Bianisotropic Huygens' Metasurface for Wideband Impedance Matching Between Two Dielectric Media

There is an established equivalence between Huygens' metasurfaces and lattice networks. This paper proposes an extension to this equivalence for bianisotropic Huygens' metasurfaces, where bianisotropy refers to the magnetoelectric coupling between the effective electric and magnetic responses. A modified version of the lattice network is proposed that exhibits a one-to-one mapping to the bianisotropic sheet transition conditions by incorporating ideal transformers. A possible realization of bianisotropic Huygens' unit cells is also proposed that relies on offsetting the wire with respect to the loop in the wire/loop topology. This is followed by deriving analytical expressions for the image impedances and the corresponding transfer function of bianisotropic Huygens' metasurfaces. Inverse analytical expressions are also derived that govern the required effective electric and magnetic responses and the magnetoelectric coupling for achieving wideband electrical transparency (symmetric case) and wideband impedance transformation (bianisotropic case) with these Huygens' metasurface unit cells. Moreover, it is shown that these expressions can be realized with transmission-line stubs. These expressions are then used to design subwavelength thin Huygens' metasurfaces immersed in an air gap between two dielectric media. It is shown, through full-wave simulations, that wideband electrical transparency and impedance transformation are realized for a normally impinging plane wave.