High Q photonic crystal cavities with tapered air holes

The quality factor (Q) of resonators defined in photonic crystal (PhC) membranes is known to be sensitive to the tapering of the air-holes. A small deviation from an ideal structure with perfectly vertical holes can result in orders of magnitude decrease in the Q, hence greatly jeopardizing applications of such devices in the field of cavity quantum electrodynamics (cQED). Tapering breaks the vertical reflection symmetry of the structure, allowing confined TE-like cavity modes to couple to the resonant continuum of lossy TM-like modes. We show, using a combination of finite-difference time-domain (FDTD) modeling and plane wave expansion method (PWE), that the membrane thickness plays a critical role in the mode coupling. By choosing the thickness about a third of the wavelength of the mode, the high Q of a PhC cavity can be restored despite 10 degrees taper. These findings show that high Q resonators relatively insensitive to the tapering are achievable.