Insight into the femtosecond laser irradiation of ZnTe crystal: Photoionization and surface eruption

The strong-field terahertz (THz) driven by ultrafast high-intensity laser has been attracting increasingly attention in THz probe and imaging. As the leading potential nonlinear optical material for strong-field terahertz (THz) generation and detection, the ZnTe tends to be damaged by focused laser irradiation. In this work, the laser ablation and morphology modification of ZnTe single crystal under a 800 nm, 100 fs Ti:Sapphire laser are investigated. The inconstant laser induced damage threshold (LIDT) of the ZnTe is precisely measured, in which a reference safety energy 38 mJ/cm² is obtained. According to the Keldysh theory, the photoionization rate in irradiated area is calculated as high as 10³⁴ cm⁻³s⁻¹, which introduces an intense plasma and in turn dominates the weak LIDT of ZnTe. In addition, the fs-laser shots induce the oxidation on the damaged surface preventing the further ablation of the crystal. A surface eruption model is proposed to elaborate the fs-laser induced damage evolution and the formation of cracks and eruption holes on ZnTe. Finally, the XPS deep etching demonstrates relative shallower ablation affected zone occurred on the surface since the interaction process happens in an ultrashort timescale. Thus, the THz response behaviors of ZnTe aged by fs-laser is able to be recovered by removing the damage layer simply. Our work provides insight into the laser damage behavior of the ZnTe single crystal and lays a foundation for further fs-laser micro-processing and laser-damage resistance improvement.