Ultrathin planar metasurface for controlling electromagnetic wave with broad bandwidth

For conventional lens to control the propagation of electromagnetic (EM) wave, the designed surface geometries or spatial variation of the refractive index distribution is a must to provide the gradual phase changes accumulated along the transmission path. The required thickness and the complication in terms of fabrication limit its applicability in some cases. The metasurface with phase discontinuity[^1] at the interface is proposed as a solution. The gradual phase changes are taken placed by the abrupt phase changes introduced at different place of the surface. If the metasurface has a constant phase gradient of dΦ/dx along X axis, the transmitted wave can be bended in the plane of XOZ with an anomalous refraction angle θ_t, which can be predicted by the generalized Snell's law (GSL)[1]. For the published designs, the realized efficiency is only a few percent, which can be hardly used.