Dynamics of pressure waves during femtosecond laser processing of glass.

Although femtosecond lasers enable microfabrication of transparent materials, precise processing is difficult owing to the inevitable damage caused to the surroundings of the processed region. In the present work, we combine pump-probe imaging with a high-speed camera to capture the dynamics of pressure waves varying from pulse to pulse, before a desired shape is created by hundreds of pulses. The results demonstrate that the pressure waves change their forms and locations as the number of pulses increases. The numerical analyses explain that a tensile stress, much greater than the tensile strength, is distributed around the processed region during the travel of the pressure wave, and that repetitive pressure waves result in cracks. The identified mechanism of the damage generation will contribute to developing strategies to prevent damage and expand the range of applications of femtosecond laser processing.