Comparison Between Different Approaches for the Design of Anomalous Refractors

Metasurfaces (MTSs) are efficient tools for controlling electromagnetic radiation and manipulating local aperture fields. Anomalous refractors, electromagnetic devices capable of altering the direction of incident plane-waves, have gained significant attention in recent research. Various design approaches have been explored, necessitating a comprehensive comparison to identify the optimal approach. This study evaluates the effectiveness of three different approaches for the design of three-layer anomalous refractors and provides criteria for the selection of the optimal one. The first two approaches employ local approximations, deriving sheet admittance profiles analytically based on the uniform problem matching the local structure. These approaches are referred to as impedance-equalized Huygens' (IEH) MTS and omega-type bianisotropic (O-BMS) MTS. The third approach, denoted as Floquet-modes optimization (FM-OPT), introduces a numerical technique based on surface field harmonics optimization. In this case, a user-specified number of evanescent Floquet modes (FMs) is employed in the optimization procedure for minimizing the real part of the admittance sheets. The aforementioned approaches are compared for the design of three-layer anomalous refractors performing three different anomalous refractions 0(degrees)-to-20(degrees)/50(degrees)/80(degrees) for two different interlayer distances. An ad hoc multilayer analysis tool based on FM expansion is developed to accurately analyze and compare the designed anomalous refractors. The numerical results indicate that, for practical applications, the FM-OPT approach outperforms both the IEH and O-BMS approaches in terms of performance and accuracy.