(Key Laboratory of Modern Acoustics of Ministry of Education， Collaborative Innovation Center of Advanced Microstructures， Department of Physics，Nanjing University，Nanjing 210093，China)
Complicated manipulation of reflected acoustic waves is of fundamental significance in acoustics, with wide applications in various important situations, such as the improvement of acoustic quality in rooms and elimination of undesired noise. In recent years, the acoustic metasurface has provided new inspiration for the miniaturization of acoustic functional devices, therefore it would be physically and practically important to further reduce the size and weight of acoustic metasurfaces. In this paper, a lightweight and ultrathin acoustic metasurface for high-efficiency and precise manipulation of low-frequency reflected airborne sound is introduced. Based on theoretical calculation, it is proven that the metasurface unit cell with a simple planar hollow structure enables free control of reflected phase over the full 0~2π range via adjustment of one sole structural parameter, with no need of sacrifice of energy reflectivity and mechanical rigidity or sophisticated inner structures that would increase the fabrication difficulty and device weight. Hence the resulting device has advantages of ultrathin thickness (of λ0/20), light weight, high reflection efficiency and simple and low-cost design. The performance of the designed metasurface is demonstrated via three distinctive examples of anomalous reflection with arbitrary angle, ultrathin acoustic planar lens for focusing acoustic energy and production of Bessel-like beam based on ultrathin flat acoustic axicon. The realization of flexible manipulation of reflected sound by using lightweight and ultrathin metasurface helps to promote the novel acoustic planar devices and their applications, and may have important implication in many diverse scenarios ranging from architectural acoustics to noise control to loudspeaker design.