9 ∗ 10 20cm−3 , at a delay time of 50 ps. The resulting change in refractive index caused by electronic plasma is 0.05, while the reflectivity of the probe beam is ≈ 0.03 ‰. The blast wave model described the plasma and cylindrical SW expansion. A peak pressure and temperature of 65 GPa and 15 eV were registered at a delay time of 150 ps, respectively. The resulting extreme conditions mapped to the cross-section analysis of the void and SW-affected area inside the fused silica reveal polycrystalline structures near the periphery of the cavity at geometrical focus. However, all other points distinctively show the presence of an amorphous phase. These findings enrich the interpretation of femtosecond laser-dielectric interactions, with significant implications for micromachining, optical data storage, and the advancement of photonic device technologies.