“…The transmission phases of the MRRs at resonance change abruptly and the group velocities reduce dramatically, providing an efficient way to slow the light wave. Such slow-light phenomenon has been explored to realize many critical optical components/devices, including the high-performance filter such as add-drop filters [64] and wavelength division multiplexers [65], [66], optical delay lines [67], Parity-Time (PT) symmetric devices [68], nonlinear light-wave and materials interaction such as four wave mixing [69], optical parametric generation [70], [86], single-photon/photon-pair source [71], [81], [88], frequency comb generation [92], optical quantum computing [84], as well as high-quality sensors [90], [85], [98], [96]. Depending on its geometry, two typical MRRs are actively studied, i.e., the circular MRRs and the racetrack MRRs.…”