Multiband and bidirectional multiplexing asymmetric optical transmission empowered by nanograting-coupled defective multilayer photonic crystal.

Asymmetric optical transmission (AOT) has been an enduring hot topic of interest in various fields, including optical communication, information processing, and so on. Particularly, the development of reciprocal micro-nanostructures achieving AOT further facilitates and accelerates the miniaturization and integration of traditional optical components. However, most of these optical components merely consider a single AOT band and transmission in a specified direction, limiting the development of their versatile functions. In this paper, we theoretically propose an all-dielectric metamaterial consisting of a nanograting and a defective multilayer photonic crystal, exhibiting multi-band and bidirectional multiplexing AOT. More specifically, the proposed metamaterial demonstrates both narrowband and wideband AOT for incidence from the nanograting to the photonic crystal, and a completely different narrowband AOT for the opposite incidence, namely, from the photonic crystal to the nanograting. These distinctive AOT spectral features are achieved by matching the diffraction effect of the nanograting with the special energy band of the defective multilayer photonic crystal. Remarkably, the device exhibits a transmittance difference of up to 0.974 and a contrast ratio of up to 0.997 (transmittance ratio of up to 673), with a transmission bandwidth of 62.7 nm for incident light with a wavelength of 624 nm illuminating from the nanograting to the defective multilayer photonic crystal. Furthermore, the bandwidth and number of transmission bands can be flexibly tuned by changing the polarization angle of the incident light, showcasing its excellent polarization multiplexing characteristics. The designed metamaterial provides an effective strategy for the realization of versatile AOT devices and is conducive to expanding the application scenarios of AOT devices.