Nonreciprocity through gain saturation in coupled nanocavities

The nonreciprocal properties of coupled nanocavities incorporating a gain medium were investigated. The proposed coupled nanocavities can be considered as a photonic non-Hermitian heterostructure. Our study shows that the nonlinearity arising from gain saturation plays a crucial role in dictating the transmittance characteristics. With increasing optical excitation intensity, the transmittance for the forward and backward incident directions exhibits a distinct evolution, ultimately leading to broken reciprocity. Furthermore, there exist both stable and metastable states of cavity modes, and these two states coalesce under a strong optical nonlinear effect. The coalescence of these two states can be attributed to the intricate interplay between the gain saturation nonlinearity and mode coupling, thus giving rise to different nonlinear optical characteristics. Our findings offer valuable insights into nonreciprocal phenomena such as optical bistability, and have potential applications for innovative devices with tunable optical properties.