Ultrathin broadband acoustic reflection metasurface based on meta-molecule clusters

The control of sound waves by metasurfaces is an interesting research topic in the metamaterial technology field. Based on existing research about the acoustic metamaterials constructed using meta-molecules, we propose a theoretical model of a deep subwavelength acoustic reflection metasurface. We further verify this model via simulations and experiments. The designed broadband acoustic metasurface consists of 'meta-atoms', 'meta-molecules', 'meta-molecule clusters' and 'meta-molecule cluster sets', in the aforementioned order. The structural parameters of the two meta-atom types can be adjusted to regulate the acoustic reflection phase, and the meta-molecule are sufficient to realise the discrete phase shifts that cover the complete 2π span. Meta-molecule clusters comprise meta-molecules that can adjust certain geometric parameters of the meta-atoms to achieve abnormal and negative reflections. In addition, 'meta-molecule cluster sets' enable the metasurface to achieve a broad multi-frequency response by combining two types of meta-atoms' regulative modes. We construct a meta-molecule cluster set consist of two clusters enable the broadband metasurface achieve 800 Hz bandwidth abnormal reflection. Our ultrathin acoustic meta-molecule metasurface combines a simple and highly-integrated structure with a thickness equal to only 1/16th of the acoustic wavelength. We additionally design a subwavelength-thickness acoustic axicon by arranging meta-molecule structure. The proposed ultrathin acoustic metasurface exhibits considerable potential for applications in acoustic devices such as acoustic cloaking and absorption.