High-fluence femtosecond laser ablation of silica glass: effects of laser-induced pressure

A pump-probe technique is employed to investigate the dynamic process of femtosecond laser ablation of silica glass, and three stress waves are observed inside the silica glass in the time-resolved shadowgraphs at a fluence of 40 J cm(-2). It is believed that the first stress wave is a thermoelastic wave generated by thermal expansion, while the second and the third are generated subsequently by the mechanical expansions as a result of the extremely high pressure induced by the laser. By employing digital holography, the ablation crater is investigated to identify the ablation results, and high-density silica glass is found beneath the ablation crater, which is also the result of the laser-induced pressure. Furthermore, it is found that more stress waves will be generated at a higher fluence, and the thresholds for the generation of the first, second and third stress wave are measured to be about 2 J cm(-2), 6 J cm(-2) and 31 J cm(-2), respectively. The theoretical calculation result shows that the laser-induced pressure increases with the increase in fluence and can be on the scale of TPa, and we believe a higher pressure will induce more mechanical expansions and therefore more stress waves.