Chirality Induced Nonreciprocity in a Nonlinear Optical Microresonator

On-chip optical nonreciprocity is one of the essential functions to fully advance the development of integrated optical systems, which remains technically challenging in many aspects. There is a great need for mechanisms and approaches to facilitate the large-scale implementation of nonreciprocal light propagation. Recently, unconventional phenomena, such as chiral optical modes and directional light propagation, have been unraveled at exceptional points (EPs), which are unique degeneracies in the energy spectrum and eigenspace of non-Hermitian systems. Here, this work theoretically and experimentally demonstrates that by steering a single microresonator with thermo-optic nonlinearity to chiral EPs, nonreciprocal light propagation is achieved with an isolation ratio up to 24 dB and insertion loss less than 0.5 dB. The nonreciprocity is dependent on the chirality and could be optimized near the EPs. Their results pave new avenues for the nonreciprocal control of light propagation enabled by non-Hermitian degeneracies and hold great potential for microscale and nanoscale on-chip nonreciprocal devices.