Fig. 1. Schematic diagram of double-pulse laser irradiated temperature field distributions at the FP laser fluence of (a) higher than ablation threshold, (b) lower than ablation threshold, and (c) lower than melting threshold.

Fig. 2. Schematic diagram of the experimental setup of temporal overlapping double-pulse laser ablation.

Fig. 3. Surface ablation on PCL film generated by single-pulse femtosecond laser irradiation at different laser fluences. (a) 80, (b) 96, (c) 112, (d) 128, (e) 144, and (f) 160  μJ/cm2; surface ablation on PCL film generated by double-pulse femtosecond laser irradiation at 264-fs time delay and different total laser fluences: (g) 80, (h) 96, (i) 112, (j) 128, (k) 144, and (l) 160  μJ/cm2.

Fig. 4. (a) Horizontal and (c) vertical cross section of surface ablation on PCL film generated by single-pulse femtosecond laser irradiation at different laser fluences; (b) horizontal and (d) vertical cross section of surface ablation on PCL film generated by double-pulse femtosecond laser irradiation at 264-fs time delay and different laser fluences. (e) Ablation volume as a function of laser fluence for single and double pulses.

Fig. 5. Ablation enhancement factor by double-pulse laser ablation on PCL film at different laser fluences. (a) FP, 40  μJ/cm2 and SP, 40  μJ/cm2; (b) FP, 48  μJ/cm2 and SP, 48  μJ/cm2; (c) FP, 56  μJ/cm2 and SP, 56  μJ/cm2; (d) FP, 64  μJ/cm2 and SP, 64  μJ/cm2; (e) FP, 72  μJ/cm2 and SP, 72  μJ/cm2; (f) FP, 80  μJ/cm2 and SP, 80  μJ/cm2.

Fig. 6. Surface ablation on PCL film generated by single-pulse femtosecond laser irradiation at different laser fluences. (a) 48, (b) 40, and (c) 32  μJ/cm2; (d), (e) ablation volumes at different laser fluence ratios for double-pulse irradiation on PCL film. The total laser fluence is kept at 112  μJ/cm2.

Fig. 7. Calculated evolution of free-electron density for the single pulse and double pulses (75 fs time delay) at a 150 fs, 800 nm pulse of different total peak intensities. (a) 2.5, (b) 5, (c) 7.5, and (d) 10  TW/cm2.

Fig. 8. Surface ablation generated by double-pulse femtosecond laser irradiation on PCL film at a total laser fluence of 160  μJ/cm2 (FP, 80  μJ/cm2 and SP, 80  μJ/cm2) and different time delays of (a) 0, (b) 66, (c) 132, (d) 198, (e) 330, (f) 396, (g) 462, (h) 528, and (i) 594 fs; (j) corresponding ablation volume as a function of time delay.

Fig. 9. Surface ablation generated by double-pulse femtosecond laser irradiation on PCL film at a total laser fluence of 112  μJ/cm2 (FP, 56  μJ/cm2 and SP, 56  μJ/cm2) and different time delays of (a) 0, (b) 66, (c) 132, (d) 198, (e) 330, (f) 396, (g) 462, (h) 528, and (i) 594 fs; (j) corresponding ablation volume as a function of time delay.