Ultrahigh-Q silicon racetrack resonators

An ultrahigh- Q silicon racetrack resonator is proposed and demonstrated with uniform multimode silicon photonic waveguides. It consists of two multimode straight waveguides connected by two multimode waveguide bends (MWBs). In particular, the MWBs are based on modified Euler curves, and a bent directional coupler is used to achieve the selective mode coupling for the fundamental mode and not exciting the higher-order mode in the racetrack. In this way, the fundamental mode is excited and propagates in the multimode racetrack resonator with ultralow loss and low intermode coupling. Meanwhile, it helps achieve a compact 180° bend to make a compact resonator with a maximized free spectral range (FSR). In this paper, for the chosen 1.6 μm wide silicon photonic waveguide, the effective radius R eff of the designed 180° bend is as small as 29 μm. The corresponding FSR is about 0.9 nm when choosing 260 μm long straight waveguides in the racetrack. The present high- Q resonator is realized with a simple standard single-etching process provided by a multiproject wafer foundry. The fabricated device, which has a measured intrinsic Q -factor as high as 2.3 × 10 6 , is the smallest silicon resonator with a > 10 6 Q -factor.