Fig. 1. Absorption of light by matter^[21]

Fig. 2. Schematic diagram of two-photon lithography process^[36]

Fig. 3. Comparison of one-photon absorption resolution and two-photon absorption resolution^[38]

Fig. 4. Comparison of spatial selectivity^[36]. (a) One-photon lithography; (b) two-photon lithography

Fig. 5. None extended-π system^[44], D-π-A type^[45-46], D-π-D type^[47], D-π-A-π-D type^[48-49], and A-π-D-π-A type^[50-51] free radical initiators

Fig. 6. D-π-A type^[54], D-π-D type^[55], A-π-A type^[56], and D-π-A-π-D type^[57] photoacids

Fig. 7. Acrylate/methacrylate based polymerisation monomers^[59-60]

Fig. 8. Thiol polymeric monomers for thiol-alkene/alkyne reaction^[60-61]

Fig. 9. Alkene/alkyne monomers for thiol-alkene/alkyne reaction^[60-62]

Fig. 10. Comparison of PTTA self-polymerisation and polymerisation of PTTA and PETMP via thiol-olefin reaction^[63]

Fig. 11. Photolithography pattern of thiol-olefin photoresist^[63]. (a) Repeatability test of line graphics; (b) square bench support structure

Fig. 12. Schematic diagram of hydrogel cross-linking to form three-dimensional structure^[64]. (a) Cross-linked cured material system of PEGDA hydrogel; (b) scanning electron microscope (SEM) image of hydrogel microstructure

Fig. 13. Schematic of microstructure formation by two-photon cross-linking of P(DMAA-AAHAQ)^[66]

Fig. 14. Schematic diagram of cationic polymerization mechanism of epoxy resin^[69]. (a) Generation of Bronsted acid from two-photon initiator; (b) Bronsted acid initiate cationic polymerization of epoxy monomer

Fig. 15. Epoxy resin polymerisation monomers^[69]

Fig. 16. Three-dimensional frameworks of alumina-coated polymer skeletons with different structures^[72]

Fig. 17. Polymer scaffold assisted fabrication of three-dimensional Al₂O₃ structure^[73]. (a) Two-photon lithography to construct polymer skeleton; (b) atomic layer deposition; (c) skeleton after milling two sides with focused ion beam; (d) oxygen plasma etching to remove polymer skeleton

Fig. 18. 8 trussed three-dimensional stereoscopic nanolattice of Al₂O₃^[73]. (a) Overall structure; (b) enlarged cross-section, inset shows isolated hollow tube

Fig. 19. Schematic diagram of process for fabricating three-dimensional structures from SiO₂ nanocomposites^[75]. (a) Three-dimensional structure obtained by two-photon polymerisation; (b) 600 ℃ heat treatment to remove polymer components; (c) 1300 ℃ sintering to obtain pure quartz 3D structure

Fig. 20. Schematic diagram of process for fabricating three-dimensional structures from half-siloxane composites^[76]

Fig. 21. Silica nanostructures fabricated by POSS resin^[76]. (a) Woodpile photonic crystal; (b) top-view close-up of line pattern with line width of 97 nm wide

Fig. 22. Schematic diagram of process for fabricating three-dimensional structure from ZnO composites^[77]. (a) Formulation of organic photoresist containing zinc ions; (b) three-dimensional structure of zinc-containing polymer obtained by two-photon lithography; (c) 500 ℃ sintering to remove organic components and obtain pure zinc oxide ceramic structure

Fig. 23. Schematic diagram of process for preparation of three-dimensional structure from nickel acrylate composites^[78]. (a) Ligand exchange reaction for preparation of nickel acrylate; (b) preparation of nickel acrylate hybrid photoresist; (c) two-photon lithography; (d) removing organic matter during heat treatment to obtain pure Ni three-dimensional structure

Fig. 24. Copolymerisation mechanism and final hybrid structure of surface functionalised zirconia nanoparticles^[81]

Fig. 25. Three-dimensional microfocusing pattern of zirconia ceramic micromesh with period of 0.8 μm^[81]

Fig. 26. Reaction mechanism of ZrO₂-BTMST photoresist^[31]. (a) Photolysis of BTMST initiator; (b) surface ligands of ZrO₂ nanoclusters under BTMST cation exchange; (c) aggregation of positively charged ZrO₂ nanoclusters with neutral clusters

Fig. 27. Two-photon induced chemical bonding three-dimensional fabrication^[83]. (a) Schematic of 3D nanoprinting of CdSe/ZnS quantum dots capped with 3-mercaptopropionic acid; (b) schematic of photon-induced chemical bonding mechanism

Fig. 28. Three-dimensional structures of quantum dot materials with two-photon lithography^[83]. (a) Linear 3D structure with scale of 5 μm; (b) curved 3D structure with scale of 5 μm; (c) volumetric structure with scale of 10 μm

Fig. 29. Schematic diagram of azide initiator for nanocrystal photoinduced chemical bonding mechanism^[84]

Fig. 30. Schematic of two-photon polymerization within DMOF crystal by femtosecond laser light^[87]. (a) Fill DETC and PEGDA into DMOF; (b) two-photon lithograph; (c) develop to wash away unexposed part

Fig. 31. Three-step fabrication of As₂S₃ three-dimensional photonic crystal^[88]. (a) Thermal evaporation of As₂S₃ to prepare molecular cage of As₄S₆; (b) ring opening of As₄S₆ in exposed region to As₂S₃ under two-photon excitation; (c) designed three-dimensional structure after development

Fig. 32. Three-dimensional structure of As₂S₃ woodpile^[88]. (a) Woodpile with rod spacing of 2 μm, and each rod consists of 8 parallel sub rods with aspect ratio close to 1.0; (b) top view of woodpile with rod spacing of 1 μm; (c) cross-section of focused ion beam of woodpile in Fig. (b); (d) top view of woodpile in Fig. (b)