Photomechanical effect on femtosecond laser ablation of transparent dielectrics extracted from oscillation of time-resolved reflectivity

Kumada, Takayuki   ; Akagi, Hiroshi; Itakura, Ryuji; Otobe, Tomohito; Nishikino, Masaharu; Yokoyama, Atsushi

Dynamics of femtosecond laser ablation of transparent polymers was examined using time-resolved reflectivity. When a pump pulse with fluence above ablation thresholds of 0.8-2.0 J/cm was irradiated to these polymers, we observed oscillation of the reflectivity, which was caused by the interference between the reflected probe pulses from the sample surface and thin layer due to non-thermal photomechanical effect of spallation. As the fluence of the pump pulse increased, the separation velocity of the thin layer increased from 6 km/s to the asymptotic value of 11 km/s. We consider that the velocities are determined by shock-wave velocities of the photo-excited layer.