Photonic band structures for three-dimensionally periodic arrays of coated spheres

Photonic band structures of three-dimensional periodic structures of opals composed of nanoscale spheres coated with various materials have been studied theoretically. Opals composed of silica spheres coated with different materials can have smaller and larger band gaps at the L and X points, respectively, in the Brillouin zone of the face-centered cubic structure, although the magnitude of the gap at the X point is generally much smaller than that at the L point in conventional opals. Moreover, such a behavior can be explained by applying the same method as the free electron approximation in solid-state physics to photonic crystals. These phenomena may be suitable for practical application of photonic crystals even when the refractive indices of the materials are low. (C) 2003 American Institute of Physics.