Calculation and fabrication of two-dimensional complete photonic bandgap structures composed of rutile TiO2 single crystals in air/liquid

Photoelectrochemical applications of photonic crystals are gathering great interests both from physicists and chemists. Here, we theoretically and experimentally present two-dimensional photonic bandgap (2D-PBG) structures based on rutile titanium dioxide (TiO₂) single crystal that is a famous material because of the photoelectrochemical ability. The structures were the arrays of hollow hexagonal rutile TiO₂ pillars in contact with air or a typical nonaqueous electrolyte solution, acetonitrile. Since the TiO₂ refractive indices exhibit a strong dispersive behavior, the bandgap width was discussed from the viewpoint of the refractive index map that would be helpful for the real application of this structure. The 2D-PBG structures for both infrared light and visible light were fabricated by our established lithography technique for rutile TiO₂ with and without Nb doping, i.e., photocatalytic TiO₂ and high electron conductive TiO₂, respectively. These structures show characteristic absorbance peaks or reflectance dips at wavelengths predicted by our theoretical calculations.