Strong coupling enhanced optical nonreciprocal transmission

Optical nonreciprocal transmission based on nonlinear platforms holds promise for miniaturized and highly integrated photonic devices. However, limited on key metrics like nonreciprocal contrast and isolation due to weak nonlinear coefficients of most materials, hindering practical applications. Here, we theoretically present a feasible strategy for enhancing nonreciprocal transmission in an epsilon-near-zero (ENZ) material (ITO)-based nonlinear grating structure via strong coupling. We validate the strong coupling behavior occurring between the guided mode resonance (GMR) and the ENZ mode with a large Rabi splitting of 124 meV. Strong coupling enables extremely strong electric field energy localized in the ITO thin film which contributes to the nonlinear enlargement. Leveraging the strong coupling effect, we achieve significant nonreciprocal transmission in the near-infrared region, with well-balanced key performance metrics at the maximum coupling point, such as a broad nonreciprocal intensity range of 2.7, a high isolation of 12.9 dB, and a low insertion of 2 dB. Our work offers an advisable approach for enhancing optical nonreciprocity and developing on-chip integrated nonreciprocal optical devices.