Damage mechanism in plasma evolution of nanosecond laser-induced damage of germanium sheets in air and vacuum

Laser-induced damage of germanium (Ge) sheets plays a key role to explore the laser irradiation effect with the development of high-power laser and precision machining technology. However, the impact of plasma evolution on the damage mechanism is still a lack of detailed analysis for Ge sheet. Here, we investigate the change of the morphology and size of the plasma plume and compare it with the damage morphology of Ge sheets versus the laser pulse number in air and vacuum conditions. The plasma plume area reveals a gradual increase and then a slow decrease trend in the air and the final damage morphology is a shallow concavity. Instead, it reveals a fast rise and fall trend in the vacuum and the final morphology is a complete breakdown pit. The damage process of Ge sheet is divided into the initial damage, developmental damage, breakdown damage and stable damage according to the morphological changes of scattering light. The plasma shielding effect and thermal ablation play a more important role in formation of the damage morphology in the air, and the plasma shock wave and thermo-mechanical stress dominate the damage morphology in the vacuum. The significant contrasts and the combined experimental and theoretical works provide an insight into the laser irradiation effect and material degradation behavior, hence broaden the potential application of Ge sheets in laser micro/nano-fabrication.