Traditional asymmetric transmission of elastic waves is mostly based on mode conversion, which presents a real challenge to get pure-mode elastic waves. In this letter, without the aid of mode conversion, we present an innovative concept of asymmetric flexural wave transmission within a wide frequency band, based on dual-layer elastic wave gradient metasurfaces. According to the generalized Snell's law, we theoretically and numerically design and experimentally demonstrate the asymmetric flexural wave transmission by tuning the supercell lengths of dual-layer metasurfaces. The experimental results confirm that the proposed design provides a wide effective frequency band feature, which agrees well with the theoretical analysis and predictions. Our concept offers the flexibility to control the wave energy flow, opening the route to pragmatic applications in many fields, such as ultrasonic detection, energy harvesting, and vibration control.

This paper proposes an innovative and simple elastic metasurface, which is one kind of ultrathin metamaterial, to abnormally deflect normally and obliquely incident flexural waves. In particular, negative refraction of flexural wave is also realized. The proposed metasurface can support high transmission and full phase control for transmitted waves. The metasurface is composed of three-dimensional (3D) functional units which are combined with multiple slender pillars for phase shift accumulation. The slots are filled in the 3D functional units to avoid coupling between adjacent functional units. Firstly, based on the generalized Snell’s law, the metasurface in the sub-wavelength regime is theoretically designed. Secondly, the deflecting functionality of the metasurface for incident flexural waves is numerically demonstrated. Finally, the deflecting functionality is verified by the corresponding experiments. The proposed metasurface has a great application potential in many fields, such as ultrasonic detection, energy harvesting, and vibration control.