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

Abstract In this study, we employ femtosecond pump–probe transmission microscopy and complex Nomarski interferometry to investigate the ultrafast dynamics of laser-induced plasma and shock waves in bulk fused silica subjected to high laser intensities. It was found that the plasma channel, in its development, takes on a columnar shape with minimal variations during the early expansion stage between 130 and 200 ps. However, after 200 ps, the cylindrical shock wave commenced its departure from the plasma channel and gradually expanded outward along the radial direction. An electron density in the evolving plasma channel approached a level of ∼ 1.9 ∗ 10 20 cm − 3 , at a delay time of 50 ps. The resulting change in refractive index caused by electronic plasma was 0.05, while the reflectivity of the probe beam was ≈0.03 % . The blast wave model described the plasma and cylindrical shock wave expansion. A pressure of 65 GPa and a plasma temperature of a few eV were registered at a delay time of 150 ps. The resulting extreme conditions mapped to the cross-section analysis of the void and shock wave-affected area reveal polycrystalline structures near the periphery of the cavity at the geometrical focus. The areas more distant from the focus tend to show an amorphous character. 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.