Proposal and Simulation of Microring Resonators With Partially Distributed Gratings for Realizing Unidirectional Emission

Microring resonators with partially distributed gratings along the inner wall are proposed and numerically investigated based on the finite-element method for realizing the in-plane unidirectional emission with a narrow beam divergence angle. The partially distributed gratings break the rotation symmetry of the resonators, and thus the double-degenerate whispering gallery modes (WGMs) split into two nondegenerate modes with different Q factors. Through proper arrangements of the period and number of the inner-wall gratings, the highQ mode light can be scattered to lower angular momentum components and result in highly unidirectional emission. The farfield divergence angle is mainly determined by the number of the gratings and the azimuthal mode number of the WGM. Furthermore, the far-field patterns and the mode Q factors are optimized by regulating the grating depth and the microring width. With fixed grating numbers, the in-plane beam divergence substantially is not sensitive to the variation of grating depth and microring width, which illustrates the robustness of the unidirectional emission.