Theoretical and experimental investigation of femtosecond laser processing fused silica

By tracking the spatiotemporal distribution of the free electron density/temperature and laser intensity, the ablation threshold, depth and crater shape of fused silica for femtosecond laser processing are investigated theoretically and experimentally. The electron dynamics as well as the transient optical and thermophysical properties of laser irradiated fused silica is quantitatively determined. The numerical model is validated by comparing the calculated threshold fluence, depth and crater shape of ablation with the experimental data at the wavelength of 800 nm. The electron relaxation time at different laser fluences and pulse durations throughout the photoionization and impact ionization processes are probed. In the present paper, it is found that (1) the electron relaxation time significantly affects the material optical properties and femtosecond laser energy absorption. The optical properties change dramatically, and the fused silica becomes opaque during laser irradiation. Moreover, the transition from electron-phonon collision to electron-ion collision accompanies by the laser ablation of fused silica in the femtosecond laser irradiation process. (2) The experimentally observed saturation of the ablation depth at high laser fluence is elucidated by the proposed model, which is due to the significantly changed optical reflectivity and absorption coefficient. Both theoretical simulations and experimental observations found that laser fluence has a strong influence on the shape of the ablation crater. The ablation volume increases sharply with increasing laser fluence for femtosecond laser irradiation compared to that for picosecond laser irradiation. (3) With the increment of laser fluence, a saturation of the ablation depth removal efficiency and ablation efficiency occurs, followed by slight decrements. The ablation depth removal efficiency peaks at laser fluence close to 1.4 times of the ablation threshold. Whereas, the accuracy is slightly low due to the higher sensitivity of the ablation characteristics (ablation crater depth and ablation volume) to the shorter pulse laser. For the laser fluence higher than 3.5 times of the ablation threshold, good repeatability over a very wide fluence range enables accurate processing results, because a more consistent flat-bottom ablation profile tends to appear. However, the heat-affected zone leads to a decrement of the processing quality compared to that of laser close to the ablation threshold.