Topology-optimized reflection-type pentamode metasurfaces for broadband underwater beam regulation

Pentamode metamaterials (PMs), a type of customizable artificial liquid-like material, consist of intricate solid microstructural units, offering promising applications in manipulating underwater waves. With their excellent acoustic impedance matching properties with water and customizable equivalent parameters, PMs can surpass the narrowband limitations and facilitate the design of broadband acoustic metasurfaces. However, due to a lack of comprehensive research on PM mechanisms, many researchers opt for conventional regular triangle lattices, limiting both structural diversity and the potential for obtaining precise equivalent parameters. In this study, we propose an inverse optimization strategy to design a series of PM units featuring square lattices. Leveraging the generalized acoustic Snell's law and impedance matching properties of PMs, we construct several reflective broadband subwavelength acoustic metasurfaces. These metasurfaces enable various functionalities based on wavefront manipulation, including an acoustic shielding device capable of converting reflected waves into surface waves across a broad frequency spectrum. Additionally, we achieve broadband anomalous reflection, achromatic focusing, and non-diffracting Bessel beams. Notably, all these achromatic functionalities exhibit relative bandwidths exceeding 100%, indicating promising application prospects.