Fig. 1. Microholes drilled by femtosecond laser temporally shaped pulse. (a) Microholes drilled by unshaped pulse (up) and double-pulse (down) with an interval of 500 fs assisted by water^[54]; (b) microholes drilled in α-SiO₂ by single unshaped pulse and three-pulses with different interval^[59]; (c) microholes drilled in PMMA by 1000 unshaped pulses (up) and double-pulses (down) with an interva

Fig. 2. Bessel beam^[63]. (a) Schematic of focal spot of Bessel beam; (b) schematic of wave vector relationship of Bessel beam; (c) focal spot intensity distribution of Bessel beam and Gaussian beam along propagation direction

Fig. 3. Microholes drilled by femtosecond laser Bessel beam. (a) Microholes drilled in PMMA by single Bessel pulse (left) and Gaussian pulse (right) when the energy is 20 μJ^[64]; (b) microholes drilled in PMMA by three types of Bessel-like beam with adjustable focusing length^[65]; (c) microholes array processed by Bessel pulse when the energy is 0.70 μJ^[66]

Fig. 4. Microholes drilled by FLICE using unshaped femtosecond laser. (a) Schematic of glass modified by femtosecond laser scanning^[83]; (b) etching microholes in glass using transverse scanning^[83]; (c) optimizing the cross section shape of microhole by beam shaping^[81]; (d) etching microholes with uniform cross section obtained by scanning path compensat

Fig. 5. Microholes drilled by FLICE using shaped and unshaped femtosecond laser. (a) Microholes formed by FLICE using unshaped pulse (up) and double-pulse with an interval of 500 fs (down)^[55]; (b) microhole morphologies vary with pulse number which formed by FLICE using Bessel pulse (up) and Bessel-double pulse (down)^[56]; (c) crater cross section and morphologies formed by FLICE using unshaped pulse and

Fig. 6. Microholes drilled by femtosecond laser in different atmospheric pressure environments^[44,86]. (a) Variatin of microholes depth drilled in air and vacuum with pulse number; (b) and (c) are the microholes processed in air and vacuum, corresponding to the data in (a); (d) variation of microholes morphology drilled by 5000 pulses with energy of 50 μJ with ambient pressure; (e) and (f) are the outlet of arrayed

Fig. 7. Microholes drilled by femtosecond laser assisted by liquid. (a) Schematic of microhole processing assisted by water on the rear surface^[87]; (b) helical microholes array processed in silica glass by femtosecond laser pulse assisted by distilled water^[89]; (c) high-aspect-ratio microholes processed using double-pulse assisted by water on the rear surface^[54

Fig. 8. Microholes drilled in variable temperature environment^[97]. (a) Schematic of changing the ambient temperature of microhole processing; (b) variation of microhole morphology with temperature