Large and efficient unidirectional plane-wave–surface-wave metasurface couplers based on modulated reactance surfaces

Planar metasurfaces for efficient unidirectional conversion of a plane wave into a surface wave are presented. They are designed as a reciprocal device of a decoupler or leaky-wave antenna based on spatially modulated reactance surfaces. In a surface wave-to-leaky wave conversion, the surface reactance is determined from optimization of the complete aperture field over the propagating and evanecent spectra such that the conversion surface becomes locally and globally passive and lossless, while the desired wave conversion is achieved. The coupling efficiency is given by the aperture efficiency of the reciprocal antenna, which can closely approach unity for large apertures. The optimized surface reactance is spatially discretized and translated into physical metasurfaces, comprising an array of subwavelength inclusions. In TM polarization, three lossless metasurface design examples of 40 and 80 wavelengths are provided with performance validation using full-wave simulations: a metallic groove array, a thickness-modulated grounded dielectric substrate, and a width-modulated grounded dielectric ridge array. Deviations of the surface reactance from the optimized distribution reduce the conversion efficiency, but high coupling efficiencies in excess of 98% for the metallic structure and 60% for dielectric structures are predicted.