Fig. 1. Effects of pulse energy on breakthrough time and maximum aspect ratio^[13-14]. (a) Effect on breakthrough time^[13]; (b) effect on maximum-aspect ratio^[14]

Fig. 2. Shadowgraphs taken at 6 ns after 1st, 250th, 500th, 1000th, and 2000th pulses with different pulse widths^[16]

Fig. 3. Variation of rim height, size of oxidized area and average drilling rate in dependence on pulse repetition rate for fixed pulse energy (E = 0.23 mJ)^[18]

Fig. 4. Effect of polarization on laser beam traveling in the hole^[22]. (a) Reflections inside the deep hole with an angle of incidence of 85°; (b) azimuth angle varies with the change of positions; (c) entrance and exit of drilled holes using respective a linearly and circularly polarized beam; (d) cross-section of drilled hole under linear polarization with pulse energy of 0.5 mJ; (e) cross-section of drilled hole under linear polarization with pulse energy of 2 mJ (the rectangular zone enclosed by dotted line represents the polarization removal)

Fig. 5. Two typical helical drilling systems^[37]. (a) Helical drilling base on three wedge plates; (b) helical drilling base on Dove prism

Fig. 6. Surface morphology of the entrance and exit ends^[38]

Fig. 7. Cross-section of micro-holes machined with different scanning paths^[39]. (a) Circular scanning; (b) linear scanning; (c) linear+circular scanning

Fig. 8. Schematic of different auxiliary processing methods. (a) Auxiliary processing with vacuum environment^[14]; (b) auxiliary processing with heat^[15]; (c) auxiliary processing with water ^[42]; (d) auxiliary processing with electric^[43]

Table 1. Main materials and typical applications of ultrafast laser micro-hole drilling

Table 2. Influence of main process parameters on ultrafast laser micro-hole drilling