Three-dimensional structure analysis of opaline photonic crystals by angle-resolved reflection spectroscopy

It is important to check distortion in an opaline photonic crystal, since it may affect the photonic band structure and optical properties of the crystal. In this paper, we demonstrate that the existence of distortion can be checked by measuring both angle-resolved reflection spectra produced by two-dimensional (2D) diffraction and mirror reflection spectra. The lattice constant (a2D) determined by 2D diffraction is derived from the periodicity in the surface plane of the opaline crystal. In contrast, the lattice constant (am) determined from the mirror reflection spectra results from the periodicity in the direction perpendicular to the surface. Therefore, the existence of distortion can be checked by comparing these two lattice constants (a2D and am). The measurements revealed that the structure of an opaline photonic crystal composed of polystyrene (PS) spheres had the structure shrunken approximately 4% in the direction perpendicular to the surface. The origin of this distortion is discussed in terms of shrinkage of the PS spheres.