1. A method for fabricating solid photonic crystals comprising:preparing a copolymer solution by dissolving a polystyrene-block-polyvinylpyridine (PS-PVP) copolymer in a chloride-containing solvent;
casting the copolymer solution on a substrate to form an initial film, the PS-PVP copolymer self-assembles into 3D periodic network structures having a periodicity corresponding to non-visible wavelengths in the initial film;
swelling the initial film, the initial film is soaked in a polar solvent to swell the PS-PVP copolymer such that the initial film becomes a solvated film, wherein the periodicity of the 3D network structures in the solvated film is higher than that in the initial film; and
drying the solvated film, the solvated film becomes a solid photonic crystal when the polar solvent is evaporated completely, wherein PVP blocks in the PS-PVP copolymer become glassy during evaporation of the polar solvent such that the periodicity of the 3D network structures in the solid photonic crystal is preserved between that in the initial film and the solvated film such that the periodicity of the 3D network structures is between 300 nm and 700 nm in the solid photonic crystal.