Broadband absorption and asymmetric reflection of flexural wave by deep-subwavelength lossy elastic metasurface

Abstract

The elastic metasurface with phase discontinuities has received much attention for realizing diverse wave steering applications. To date, the elastic metasurfaces capable of realizing the multi-reflection-enhanced absorption of flexural waves at a deep subwavelength scale have rarely been reported. Here, we propose a novel concept of deep-subwavelength lossy elastic metasurface (DLEM) composed of gradient lossy force-moment resonators with a small amount of loss. Based on the diffraction theorem, we provide an accurate approach for predicting diffracted flexural wave behaviors considering the coupling of flexural and longitudinal diffraction modes in a plate and DLEM. The underlying mechanism of high-efficiency broadband absorption and asymmetric reflection of flexural wave is theoretically revealed, which stems from multi-reflection-enhanced absorption of the n=0 and n=-1 order diffractions. For the composite subunit of the DLEM implemented by a lead block atop damping silicone rubber block, a simplified two degree-of-freedom (2DOF) mass-spring-damping model and the corresponding accurate model are both theoretically derived to reveal the deep-subwavelength mechanism of controlling the reflected wave phase. The experiments of the broadband wave absorption and asymmetric reflection are further carried out to validate the performance of the designed DLEMs. The proposed methodology opens a new perspective for broadband vibration suppression and wave manipulation of lossy elastic metasurface with a deep-subwavelength thickness.