Refractive index dependence of the transmission properties for a photonic crystal array of dielectric spheres

The refractive index dependence of the transmission properties for a two-dimensional photonic crystal array of dielectric spheres has been investigated. The transmission properties of a ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ $(n\ensuremath{\sim}2.99)$ sphere-array photonic crystal and a polytetrafluoroethylene (PTFE, $n\ensuremath{\sim}1.44$) sphere-array photonic crystal are measured in the sub-THz frequency region. The experimental photonic band structures show good agreement with those calculated by the vector Korringa-Kohn-Rostoker method. They are also compared with the optical density of states of the gallery mode localized in an isolated sphere and the photonic band structure based on the guided wave mode in a dielectric slab. The results indicate that the characteristics of the gallery mode appear in the photonic band structure of the ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ sphere-array photonic crystal. On the other hand, the characteristics of the guided wave mode appear dominantly in the photonic band structure of the PTFE sphere-array photonic crystal. The crossover from the guided-wave-like to the localized-gallery-mode-like characteristics occurs with increasing refractive index of the material.