Exploiting metasurface anisotropy for achieving near-perfect low-profile cloaks beyond the quasi-static limit

In this paper, we report an approach for achieving near-perfect low-profile electromagnetic cloaking beyond the quasi-static limit. In contrast to previous works on metasurface cloaks where only the Leontovich surface impedance boundary condition has been considered, we employ a second-order surface impedance boundary condition to account for the radial response of the proposed anisotropic metasurfaces. This radial surface polarizability tensor parameter can be tailored to eliminate the higher order scattering modes. It is shown from analytical expressions that for a moderate-sized perfect electric conducting or dielectric cylinder (similar to 0.4 lambda(0) in width), near-perfect scattering reduction, i.e. >98%, can be achieved by a single anisotropic metasurface with non-vanishing radial response, which is far superior to the conventional scalar impedance surface. A practical design of the metasurface is also presented and further validated by full-wave simulations. The physical mechanism of the metasurface cloaks is investigated in detail, revealing that the radiation cancellation of the induced surface currents is responsible for the scattering reduction. Importantly, it is shown that in addition to a 'low-visibility coating', the metasurface also functions to provide induced current enhancement which would be beneficial for the operation of a 'cloaked sensor'. These findings will broaden the usage of metasurfaces to applications ranging from scattering reduction to noninvasive probing for objects beyond the quasi-static limit.