Broadband Coding Metasurfaces with 2-bit Manipulations

Due to their limited number of units but outstanding ability to control rather complex wave-propagation phenomena, acoustic coding metasurfaces (ACMs) as two-dimensional metamaterials show a stronger competitiveness in metamaterial applications. However, hindered by their narrow-band modulation capability, the previously reported ACMs do not exhibit a broadband applicability. To address the frequency-based coding capability, in this paper we report broadband acoustic coding metasurfaces (BACMs) whose units are designed by the bottom-up topology optimization method. Subsequently, by utilizing our optimization strategy, we design the 1-bit coding units ``0'' and ``1'' with out-of-phase responses and the 2-bit coding units ``00'', ``01'', ``10,'' and ``11'' with four different phase shifts of 0\ifmmode^\circ\else\textdegree\fi{}, 90\ifmmode^\circ\else\textdegree\fi{}, 180\ifmmode^\circ\else\textdegree\fi{}, and 270\ifmmode^\circ\else\textdegree\fi{}. The topological features show constant phase differences, and in the following analysis, we attempt to explain this phenomenon by the related mechanisms of the internal resonances and the bianisotropy effect. The optimized BACMs are beneficial to improve the functions of a fixed coding metasurface in the frequency range. This idea provides an inspiration for fabricating fast-sector-scanning sound antenna. In combination with the convolution method, we also present the design strategy for the broadband fast-scanning multiple-beam antenna. The design of the acoustic antenna is a fixed coding sequence, and its scanning and echolocation mode does not depend on the reconfiguration of the coding units. Furthermore, we also demonstrate a pair of opposite functions, namely the broadband sound focusing and diffusion-scattering characteristics, which have potential applications in ultrasonic therapy, low-scattering target design and noise control, and so on.