Characteristics of plume plasma and its effects on ablation depth during ultrashort laser ablation of copper in air

The characteristics of plume plasma induced by ultrashort laser ablation of copper in air are studied experimentally and theoretically in this paper. The plasma front evolutions are captured using a high-speed intensified charge-coupled device camera, while the plasma temperature and electron number density evolutions are obtained from the plasma emission spectra measured by a high-precision spectrometer. For the theoretical study, a hybrid simulation model is proposed to combine an atomistic model for the early stage and a hydrodynamic model for the later stage. The simulation model is validated against the measurement results and shown to be capable of predicting plasma properties for a broader range of fluence, without being constrained by the signal strength as in the measurements. Both experimental and theoretical results show that the nonlinear relationship between plasma properties and laser fluence directly relates to the relationship between ablation depth and laser fluence. Therefore, the measurable and predictable plasma properties reveal the underlying ablation mechanism that has been studied primarily by theory.