Shock induced phenomena in high fluence femtosecond laser ablation of silica glass

Abstract

Shadowgraphs of dynamic processes outside and inside the target during the intense femtosecond laser ablation of silica glass at different energy fluences are recorded. Two material ejections outside the target and two corresponding stress waves inside the target are observed. In particular, a third stress wave can be observed at energy fluence as high as 40 J/cm2. The pressure, the temperature, the free electron density, and the ionic components at the laser pulse end are estimated, based on which the mechanical reaction of the laser heated material is investigated. According to our analysis, the first wave is a thermoelastic wave, while the second and the third may be generated subsequently by the mechanical expansions. Besides, the velocities of the stress waves are deduced from the time-resolved shadowgraphs, and it is found that the first stress wave propagates with a velocity greater than the sound velocity, while the second stress wave propagates with a velocity less than the sound velocity. Therefore, the first wave is a supersonic shockwave with a high stress magnitude, while the second may be the plastic stress wave or subsonic shockwave with a lower stress magnitude. Further more, the temporal evolution the second stress wave is investigated, and its velocity is found to increases gradually at large delay times. According to the extrapolation curve, however, it is speculated that the velocity decreases from a high value initially, which could be due to the interaction between the first and second stress waves at small delay times. These results can provide a further support to the theory of highpressure shock phenomena in femtosecond laser ablations.