Reflectionless Wide-Angle Refracting Metasurfaces

We hereby report a solution to the problem of reflections from passive lossless refracting metasurfaces that works even at wide deflection angles. The phenomenon of reflections is due to not accounting for the local wave-impedance mismatch at the source and load faces of the metasurface; this cannot be fully mitigated even with an impedance-equalized Huygens' (IEH) metasurface. The solution is demonstrated to be enabled through the correct application of generalized, instead of canonical, scattering parameters. The canonical and IEH approaches are compared to the generalized scattering parameter approach. It is seen that though the approaches are similar at low-to-moderate levels of refraction, at wide-angle refraction only the generalized S-matrix approach is capable of good matching. This is demonstrated via full-wave simulations, for refraction from normal incidence, using an asymmetric structure comprising three cascaded admittance surfaces.