Observation of Femtosecond Laser-Induced Ablation in Crystalline Silicon

The ablation of crystalline silicon by ultrashort laser pulses is studied experimentally. A pump-and-probe experiment is implemented in a collinear arrangement, utilizing a time-delayed frequency-doubled probe beam for in situ reflectance measurement and ultrafast microscopy observation. Enhanced surface reflectivity in sub-picosecond time scale at the center of the irradiated spot indicates nonthermal liquid layer formation. A short-lived nonthermal liquid phase was detected at fluence of 1.5J/cm2. In addition to this observation, the reflected images for pump beam fluences ranging from 1.5 to 4.6J/cm2 provide evidence of plasma expansion above the irradiated target.