Frequency-Selected Bifunctional Coding Acoustic Metasurfaces

Coding metasurfaces are a kind of metasurface that can combine artificial structures with digital codes 0 and 1. By digitally programming the arrangements of structural units, they can manipulate the transmitted or reflected waves into arbitrary patterns. Recently, coding metasurfaces have been demonstrated to realize fascinating functions in acoustics, including far-field modulation, focusing, and vortex beam generation. However, the previous acoustic coding metasurfaces work at a single frequency and have only one function under a specific arrangement, limiting the efficiency and capacity of manipulation to a low quality. To overcome these shortcomings, here we propose frequency-selected bifunctional coding acoustic metasurfaces. Dual-layer Helmholtz-like resonators are utilized to produce a 0 and pi phase difference, corresponding to the 0 and 1 codes, respectively. Owing to the design of the dual-layer resonators, the two working frequencies are integrated on a single unit with a subwavelength thickness, which eliminates the long-wavelength limitations. Frequency-selected anomalous reflection, diffusion, and Airy-beam generation are further demonstrated to prove its availability. The results are shown through theoretical predictions, simulations, and experiments, which show good correspondence. The proposed frequency-selected bifunctional coding acoustic metasurfaces can be used in acoustic information preservation and provide an alternative way to achieve multispectral acoustic devices.