Chip‐Based Optical Isolator and Nonreciprocal Parity‐Time Symmetry Induced by Stimulated Brillouin Scattering

Realization of chip‐scale nonreciprocal optics such as isolators and circulators is highly demanding for all‐optical signal routing and protection with standard photonics foundry process. Owing to the significant challenge for incorporating magneto‐optical materials on chip, the exploration of magnetic‐free alternatives has become exceedingly imperative in integrated photonics. Here, a chip‐based, tunable all‐optical isolator at the telecommunication band is demonstrated, which is based upon bulk stimulated Brillouin scattering (SBS) in a high‐Q silica microtoroid resonator. This device exhibits remarkable characteristics over most state‐of‐the‐art implements, including high isolation ratio, no insertion loss, and large working power range. Thanks to the guided acoustic wave and accompanying momentum‐conservation condition, bulk SBS also assist in realizing the nonreciprocal parity‐time symmetry in two directly coupled microresonators. The breach of time‐reversal symmetry further makes the design a versatile arena for developing many formidable ultra‐compact devices such as unidirectional single‐mode Brillouin lasers and supersensitive photonic sensors. By employing stimulated Brillouin scattering, a nonmagnetic isolator in an on‐chip microtoroid resonator is demonstrated. This device bears many compelling features including high isolation ratio, no insertion loss, and isolation tunability. Furthermore, by coupling the active Brillouin microtoroid with a passive one, nonreciprocal parity‐time symmetry is experimentally realized, which may find applications in ultra‐sensitive bi‐scale nanoparticle sensors.