![]() Based on these acoustic meta-atoms, a great number of acoustic wavefront manipulation devices have been constructed and operated successfully, for example, the acoustic focusing lens 21, 22, acoustic vortex beam generator 23, acoustic Airy beam generator 24, 25, acoustic carpet cloaking 26, 27 and so on. Numerous types of acoustic meta-atoms have been proposed to construct the functional acoustic metasurfaces, such as tapered labyrinthine structure 14, coiling-up slit structure 15, 16, zigzag channel 17, Helmholtz resonator array 18, split sphere 19 and membrane based structure 20. Similar to their electromagnetic counterparts, acoustic metasurfaces have become attractive for they are able to engineer the phase profiles of the impinging waves by the artificial designed structures with subwavelength thickness instead of the space consuming solutions offered by the traditional diffractive acoustic devices. Some nontrivial physical phenomenon, such as the photonic spin Hall effect 10, 11 and the generalized laws of reflection and refraction 1, 12, 13 to name a few, can be amply studied with the help of the gradient metasurfaces. 1 stimulates the intensive investigation about the electromagnetic metasurfaces and reveals the huge potentials underneath the metasurface of realizing highly integrated functional electromagnetic devices, such as the planar lens 2, 3, optical vortex generator 4, 5, holograms 6, 7 and ultrathin cloaking 8, 9. Since metasurface itself can be regarded as the application of the Huygens’ principle, arbitrary wavefront manipulation can be realized by designing these artificial secondary sources according to the detailed information of the desired wavefront. Tailoring the wavefront into arbitrary desired shape with a metasurface, the two dimensional metamaterial with subwavelength inclusions, has attracted tremendous attention in recent years. This work may provide new freedom in designing functional acoustic signal modulation devices, such as acoustic isolator and acoustic illusion device. ![]() The apparent negative reflection phenomena has been perfectly verified by the calculated scattered acoustic waves of the reflected gradient acoustic metasurface. The underlying mechanism of the apparent negative reflection is understood as the higher order diffraction arising from the interaction between the local phase modulation and the non-local effects introduced by the supercell periodicity. Here we theoretically demonstrate that apparent negative reflection can be realized by a gradient acoustic metasurface when the incident angle is beyond the critical angle. However, the critical angle that derived from the generalized Snell’s law circles the domain of the incident angles that allow the occurrence of the anomalous reflection and refraction, and no free space scattering waves could exist when the incident angle is beyond the critical angle. The wave scattered by the gradient metasurface, which is composed by the periodic supercells, is governed by the generalized Snell’s law. As the two dimensional version of the functional wavefront manipulation metamaterial, metasurface has become a research hot spot for engineering the wavefront at will with a subwavelength thickness.
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