  Recent advances in gradient metasurfaces have shown that only by locally
controlling the bianisotropic response of the constituent cells can one ensure
full control of refraction, i.e., arbitrarily modify the direction of the
incident waves without producing scattering into unwanted directions. In this
work, we propose and experimentally verify the use of a new acoustic cell
architecture that provides enough degrees of freedom to fully control the
bianisotropic response and minimizes the implementation losses produced by
resonant elements. The versatility of the approach is shown through the design
of three different anomalous refractive metasurfaces capable of redirecting a
normally incident plane wave to 60, 70 and 80 degrees on transmission. The
effciency of the bianisotropic designs is over 90%, much higher than the
corresponding designs based on the conventional generalized Snells law (81%,
58%, and 35%). The proposed strategy opens a new way of designing practical and
highly effcient bianisotropic metasurfaces for different functionalities,
enabling nearly ideal control over the acoustic energy flow through thin
metasurfaces.
