Spatiotemporal probe into the femtosecond laser processing of fused silica

Femtosecond laser processing of fused silica has been widely investigated. Nevertheless, a theoretical model to provide effective and direct guidance to the actual processing is still essential. By investigating the ablation threshold, depth, and crater shape of fused silica for femtosecond laser processing experimentally and numerically, a theoretical model is proposed and validated at the wavelength of 800 nm. It is carried out based on tracing the spatiotemporal distribution of the free electron density, electron temperature, and laser intensity. The electron temperature and electron density, as well as the transient optical and thermo-physical properties of femtosecond laser-irradiated fused silica are quantitatively determined. The experimentally measured saturation of the ablation depth at high laser fluence is predicted by the proposed model. The electron relaxation time at different laser fluences and pulse durations throughout the photoionization and impact ionization processes are probed. The results show that electron relaxation time plays a crucially important role in determining the evolutions of material optical properties and femtosecond laser energy absorption. The laser fluence is proved to affect the shape of the ablation crater strongly. With the increment of laser fluence for a given laser pulse duration, the ablation volume is more sensitive for the case with shorter pulse durations. Suitable laser parameters should be taken according to actual processing needs.