Apparent Negative Reflection with the Gradient Acoustic Metasurface by Integrating Supercell Periodicity into the Generalized Law of Reflection

Abstract

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. The wave scattered by the gradient metasurface, which is composed by the periodic supercells, is governed by the generalized Snell’s law. 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. 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. 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. The apparent negative reflection phenomena has been perfectly verified by the calculated scattered acoustic waves of the reflected gradient acoustic metasurface. This work may provide new freedom in designing functional acoustic signal modulation devices, such as acoustic isolator and acoustic illusion device.

Manipulation of acoustic wavefront by gradient metasurface based on Helmholtz Resonators

“…Recent years have witnessed intense investigation of acoustic metasurface capable of realizing general wavefront modulation 1 – 9 . According to the generalized Snell’s law 10 , the angles of reflected, refracted and diffracted waves can be artificially operated by the metasurface with gradient change of acoustical phase 7 – 9 , 11 , 12 . In addition, the fascinating phenomena and capabilities, such as acoustic bending 13 , 14 , anomalous refraction 15 , ultrathin flat lens 4 , 16 – 18 , conversion of propagating wave to surface wave 19 – 21 , tunable acoustic negative refraction 22 , etc., have been exhibited by different kinds of gradient metasurfaces.…”

Acoustic metasurfaces

“…As a consequence of the association of the reflected phase shift with the propagating path, a coiling-up space structure with a tunable propagating path is a feasible way to construct acoustic-reflection-type metasurfaces 2,10,11,[71][72][73][74][75][76][77] . A simple type of coiling-up space structure, namely, a labyrinthine-like structure, constitutes the building block of an acoustic metasurface for controlling sound reflection ( Fig.…”

Anomalous Reflection of Acoustic Waves in Air with Metasurfaces at Low Frequency

“…The generalized Snell's law (GSL) was introduced to predict the anomalous propagation of incident wave across material interfaces characterized by a phase gradient [14,20,33,34], which is suitable for both reflected wave and transmitted wave. The metasurfaces proposed in this study are also built based on GSL.…”

“…In the field of optics, the metasurfaces have developed to realize many remarkable functions, such as light beam steering [14], ultrathin optical skin cloak [15], and holograms [16]. As the wavelengths of sound waves are much larger than that of light wave, the acoustic metasurfaces [17][18][19][20][21][22][23][24] are especially important for acoustic devices. Faure et al realized an acoustic carpet cloaking by using a metasurface made of graded Helmholtz resonators [18].…”