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

The influence of pulse-separation (tau(s)) between a pair of temporally separated femtosecond laser pulses (with near ablation-threshold energy) on surface ablation of SiO2 were experimentally studied. A tau(s) range of tau(s) <= 20 ps was considered. It was shown that a tau(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-tau(s) -independent at tau(s) > similar to 1 ps. This tau(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 tau(s)-independent phenomena. For physical interest, it is inferred that destruction of STEs will tend to break the tau(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.