A broadband elastic wave absorption by a sub-wavelength and lightweight structure is of considerable significance in vibration suppression, especially for low frequencies in plate-like structure. However, it has always been a great challenge. In this research, we systematically study the flexural wave diffraction in a thin plate. Based on the diffraction mechanism, we propose the concept of sub-wavelength lossy gradient elastic metasurface for flexural wave absorption. We theoretically reveal high-efficiency and quasi-omnidirectional absorption behavior, which stem from maximum multireflection-enhanced absorption of the 0th order diffraction. We experimentally demonstrate a robust high-efficiency absorption in the frequency range from 343 to 1000 Hz (larger than 1.5 octaves). In addition, we propose a general approach which involves new physics of adjusting an arrangement sequence of subunits to suppress the first-order diffraction mode. This allows to further reduce the sub-wavelength thickness of the metasurface while maintaining its high-efficiency absorption. Our designs could provide new routes to broadband vibration suppression and cancelation in low frequency by lossy elastic metamaterials and metasurfaces.

亚波长和轻量结构对宽带弹性波的吸收在抑制振动方面具有重要意义，尤其是对于板状结构中的低频而言。但是，这一直是一个巨大的挑战。在这项研究中，我们系统地研究了薄板中的弯曲波衍射。基于衍射机理，提出了亚波长有损梯度弹性超表面的概念，以吸收弯曲波。我们从理论上揭示了高效和准全向的吸收行为，这是由于0阶衍射的最大多次反射增强了的吸收。我们通过实验证明了在343至1000 Hz（大于1.5个八度）的频率范围内具有强大的高效吸收能力。此外，我们提出了一种通用方法，该方法涉及调整子单元的排列顺序以抑制一阶衍射模式的新物理学。这允许进一步减小超颖表面的亚波长厚度，同时保持其高效吸收。我们的设计可以为有损弹性超材料和超表面提供宽带抑制和消除低频低频振动的新途径。