Multi-band unidirectional reflectionlessness of single photons via intermode backscattering

Nonreciprocal light transport is of great importance due to its potential applications in optical communication. We investigate theoretically the unidirectional reflectionlessness behaviors of single photons in a waveguide system coupled with whispering-gallery-mode microresonators. Numerical results show that multi-band unidirectional reflectionlessness can be effectively implemented at the exceptional points by adjusting asymmetric intermode backscatterings of the coupled resonators, and reflectionless frequency-band number increased by multiplication with coupled resonators is also predicted. This suggests a novel multi-band device to effectively perform single-photon unidirectional reflectionlessness, which provides possible applications in quantum communication for designing high-efficiency optical filters, diodes, and so on.