Investigation of defect cavities formed in inverse three-dimensional rod-connected diamond photonic crystals

We report the optical properties of single defects in inverse three-dimensional (3D) rod-connected diamond (RCD) photonic crystal (PhC) cavities by using plane-wave expansion (PWE) and finite-difference time domain (FDTD) methods. By optimizing the dimensions of a 3D inverse RCD PhC, wide photonic band gaps (PBG) are obtained. Optical cavities with resonances in the bandgap arise when point defects are introduced in the crystal. A variety of shapes and locations of single defects are investigated in high-refractive-index contrast (gallium phosphide-air or chalcogenide-air) inverse RCD structures, and Q-factors and mode volumes (V_eff) of the resonant cavity modes are calculated. By choosing a single sphere defect located at the center of a tetrahedral arrangement, small mode volumes can be obtained. Here, high Q-factors up to 724,000 and cavity mode volumes down to V_eff <; 0.3(λ/n)³ have been demonstrated.