Fig. 1. Experimental schematic diagram. (a) Schematic diagram of the experimental device; (b) Timing diagram of femtosecond laser output pulse number control

Fig. 2. (a) Optical micrograph of YAG crystal ablation craters under the shot of single pulse femtosecond laser (3000×); (b) Semi-log plot of D² vs E_P; (c) SEM image under single pulse (E_P=3.00 μJ); (d) Cross section of ablation craters under single pulse shot

Fig. 3. (a) Optical micrographs (1000×) of YAG crystals after ablation with different pulse energies, with pulse number N=3 000; (b) Optical micrographs (1000×) of YAG crystals ablated with different pulse numbers, with pulse energy E_P 1.67=μJ; (c) AFM ablation morphology of YAG crystals under multi-pulse action; (d) Cross section diagram of ablation craters under multi-pulses

Fig. 4. The semi-log plot of D² vs E_P when the number of pulses N is 3, 30, 600 and 50 000 respectively under the shot of femtosecond laser

Fig. 5. Experimental and fitting results of ablation threshold energy flux density (black sphere) \begin{document}$ {{F}}_{\rm{t}\rm{h}} $\end{document}(N) as a semi logarithmic function of laser pulse number N under the shot of multi pulse femtosecond laser (The red curve (Model I), blue curve (Model II), and green curve (Model III) correspond to the linear fitting of the three models, respectively)