Direct Synthesis of Metasurfaces for Efficient Space- to Surface-Wave Conversion and Beamforming

This article presents an accurate analytical-based approach for the synthesis of modulated metasurfaces (MTSs) that efficiently convert an arbitrary impinging field into a surface wave (SW) with a given wavenumber and convert a given SW into an arbitrary amplitude-phase shaped aperture leaky wave (LW) that produces a desired radiation pattern. The considered MTSs consist of dense metallic claddings printed over a dielectric slab, which are modeled through homogenized penetrable impedance boundary conditions (PIBCs). The desired conversion is obtained through a proper locally periodic modulation of the PIBC, whose profile is designed through an effective systematic procedure based on analytical formulas. These latter are derived from the Floquet-wave (FW) expansion of fields and currents over the sinusoidally modulated impedance surface that locally matches the actual structure. This approach provides an accurate control of both the amplitude and phase of the aperture field, which allows for high conversion efficiencies and a general radiation pattern. Another advantage of the proposed analytically driven procedure is that it leads to impedance profiles with smooth spatial variations, which can be easily implemented through subwavelength patches of variable size. The effectiveness of the procedure is demonstrated by designing MTSs that couple SWs and space waves (SPWs) in different scenarios. Moreover, MTSs are designed, which first convert the impinging signal into an SW and then convert it back to an SPW with prescribed characteristics. This latter example can find application in future smart radio environments (SREs), making it a valid alternative to reflecting intelligent surfaces (RISs).