Acoustic bilayer gradient metasurfaces for perfect and asymmetric beam splitting

We experimentally and theoretically present a paradigm for the accurate bilayer design of gradient metasurfaces for wave beam manipulation, producing extremely asymmetric splitting effect by simply tailoring the interlayer size. This concept arises from the anomalous diffraction in phase gradient metasurfaces and the precise combination of the phase gradient in bilayer metasurfaces. Ensured by different diffraction routes in momentum space for incident beam from opposite direction, extremely asymmetric acoustic beam splitting can be generated in a robust way, as demonstrated through a designed bilayer system in experiments. Our work provides a novel approach and feasible platform to design tunable devices for the control of wave propagation.