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An acoustic metasurface made of a composite structure of cavity and membrane is proposed and numerically investigated. The target frequency is in the low frequency regime (570 Hz). The unit cells, which provide precise local phase modulation, are rather thin with thickness in the order around 1/5 of the working wavelength. The numerical simulations show that the designed metasurface can steer the reflected waves at will. By taking the advantage of this metasurface, an ultrathin planar acoustic axicon, acoustic lens, and acoustic nondiffracting Airy beam generator are realized. Our design method provides a new approach for the revolution of future acoustic devices.

The rising acoustic metamaterials, whose structures are on a subwavelength scale, exhibit many novel properties that can not be realized by natural materials in controlling sound waves, such as negative mass density, negative modulus, and double-negative parameters [

Figure

(a) Schematic map of acoustic metasurface unit cell and (b) longitudinal section of the unit cell. The unit cell is composed of a cavity with acoustically hard walls, which is capped with a rigid circular ring. The inside of the ring is a clamped membrane, and the outside is an air gap.

When there is no incident sound wave, the elastic membrane will be tensioned to be a straight line; however, when there are incident waves along the ^{3}, individually. The mass density, Young’s modulus, and Poisson’s ratio of the membrane are 920 kg/m^{3}, 9.6 × 10^{9} Pa, and 0.36, individually, so that traditional polyethylene can be suitable for the practical fabrication of this membrane. Based on [

The relation between the simulated reflected phase of the unit and the membrane radius is shown with the red curve in Figure

Reflected phases and ratios of the eight subunits as a function of

The generalized Snell’s law (GSL) was introduced to predict the anomalous propagation of incident wave across material interfaces characterized by a phase gradient [

Simulated snapshots of pressure map for the reflected beams of the metasurfaces with seven different unit spaces ((a)~(g) correspond to 120, 130, 140, 150, 160, 170, and 180 mm, respectively). The metasurfaces are located on the

When acoustic waves impinge on a metasurface, the distribution of scattered pressure field follows generalized Snell’s law. However, if we apply a slight modification to the metasurface, such as changing the phase gradients of different parts on it, the scattered field will be modulated to specific distribution. In the following, we show different wave manipulation effects by exploiting the proposed acoustic metasurfaces based on different combinations of fundamental units. In particular, we will show three mechanisms that could promote the development of future ultracompact acoustic devices, including an ultrathin planar acoustic axicon, an ultrathin planar lens, and a nondiffracting Airy beam [

In order to design the acoustic axicon, the reflected angle of sound wave is set to be 10°, which is symmetrical along

Based on (

Acoustic axicon realized by the designed planar metasurface. (a) The normalized squared absolute pressure distribution of the reflected waves. The red arrow indicates the propagation direction of incident wave. (b) Transient sound pressure field distribution of reflected wave. The black arrows refer to the reflected directions. The angle between equiphase surface and

To design a planar acoustic lens, a hyperboloidal phase profile is employed on the metasurfaces. For a given focal length

Planar acoustic lens. (a) The squared absolute pressure field of reflected waves. The red arrows represent the propagation direction of incident plane wave. (b) The transient pressure field distribution of reflected waves. The black dashed arc indicates the shape of reflected wavefront. (c) The longitudinal sound intensity distribution at

The acoustic nondiffracting Airy beam has unique features, such as self-bending and self-healing [

Acoustic nondiffracting Airy beam. (a) The squared absolute pressure field of reflected waves. The red arrows represent the propagation direction of incident plane wave. (b) The transient pressure field distribution of reflected waves. The black dashed arrow indicates the travel trajectory of the acoustic energy.

In summary, we designed a new kind of acoustic metasurface consisting of cubes with cylindrical cavities, whose endings are covered with elastic membrane and air gap. The simulated results demonstrate that this ultrathin acoustic metasurface (about

The author declares that there are no conflicts of interest.

This work was supported by the National Natural Science Foundation of China under Grant no. 11764033, Natural Science Foundation of Ningxia under Grant no. NZ17253, Higher School Science Research Project of Ningxia under Grant no. NGY2016173, Undergraduate Teaching Engineering Project in 2017 of Ningxia Normal University, and Ningxia Higher School First-Class Discipline Construction (Pedagogy) under Grant no. NXYLXK2017B11.