Evaluation of 3D photonic crystal cavities on a volumetric basis

Studies on three-dimensional (3D) photonic crystals were intensively carried out for a while after the theoretical predictions that 3D photonic crystals can fully control behaviors of light regardless of its propagation direction, polarization direction, and wavelength. Though, immature fine processing technologies of the time could not meet requirements for producing the crystals operating in optical wavelengths. Thus, interests of majorities of researchers moved away from 3D crystals toward two-dimensional photonic crystals which are far easier to be made compared to 3D structures, and provide us quasi all directional light controllability with the aid of total internal reflection. Interestingly, 3D photonic crystals are receiving renewed interests in recent years thanks to current well-matured fine structure processing technologies and emerging new 3D fine structure fabrication techniques. Coupling of photons and electrons in zero-dimensional systems, high-Q resonant cavities, and so on have been realized within the last couple of years. Light confining efficiencies of finite-sized 3D photonic crystals exponentially increase according to expansion of their crystal dimensions. Thus, crystal dimensions tend to be expanded to pull up a Q-factor of a resonator. This tendency, enlarging device sizes, is totally running counter to the recent trend toward downsizing components in any devices. In this study, enhancement of light confining efficiency of a 3D photonic crystal as well as shrinkage of crystal dimensions was attempted. A point-defect cavity embedded in an air-rod connected diamond structure was numerically revealed to exhibit three-times larger Q-factor than the one in a woodpile structure under the condition where dimensions of both crystals are the same on a basis normalized by a lattice constant. Furthermore, the point-defect cavity resonator in a rod-connected diamond structure was estimated to operate on a single-mode in an entire operating range. Despite its largest full photonic bandgap among ever-proposed 3D photonic crystals, rod-connected diamond structures had never been realized in wavelength regions shorter than microwave because of its complexity. Feasible fabrication approach to realize a rod-connected diamond structure in optical wavelengths will be proposed in this talk. Light-confining efficiency index based on crystal volume will be also proposed as a guide to fairly evaluate widely varying 3D photonic crystal resonators.