Broadband high-efficiency acoustic vortices via a topology-optimized space-coiling-cavity metasurface

Acoustic vortices carrying orbital angular momentum are significant in wave-matter interactions, allowing for versatile devices with promising applications. However, current metasurface-based vortex generators have limited bandwidth, operating within a narrow frequency range. Here, we propose a broadband acoustic metasurface with four space-coiling-cavity units through the bottom-up topology optimization. These units have constant phase differences with high-efficiency transmission (>0.8) over a desired frequency range [3.0, 4.0] kHz, indicating the unique wideband vortex response of their assembled metasurfaces. Our simulations and experiments further demonstrate this excellent performance. This breakthrough in broadband vortices allows for the development of devices like acoustic tweezers, antennas, and spanners.