Research Of High-aspect Ratio Holes Drilled By Femtosecond Laser

In the last two decades, ultrashort-pulsed laser interactions with materials have attracted great interests, because laser ablation processing by femtosecond lasers has the advantages of rapid energy deposition and reducing the size of the undesirable heataffected zone. It's difficult to drill high-aspect ratio holes due to the limited Rayleigh range of Gaussian beam. In our previous model, the single femtosecond laser pulse ablation process has been studied by two-temperature model(TTM) in thin aluminum films. However, for high-aspect-ratio holes drilled in thick metal films, the mechanism of single-pulse laser ablation does not work and cannot provide useful data for us, then an improved 3D TTM should be employed to simulate the multi-pulse femtosecond laser ablation. We calculate the multi-pulse ablation threshold and investigate the effect of other laser parameters, such as the laser fluence, the number of pulses and the beam waist, which will be of great instructive significance to femtosecond hole drilling technology.In our simulations and experiments, we found that laser fluence and the number of pulses mainly affect the crater depth, while the beam waist dominantly decides the crater diameter.In our experiment, we drilled micron holes by femtosecond laser in the aluminum and polystyrene(PS) films. We discussed the effect of vacuum degree, and found that the cracks and recasting melt decrease in vacuum chamber. The multi-pulse laser ablation threshold was measured and in good agreement with our simulation. We also investigated the effect of laser fluence and number of pulses on aspect ratio. In femtosecond laser ablation of polystyrene film experiment, we investigated the effects of the laser repetition frequency, water on the material surface and the spot focus position on the shape of a hole. At last, we analysed the properties of Bessel beams generated by axicon and their improvement on femtosecond laser hole drilling process in theoretical. In our simulation, an axicon with base angle of 1° was used to generate Bessel beam in the femtosecond laser micro processing platform, which will benefit to the laser drilling of high aspect ratio holes.