Femtosecond laser drilling controlled with laser-generated ultrasound pressure

Laser drilling of glass using tightly focused femtosecond laser pulses while monitoring laser-generated sound is demonstrated, aiming laser drilling controlled by laser-generated sound. The amount of laser ablation was found to have a monotonical relation to the intensity of the sound pressure. It was also found that when the laser pulses were focused on the glass surface, the sound pressure increased in the initial stage of the laser drilling and then declined as the hole became deeper. These behaviors were the result of increasing ablation caused by surface roughening and loss of sound propagation through the hole, respectively. It was further found that the movement of the objective lens (OL) toward the target material at an appropriate constant speed created a hole with a large depth and narrow entrance (a high aspect ratio); that is, the lens movement changed the performance of the laser drilling. A simple method for moving the lens using laser-generated sound was adopted in this study. The axial position of the OL was controlled by maximizing the sound pressure at each pulse irradiation to obtain a hole with a high aspect ratio, which was the same as the maximum hole depth obtained by the iterative experiments in the constant-speed control of the OL. More sophisticated control methods should be developed according to the given applications.