Fig. 1. Schematic diagram of one-photon absorption, two (multi-) photon absorption, multi-photon ionization, and photopolymerization process^[45-47]

Fig. 2. Cross-linking process and degree of conversion. (a) Schematic diagram of the polymerization and cross-linking process of monomer molecules^[64]; (b) Degree of conversion calculated from the Raman spectra for TPP with different scanning velocities in the commercial (acrylate-based) photoresist IP-L 780^[67]

Fig. 3. Distribution of the photopolymerization threshold and the damage threshold. (a) Laser exposure threshold intensity as a function of exposure time^[71]; (b) Laser power threshold at different scan speeds measured by darkfield microscopy^[47]; (c) Threshold laser intensities at single and multiple exposures measured by darkfield and in situ microscopy^[47]

Fig. 4. Sub-diffractive feature-scale structures by TPP and topography models of Voxel. (a) Schematic diagram of the TPP^[15]; (b) The prepared "nano bull"^[19]; (c) The nonlinear relationship between line width and exposure time^[19]; (d) Light intensity distribution of the focus, where the dotted line is the square of light intensity^[47]; (e) The relationship between voxel length and exposure power^[78]; (f) The relationship between voxel linewidth and exposure power^[78]

Fig. 5. Schematic diagram of the TPP system. (a) Optical path diagram of a typical TPP based on the piezoelectric stage scanning^[49]; (b) Schematic diagram of different scanning strategies on different substrates^[17]; (c) Galvanometer scanning, (d) immersion scanning, (e) STED assised and (f) SLM based TPP system^[17]

Fig. 6. The nanodots/wires structures fabricated by TPP.(a) 120 nm^[19], (b) 100 nm^[98], (c) 80 nm^[100], (d) 90 nm^[101], (e) 50 nm^[103], (f) 35 nm^[74], (g) 30 nm^[104], (h) 23 nm^[105], (i) 7 nm, 8 nm and 9 nm^[106] feature size in the nanodots/wires structures

Fig. 7. The nanowires structures fabricated by STED-TPP. (a) Schematic diagram of the STED-TPP and the minimum longitudinal size of 40 nm^[110]; (b) The relationship between the line width and the 532 nm CW laser and the fabricated nanowires with widths of 95 nm, 65 nm, 90 nm, and 145 nm^[111]; (c) Cross-sectional intensity distributions of the initiation and inhibition laser beam, and the fabricated nanowires with width of 54 nm^[112]; (d) Schematic diagram of improving resolution by increasing inhibition intensity and the fabricated nanowires with the minimum size of 9 nm^[113]

Fig. 8. Periodic 3D structures fabricated based on multi-beam parallel fabrication technology.(a,b) Micro-gear and micro-bull combined structures fabricated by TPP with diffractive elements^[120]; (c) 2D array structure fabricated by TPP with DMD projection^[121]; (d) Parabolic mirror structure based on SLM-TPP^[122]; (e-g) 3D mechanical metamaterials fabricated by TPP with diffractive elements^[123]

Fig. 9. High-precision, large-scale structures fabricated by DMD TPP technology.(a) DMD holographic multi-focus 3D TPP method and the fabricated high-resolution "bridge" structure, and woodpile structures^[125]; (b) The prepared Millimeter-scale structure with sub-micrometer features, micro-nano bridge structures by FP-TPL technology^[126]; (c) The prepared micro-nano suspended lines and micro-metamaterial structures by projection TPP with the spatiotemporal focusing technology^[127]; (d) The prepared nanowires and nanodots structures, cross-scale structures by femtosecond projection nanolithography technology^[128]

Fig. 10. Photonic crystals, metamaterials, and devices. (a) Woodpile photonic crystal^[131]; (b) Diamond photonic crystal^[133]; (c) Heat shrinkable woodpile photonic crystal^[134]; (d) Chiral helical metamaterial^[135]; (e) 3D invisibility cloak structure in optical band^[136]; (f) Beam deflector^[137]; (g) Composite chiral photonic crystal material^[138]; (h) Circularly polarized beam splitter^[139]

Fig. 11. Metalens device. (a) Schematic diagram of the broadband focusing of the hybrid achromatic metalens^[141]; (b) The structure of the hybrid achromatic metalens^[141]; (c) The partial enlarged view^[141]; (d) The imaging of the broadband near-infrared light^[141]; (e) The broadband metalens, which combines nanoholes with a phase plate^[142]; (f) Measured broadband focusing spot^[142]; (g) The tunable multifocal 3D metalens^[146]; (h) Measured zoom focusing spot^[146]

Fig. 12. Integrated photonics device. (a) Miniature prism coupler^[148]; (b) Low-loss fiber-on-chip coupler^[149]; (c) Polarization-rotated polymer rectangular waveguide^[150]; (d) Fiber-on-chip connector^[153]; (e) Chip-on-chip optical connector^[153]; (f) On-chip device-device optical connector^[153]; (g) Microdisk cavity structure and optical interconnect waveguide structure^[154]; (h) 3D curved surface photonic microcavity structure^[155]

Fig. 13. Micro-nano optical lens. (a) Aspherical solid immersion microlenses^[158]; (b) Ultracompact multi-lens objectives^[159]; (c) Stacked diffractive microlenses^[160]; (d) Graded index Lumberg lenses^[161]

Fig. 14. Inverse-designed micro-nano optics devices. (a) Free-form NIR polarizing beamsplitter^[163]; (b) Spectral splitting metalens^[164]; (c) 3D circularly symmetric metalens^[165]; (d) Multilayer metalens^[166]

Fig. 15. Mechanical metamaterial structures and devices.(a) Pentamode metamterials and structural unit of two connected truncated cones^[168]; (b) An elasto-mechanical unfeelability cloak made of metamaterials^[170]; (c) A twist mechanical metamaterials^[171]

Fig. 16. Drivable micromechanical devices. (a) Remote magnetically actuated micro-rotor, micro-shield machine and 3D helical thruster^[176]; (b) The pH-responsive spider micro-robot and smart micro-gripper^[179]; (c) The liquid crystal elastomer-based micro-walker^[180]; (d) Optical tweezers-driven micromechanical rotor^[181-182]

Fig. 17. Microfluidic device. (a) Micropump^[185]; (b) Microturbines^[186]; (c) Microsieves^[188]; (d) Microfilters^[189]; (e) Microvalve^[190]; (f) Micromixer^[191]; (g) Micromixer and filters^[191]; (d) Micro-overpass devices^[192]