Fig. 1. Structures and optical properties of common three-dimensional photonic crystals. (a)‒(b) Schematic diagrams of opal and inverse opal structures^[28] (The brown pellets are closely packed to form opal structure. After removing the brown pellets, the blue part left forms the structure of inverse opal); (c) diamond structure with blue and dark red balls forming a face-centered cubic lattice, as well as pink balls inside the lattice^[1]; (d) the model of woodpile structure^[31], the lower right corner shows the diamond lattice model diagram connected by rods; (e) gyroid structure^[36] (on the left is an optical photo of a Callophrys rubi butterfly and scanning electron microscopy (SEM) image of the gyroid structure found inside its wings, with a period of about 350 nm; on the upper right corner is an SEM and optical image of artificially gyroid nanostructure with a unit cell size of 360 nm; and on the lower right corner is a zoom-in SEM image of the artificially helical structure); (f) photonic band structure of opal photonic crystal formed by close-packed of SiO₂ dielectric spheres^[1]; (g) photonic band structure of inverse opal photonic crystal with dielectric constant ε=13^[1]; (h) photonic band structure of a diamond lattice with air spheres in a high dielectric (ε=13) material^[1]; (i) photonic band structure of woodpile structure ( wood rods ε≈11)^[35]

Fig. 2. TPL principle and device. (a) Energy level transitions for one-photon and two-photon absorption, where S₀, S₁, T₁, ISC, R·represent ground state, excited singlet state, triplet state, intersystem crossing and free radical, respectively^[38]; (b) schematic of typical TPL device^[42-43]

Fig. 3. Research progress on improving printing resolution. (a) Schematic diagram of dual-beam super-resolution direct laser writing nanofabrication technology^[58]; (b) SEM image (left) of the woodpile structure with a characteristic size of 58 nm and a lattice constant of 250 nm and its reflective optical micrograph (right)^[59]; (c) Eiffel Tower micro-models printed by woodpile photonic crystals based on heat treatment method, images from left to right showing the design drawing and the optical micrographs of the blue tower, red tower, gradient-color tower and further down-scaled tower prepared by 3D printing^[60]; (d) SEM images of woodpile structure before and after heating^[60]; (e) SEM images of woodpile structure after heat-shrinking on the receiving substrate with an anti-sticking layer^[61]; (f) optical images of woodpile structure obtained by writing copper complex-based photoinitiator at different laser powers^[63]

Fig. 4. Research progress on extending the TPL material library. (a) SEM images of TiO₂ woodpile structure before (left side) and after calcination (middle and right side)^[70]; (b) SEM and microscopic images of SiO₂ woodpile structures showing structural colors^[65]; (c) schematic demonstration of the sacrificial-scaffold-mediated TPL process^[88]; (d) gyroid sample containing opal microstructures prepared using the sacrificial scaffold method^[88], where image (i) is model diagram, image (ii) is SEM image, image (iii) is top-view optical photograph, and image (iv) is side-view optical photograph

Fig. 5. Three-dimensional photonic crystals for structural color display. (a) SEM and optical images of 3D photonic Merlions consisting of woodpile structures^[93]; (b) 3D photonic crystal showing different color information in ethanol and air^[95]; (c) temperature response of a woodpile structure, from left to right, the computer-aided design picture and cross-polarized optical micrographs recorded at different temperatures are shown^[97]; (d) optical micrographs of the butterfly painting which varies with the pH of the solution^[98]

Fig. 6. Other applications of three-dimensional photonic crystals. (a) Schematic of the birefringent woodpile structure (left) and the measured and simulated retardance caused by woodpile structure with different layers (right)^[102]; (b) the left image is a model diagram of the chiral woodpile structure, the middle image is a pseudo-color SEM image containing the chiral structure (blue) surrounded by the solid ring (green), and the right image is the measured transmittance profile for right- and left-handed circularly polarized light passing the photonic crystal^[32]; (c) gyroid photonic crystal with type I Weyl point, the left image is its bulk Brillouin zone diagram, the middle image is its geometric diagram in real space, the right images with red box are SEM image of TPL printed polymer photonic crystal (top) and Sb₂Te₃ gyroid photonic crystal (bottom) with observation direction of [010], and the right images with blue box are SEM image with observation direction of [1¯01]^[108]