Generation of high-directivity beams by using metasurfaces

By designing subwavelength artificial structures, metamaterials provide a physical platform to controlling the propertiesof electromagnetic waves. A recent research hot spot of metamaterial is metasurface,which is composed of thin-layer man-made structures and reliefs the significant losses and fabrication challenges due to the notable thickness of bulk metamaterials. In this talk, we present two surface lenses to generate special high-directivity beams based on man-made surfaces. First, we introduce a transformed surface Luneburg lens based on the quasi-conformal mapping and metasurface, which has nearly-isotropic electromagnetic material parameters. The designed metasurface lens was fabricated by using inhomogeneous artificial structures, which was used to realize the required surface refractive indexes. We tested the transformed metasurface Luneburg lens in three-dimensional near-field scanning platform and the measurement results demonstrate very good performance in controlling the radiated surface waves. Then, weintroduce a method to generate spatial Bessel beams in the microwave frequency regime by using backward radiations of leaky waves supported by a holographic metasurface. In real fabrication, the holographic metasurface consists of a series of metal patches with a grounded dielectric slab. By changing the size of metal patches, the surface-impedance distribution of the holographic metasurface can be modulated, and hence the radiation properties of the leaky waves can be designed to realize Bessel beams. Both numerical simulations and experimental test verify the features of spatial Bessel beams, which may have some potential applications in imaging applications or wireless power transmissions with the dynamic focal-depth controls.