The smallest possible whispering-gallery-like mode in the square lattice photonic-crystal slab single-defect cavity

The smallest possible whispering-gallery-like mode (WGM) formed in a free-standing square-lattice photonic-crystal slab single-defect cavity is studied by using the three-dimensional finite-difference time-domain method. The WGM is nondegenerate and the only existing resonant mode in the transverse electric mode bandgap of the square lattice. High-quality factors are achieved from this mode by suppressing both the in-plane and the out-of-plane optical losses. By optimizing the structural parameters, a total quality factor of 18 000 and a vertical quality factor of 40 000 are obtained from the WGM. The quality factor does not decrease significantly in the presence of a current flow path at the center of the cavity due to the null of electromagnetic energy at the cavity center, which means this mode is advantageous to achieve electrically pumped photonic-crystal lasers. In addition, by combining the high-quality factor with wavelength-size small-mode volume, very high Purcell factors >8000 are achieved, the largest value ever reported. The WGM of the square-lattice single-defect cavity is promising for both cavity quantum electrodynamics research and the low-threshold microcavity laser.