Nonreciprocal photonic composited Su–Schrieffer–Heeger chain

We explore nonreciprocity based on the topological zero modes of a finite asymmetric composited Su–Schrieffer–Heeger (SSH) chain composed of spinning ring cavities theoretically. Here, the first cavity on the short side is designed as a rotating cavity, whose frequency is related to the transmission direction of the light due to the Fizeau light-dragging effect. There are two topological edge modes (TEMs) and one topological interface mode in the middle of the bandgap for the composited SSH chain. The structural asymmetry reduces the transmittance of the system significantly due to symmetry breaking of the field distribution induced for the three modes. However, a small amount of frequency shift of the rotating cavity by Fizeau drag can greatly increase the transmittance by compensating the asymmetry of the structure for one TEM. Therefore, by introducing the input and output waveguides to couple the two outermost ring cavities, we construct an efficient nonreciprocal chain (the maximum isolation ratio can exceed 50 dB) with significant characteristics of monochromaticity and robustness in a narrow frequency range. Moreover, we prove that choosing appropriate system parameters can reduce the requirements of frequency shift. Our work combines nonreciprocity and one-dimensional topological structure organically and provides a scheme to achieve long-range nonreciprocity with topological protection, which is a crucial element in the classical and quantum technologies of computing and information.