Huygens' metasurfaces via the equivalence principle: Design and applications

We review the current trends in the design of Huygens' metasurfaces (HMSs), which are planar arrays of balanced electric and magnetic polarizable particles (meta-Atoms) of subwavelength size.We focus on schemes that follow the equivalence principle, as these can be rigorously incorporated into Maxwell's equations, leading to design specifications in the form of (electric and magnetic) surface-impedance distributions. The advantages of this approach with respect to the more common phase-shift stipulation approach are highlighted and discussed. We present a (microscopic) methodology to associate a general meta-Atom configuration with an equivalent surface impedance, and derive metasurface (macroscopic) design procedures for various beam forming applications. The methods and concepts developed in the paper provide the basic tools for understanding and designing scalar, passive, and lossless HMSs, and we indicate possible extensions applicable to more complex structures.