Fig. 1. Four methods of femtosecond laser preparation SERS substrate ^{[7, 30-32]}. Figure reproduced with permission from: ref. [7] © Wiley; ref. [30-31] © Elsevier; ref. [32] © The Royal Society of Chemistry

Fig. 2. SERS principle. (a) Inelastic light scattering of molecules on corrugated metal surfaces^[33]; (b) localized surface plasmon resonances (LSPRs) on the surface of precious metals^[36]. Figure reproduced with permission from: (a) ref. [33] © American Chemical Society; (b) ref. [36] © The Royal Society of Chemistry

Fig. 3. Preparation of SERS microstructures by top-down micromachining and particle self-assembly. (a) RIE^[45]; (b, c) EBL^[46-47]; (d-f) Nanoparticle self-assembly^[48-50]; Scale bar: (e) 20 nm; (f) 200 nm. Figure reproduced with permission from: (a) ref. [45] © American Chemical Society; (b) ref. [46], (e) ref. [49] and (f) ref. [50] © under a Creative Commons Attribution-NonCommercial-No- Derivatives 4.0 International License; (c) ref. [47] © American Chemical Society; (d) ref. [48] © The American Association for the Advancement of Science

Fig. 4. Preparation of SERS microstructure by microcolumn self-assembly methods. (a) Self-assembly of gold nanopillars^[55]; (b) Self-assembly of polymer-silver micropillars^[56]; (c) Self-assembly of polymer-silver micropillars^[57]; (d) Self-assembly of silver micropillars^[58]; (e) Self-assembly of polymer-gold micropillars^[59]. Figure reproduced with permission from: (a) ref. [55] and (e) ref. [59] © American Chemical Society; (b) ref. [56], (c) ref. [57] and (d) ref. [58] © Wiley

Fig. 5. Femtosecond two-photon reduction to prepare SERS substrates. (a) Two-photon reduction principle^[70]; (b)Two-photon reduced silver microwire^[71]; Scale bar: (b) 10 μm; (e) 1 μm. Figure reproduced with permission from: (a) ref. [70], (b) ref. [71], (c) ref. [74] and (e) ref. [71] © Wiley; (d) ref. [72] © The Royal Society of Chemistry

Fig. 6. Femtosecond laser cutting metal to prepare SERS substrate. (a) Femtosecond laser directly ablated metal surface forming nanostructure principle ^[80]; (b) Ag periodic surface^[91]; (c) Superhydrophilic - superhydrophobic patterned substrate structures were prepared directly on copper surface ^[30]; (d) S-Ag-Ar substrate^[92]; (e) Titanium alloy SERS substrate^[93]. Figure reproduced with permission from: (a) ref. [80] © Elsevier; (b) ref. [91], (c) ref. [30] and (d) ref. [92] © Elsevier; (e) ref. [93] © under a Creative Commons Attribution-NonCommercial-No- Derivatives 4.0 International License

Fig. 7. Femtosecond laser cutting-sputtering to prepare a SERS substrate. (a) Large area SERS substrate^[105]; (b) Flexible transparent SERS substrate^[31]; (c) Glass SERS substrate^[106]; (d) Hydrophobic-superhydrophobic SERS substrate^[107]; (e) Superhydrophobic-hydrophilic SERS substrate^[108] . Figure reproduced with permission from: (a) ref. [108], (b) ref. [31] and (c) ref. [106] © Elsevier; (d) ref. [107] © BioMed Central Ltd unless otherwise stated; (e) ref. [108] © American Chemical Society

Fig. 8. Two-photon direct writing combined metal evaporation. (a, b) 3D SERS structure of fiber surface ^[121-122]. Figure reproduced with permission from: (a) ref. [121] © under a Creative Commons Attribution-NonCommercial-No-Derivatives 4.0 International License; (b) ref. [122] © Wiley

Fig. 9. Femtosecond laser processing capillary self-assembly to prepare SERS substrate. (a) Capillary force self-assembly^[126]; (b) Three-dimensional SERS structure based on capillary force self-assembly microchannels^[7]. Figure reproduced with permission from: (a) ref. [126] © American Chemical Society; (b) ref. [7] © Wiley

Table 1. Comparison of four methods for processing SERS substrates by femtosecond laser