Passive non-reciprocal metasurfaces based on independently tunable nonlinear dual bound states in the continuum

Non-reciprocal devices are pivotal technologies in modern photonics, and their significance is increasingly recognized. However, achieving compact and efficient non-reciprocal devices remains a significant challenge. By leveraging optical nonlinear effects and introducing high-Q optical resonances, changes in the optical properties of the medium can be achieved, enabling non-reciprocity and reducing the required optical intensity. However, current research is limited to narrow single-wavelength ranges, hindering multi-wavelength communication applications. In this work, we propose a metasurface with the double asymmetric periodic gratings structure. This design, based on its symmetric-protected dual bound states in the continuum (BIC), enables non-reciprocal transmission and optical bistability near two wavelengths. We introduce both in-plane and out-of-plane asymmetries: in-plane asymmetry adjusts the radiation linewidth of quasi-BICs, while out-of-plane asymmetry creates asymmetric coupling of incident plane waves with the metasurface in both forward and backward directions. Furthermore, we discuss the limitations and permissible boundaries between the maximum transmission of the metasurface and the non-reciprocal intensity range (NRIR) and describe how to approach and achieve these limits. Our research broadens the application scope of non-reciprocal devices and presents a new, to the best of our knowledge, design methodology. This approach shows exciting prospects for applications such as protection in high-power lasers and non-reciprocal signal routing in simulation computations.