Few-layer metasurfaces with arbitrary scattering properties

Metasurfaces, which are planar arrays of subwavelength artificial structures, have emerged as excellent platforms for the integration and miniaturization of electromagnetic devices and provided ample possibilities for single-dimensional and multidimensional manipulations of electromagnetic waves. However, owing to the limited interactions between planar thin metallic nanostructures and electromagnetic waves as well as intrinsic losses in metals, metasurfaces exhibit disadvantages in terms of efficiency, controllability, and functionality. Recent advances in this field show that few-layer metasurfaces can overcome these drawbacks. Few-layer metasurfaces composed of more than one functional layer enable more degrees of design freedom. Hence, they possess unprecedented capabilities for electromagnetic wave manipulation, which have considerable impact in the area of nanophotonics. This article reviews recent advances in few-layer metasurfaces from the viewpoint of their scattering properties. The scattering matrix theory is briefly introduced, and the advantages and drawbacks of few-layer metasurfaces for the realization of arbitrary scattering properties are discussed. Then, a detailed overview of typical few-layer metasurfaces with various scattering properties and their design principles is provided. Finally, an outlook on the future directions and challenges in this promising research area is presented.