Control of surface ablation on fused silica with ultrafast laser double-pulse based on seed electrons dynamics control

The influence of pulse-separation (τ_s) between a pair of temporally separated femtosecond laser pulses (with near ablation-threshold energy) on surface ablation of SiO₂ were experimentally studied. A τ_s range of τ_s≤20 ps was considered. It was shown that a τ_s-independent/-dependent crater ablation area can be flexibly controlled. Once the pulse energy of the pulse pair exceeds a threshold value, crater ablation area become quasi-τ,_s-independent at τ_s> ∼1 ps. This τ_s-independent phenomenon can even be observed when each pulse within the double-pulse pair has a sub-threshold energy, which leads to a further reduction in ablation size. The experimental findings have not only confirmed our previous calculation based on a modified model, but also greatly extended the results both quantitatively and qualitatively. A dominant amount of seed electron from photoionization of self-trapped excitons (STEs) is responsible for the appearance of τ_s-independent phenomena. For physical interest, it is inferred that destruction of STEs will tend to break the τ_s-independent ablation phenomena. Experiments performed on CdWO₄, a material exhibiting similar electron dynamics to that in SiO₂ but a faster decay in STE population, support this conjecture. A possible improvement for the relevant theoretical modeling is also suggested based on the experimental findings.