Fig. 1. Picosecond laser processing platform

Fig. 2. Schematic diagram of the hole drilling process

Fig. 3. Hole diameter measurement method

Fig. 4. Optical images of entrance and exit holes at different scanning speeds. (a₁)‒(g₁) Entrance; (a₂)‒(g₂) exit

Fig. 5. Effects of scanning speed on hole quality. (a) Scanning speed versus hole diameter; (b) scanning speed versus hole roundness; (c) scanning speed versus hole taper

Fig. 6. Optical images of entrance and exit holes at different scanning times. (a₁)‒(g₁) Entrance; (a₂)‒(g₂) exit

Fig. 7. Effects of scanning times on hole quality. (a) Scanning times versus hole diameter; (b) scanning times versus hole roundness; (c) scanning times versus hole taper

Fig. 8. Optical images of entrance and exit holes at different repetition frequencies. (a₁)‒(g₁) Entrance; (a₂)‒(g₂) exit

Fig. 9. Effects of repetition frequency on hole quality. (a) Repetition frequency versus hole diameter; (b) repetition frequency versus hole roundness; (c) repetition frequency versus hole taper

Fig. 10. Optical images of entrance and exit holes at different single-pulse energies. (a₁)‒(g₁) Entrance; (a₂)‒(g₂) exit

Fig. 11. Effects of single-pulse energy on hole quality. (a) Single-pulse energy versus hole diameter; (b) single-pulse energy versus hole roundness; (c) single-pulse energy versus hole taper

Fig. 12. SEM backscattering images of longitudinal section of the air-film hole. (a) Longitudinal section profile; (b) partial enlarged image showing no visible recast and oxide layers on the sidewall of the hole

Fig. 13. SEM backscattering images and their partial magnification of the longitudinal section of the air-film holes processed at different repetition frequencies

Fig. 14. EDS line scanning results of the sidewall of the air-film holes. (a) The area without oxide layer; (b) the area with oxide layer