Surface acoustic waves over bianisotropic metasurfaces

Abstract

Metasurfaces are the two-dimensional planarized analogues of metamaterials, yielding unique sound manipulation along a surface. Our group recently proposed the concept of bianisotropic metasurfaces achieved by exploiting the strong nonlocal coupling between neighboring vibrating units, yielding anisotropic impedance in airborne acoustics, and we have applied this property to realize acoustic hyperbolic metasurfaces, which require extreme impedance anisotropy. In this talk, we investigate the wave propagation properties of bianisotropic metasurfaces and report all-angle backward-wave propagation on the surface. By carefully designing the coupling between unit cells, both negative phase index and energy index for surface acoustic waves are observed above the metasurface for all propagation directions, hence realizing converging waves for a radiating point source. Since in bianisotropic metasurfaces the energy can be transmitted not only through surface waves, but also through the coupling units, negative energy index for surface waves does not violate energy conservation laws, as it can be sustained by a positive energy flow within the connected unit cells composing the metasurface.