An ultra-thin acoustic metasurface with multiply resonant units

In this paper, we propose an ultra-thin acoustic metasurface constructed of multiply resonant units to manipulate reflected wavefront. As a counterpart to the Helmholtz resonator (with monopolar resonance) and membrane-type resonator (with dipolar resonance), the multiply resonator are used in metamaterials to induce strong quadrupolar resonance. Here we use the multiply resonator as a new kind of building blocks to make acoustic metasurfaces. The used multiply resonant unit is composed of solid materials, and the acoustic metasurface can work in a water background. We demonstrate that the proposed acoustic metasurface achieves good performance in anomalous reflection, focusing, and carpet cloaking. The thickness of the acoustic metasurface is about two orders of magnitude smaller than the acoustic wavelength in water. A design of unit group is further proposed to avoid the phase discretization becoming too fine in such a long-wavelength condition.