Compact acoustic bilayer metasurfaces for high-efficiency flexible beamsplitting

Tunable beamsplitting is important for the flexible control of sound wave radiation in acoustics, which has garnered an increasing amount of attention recently. Twisted bilayer metasurfaces, capable of dynamically manipulating acoustic waves by altering the interlayer angle, offer the significant advantage of facile adjustability. Here, we introduce a compact acoustic bilayer metasurface (ABM) with near-zero interlayer distance that enables high-efficiency flexible beamsplitting. The ABM integrates two metasurfaces with identical phase distribution, allowing for four distinct phase configurations by rotating one metasurface in 90° increments, thereby achieving beamsplitting function with four types of far-field radiation patterns. The periodic design permits the ABM to be infinitely large, while its compact structure assures stability. Both numerical simulations and experimental validations confirm the effectiveness of the ABM. Our work offers a compact and versatile solution for advanced acoustic beamsplitting and multifunctional applications.