Significant Non‐Reciprocal Transmission Achieved by Combining Nonlinear Near‐Zero Index Materials with Bound States in the Continuum

Abstract For many modern photonics applications, significant nonlinear optical interactions are crucial. However, achieving this typically demands powerful laser sources and extended interaction area because most natural materials exhibit extremely weak optical nonlinearities. Hence, integrating nonlinear optics into novel nanophotonics devices poses a challenge. In this work, an asymmetrical optical nonlinear metasurface is proposed that combines effective zero‐index media with bound states in the continuum (BIC). This scheme enables us to achieve a huge non‐reciprocal intensity range of ≈7.1, leveraging significantly enhanced optical nonlinear effects. The high‐ Q characteristics of quasi‐BIC within the zero‐index background greatly enhance light‐matter interactions and reduce operational power requirements. The non‐reciprocal metasurface relies on a straightforward set of material requirements and fabrication processes, making it a highly versatile option for various applications in optical communication, light detection, signal processing.