Asymmetric sandwich-like elements for bianisotropic acoustic metasurfaces

Abstract Conventional gradient metasurface suffers from low efficiency because of impedance mismatching, especially in large-angle manipulation. Here we propose an asymmetric sandwich-like material element to design bianisotropic metasurface for full-angle high-efficiency anomalous refractive manipulation. The proposed asymmetric element is composed of two different auxiliary layers attached to the core layer which can be viewed as a conventional gradient index metasurface, and we demonstrate analytically that these sandwich-like structures can exhibit a bianisotropic response and meet the normal impedance matching condition at both sides of the metasurface. Besides, this material design concept can be realized via the space-coiling structures. The availability of the element is first demonstrated by a nearly total refraction at an extreme refracted angle (up to 80°). Subsequently, these elements are used for the transition connection of different waveguides and dramatically improve the transmission performance. Our method brings new possibilities for the design and application of bianisotropic metasurfaces.