Fig. 1. Flow chart of femtosecond laser irradiation and wet etching. (a) Fabrication of high aspect ratio groove by femtosecond laser irradiation and wet etching; (b) fabrication of micro-lens by femtosecond laser irradiation and wet etching

Fig. 2. Schematic of sapphire transition from crystal structure to amorphous structure under femtosecond laser^[36]

Fig. 3. Morphology of structure changes and the chemical selective etching induced grooves^[44]. (a)(b) Optical micro-scope images of structure change regions before and after chemical etching induced by lens focused femtosecond laser; (c) SEM image of Fig. 3(b); (d)(e) optical micro-scope images of structure change regions before and after chemical etching induced by micro-scope objective lens focused femtosecond lase

Fig. 4. SEM images of honeycomb arrays of micro-holes fabricated by femtosecond laser irradiation and selective chemical etching^[21]. (a) Hole entrances for the sample fabricated at laser power of 30 mW and pulse number of 4000; (b) hole entrances for the sample fabricated at laser power of 45 mW and pulse number of 1600

Fig. 5. SEM images of various structures^[45]. (a) Chinese dragon; (b) line array; (c) circular ring; (d) rings array; (e) triangular; (f) square; (g) pentagon

Fig. 6. SEM images of various designed suspended structures^[45]. (a) Suspended line; (b) suspended disk; (c) suspended helix; (d) suspended hexagon

Fig. 7. Convex grid-patterned microstructures induced by wet etching assisted femtosecond laser^[47]. (a) SEM image of Si sample after femtosecond laser irradiation; (b)(c) perpendicular and sloping images of the obtained convex microstructures; (d) morphology of the microstructures achieved by LCM; (e) height of the microstructures in different position

Fig. 8. Process of imaging MLA with a fill factor of 96.6%^[26]. (a) Imaging device; (b) image array

Fig. 9. Experimental results by parallel fabrication of femtosecond laser irradiation assisted wet etching. (a) Groove with high aspect ratio^[49]; (b) micro-lens array^[50]

Fig. 10. Silicon carbide processed by femtosecond laser irradiation and wet etching. (a) Groove with high aspect ratio^[22]; (b) through hole structure^[23]

Fig. 11. SEM images of micro-channel morphology of silicon carbide. (a) SEM image after laser irradiation; (b) magnified view of the laser irradiation modified area marked with rectangle 1 in Fig. 1(a); (c) micro-channel after treatment with HF; (d) magnified view of the micro-channel^[54]

Fig. 12. Grating structure processed by femtosecond laser irradiation and wet etching^[55]. (a) SiC grating structure(100×); (b) diffraction pattern of SiC grating structure captured by camera

Fig. 13. Morphology and chemical composition of silicon carbide grating structure before and after chemical corrosion^[59]. (a) Morphology before corrosion; (b) chemical composition in zone A; (c) morphology after corrosion; (d) chemical composition in zone B

Fig. 14. Influence of polarization on nano-ripples^[27]. (a) E perpendicular to the hole-wall surface; (b) the angle of E and the hole-wall surface is 45°; (c) E is parallel to the hole-wall surface

Fig. 15. Sapphire processed by femtosecond laser. (a) 3D channel structure^[37]; (b) deep hole structure^[40]; (c) nanopore structure^[60]

Fig. 16. Sapphire micro-optical devices prepared by femtosecond laser irradiation and wet etching. (a) AFM characterization exhibiting 3D morphology and the cross section at the central line of the FZP^[62]; (b) SEM images and the diffraction efficiency of the Dammann gratings that generate 5×5 spot sources^[63]; (c) SEM image of SWSs on sapphire after etching and measured transmittance of fabricated SWSs on

Fig. 17. Imaging performance of the concave micro-lens array^[65]. (a) Schematic illustration of the imaging system; (b) optical images of the concave micro-lens array on a paper printed with JLU, the square region is the concave micro-lens array; (c)(d) image array of letter “F” using the concave micro-lens arrays with spacing of 25 μm and 40 μm, and the corresponding scale bars are 50 μm and 80 μm, respectively