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15 October 2015 Control of surface ablation on fused silica with ultrafast laser double-pulse based on seed electrons dynamics control
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The influence of pulse-separation (τs) between a pair of temporally separated femtosecond laser pulses (with near ablation-threshold energy) on surface ablation of SiO2 were experimentally studied. A τs range of τs≤20 ps was considered. It was shown that a τs-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-τs-independent at τs> ~1 ps. This τs-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 τs-independent phenomena. For physical interest, it is inferred that destruction of STEs will tend to break the τs-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.
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K. H. Zhang, X. Li, W. L. Rong, P. Ran, B. Li, P. Feng, and Q. Q. Yang "Control of surface ablation on fused silica with ultrafast laser double-pulse based on seed electrons dynamics control", Proc. SPIE 9673, AOPC 2015: Micro/Nano Optical Manufacturing Technologies; and Laser Processing and Rapid Prototyping Techniques, 96730Z (15 October 2015);

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