Bilayer interaction metasurfaces: Configuration, mechanism, fabrication, and function

As an integrated assembly comprising two metasurfaces, bilayer metasurfaces exhibit more intricate configurations, which substantially expand the design degree of freedom and enable functions beyond the reach of monolayer metasurfaces in light field manipulation. In recent years, significant progress has demonstrated their considerable potential in micro/nanophotonics, with research primarily concentrating on unit cell design, modulation mechanisms, and advanced functions. Given this rapidly evolving landscape, a systematic examination of how distinct configurations affect the overall performance of bilayer metasurface systems is urgently needed. In this review, we first introduce the general architectures of bilayer metasurfaces and categorize them into three primary types, which are further divided into six representative sub-configurations. For each sub-configuration, we discuss and evaluate the characteristic structural features, underlying coupling mechanisms, common fabrication strategies, and corresponding advanced optical functions. Finally, we summarize the key advantages and inherent limitations of bilayer metasurfaces, and offer perspectives on their future development.