Controlling Radiation Beams by High-Efficiency Metasurfaces with Enhanced Refractive Index Elements.

The efficient manipulation of the electromagnetic wavefront using metasurfaces (MSs) has attracted a continuous amount of attention in recent years. MSs with arbitrarily controllable amplitude and phase are highly desirable in many emerging fields. However, in previous research, the resonance effect has mainly been used to accomplish the task. This article presents a nonresonant metamaterial element with stronger capacitive coupling. The broadband sandwich-structured meta-atom achieves an enhanced refractive index ranging from 2.44 to 8.90. By using the enhanced refractive index elements, the proposed transmissive MS has an ultrathin thickness (0.23 free-space wavelength). It consists of a specially designed gradient index MS placed on top of the transmitting antenna. The MS is designed to convert the spherical surface wave into a plane wave with high efficiency. One core layer (CL) gradient index MS and two impedance matching layers (IMLs) have been designed to transform the magnitude and phase of the radiation beam from the transmitting antenna. An efficient transmissive MS has been designed and experimentally evaluated with a maximum measured efficiency of over 85%. The proposed control strategy increases the design flexibility of the MS and shows great potential for broadband applications in the realization of efficient functional devices.