Bloch-wave engineering for high-Q, small-V microcavities

The overall decay rate of the mode in an optical microcavity formed by a defect surrounded by two Bragg mirrors in a monomode waveguide is driven by two mechanisms, the desired coupling to a guided mode and the detrimental coupling to radiation modes. We propose two approaches fully compatible with planar fabrication, which allow to increase the cavity Q's by several orders of magnitude while keeping constant the mode volume V of the cavity. The first approach consists of engineering the mirror to taper the guided mode into the mirror Bloch mode, thus decreasing losses. The second approach is less intuitive and relies on a recycling mechanism of the radiation losses. The study is supported by computational results obtained for two-dimensional silicon-on-insulator geometries, but the results apply as well to other related geometries like three-dimensional photonic-wire cavities.