Focusing Microwaves in the Fresnel Zone With a Cavity-Backed Holographic Metasurface

We present the design and experimental demonstration of a cavity-backed, holographic metasurface capable of focusing microwaves in the Fresnel zone. The circular cavity consists of two stacked plates: microwaves are injected into the bottom plate via a coaxial connector, which forms the feed layer, and are coupled to the top holographic metasurface layer via an annular ring on the periphery of the cavity. This coupling results in an inward traveling cylindrical wave in the top layer, which serves as the reference wave for the hologram. A sparse array of slots, patterned into the upper plate, constitutes the hologram that produces the focal spot. To mitigate high sidelobe levels and improve performance, a tapered design, facilitated by varying the slot size, is also introduced. The proposed designs-which have a 10-cm diameter, operate at 20 GHz, and form a focal spot at a distance of 10 cm-are validated using full-wave simulations as well as measurements of fabricated samples. The proposed focusing metasurface may find application as a compact source for Fresnel zone wireless power transfer and remote sensing schemes.