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Title: Inherent losses induced absorptive acoustic rainbow trapping with a gradient metasurface
Authors: Liu, T 
Liang, S 
Chen, F 
Zhu, J 
Issue Date: 7-Mar-2018
Source: Journal of applied physics, 7 Mar. 2018, v. 123, no. 991702, p. 091702-1-091702-9
Abstract: Acoustic rainbow trapping represents the phenomenon of strong acoustic dispersion similar to the optical "trapped rainbow," which allows spatial-spectral modulation and broadband trapping of sound. It can be realized with metamaterials that provide the required strong dispersion absent in natural materials. However, as the group velocity cannot be reduced to exactly zero before the forward mode being coupled to the backward mode, such trapping is temporary and the local sound oscillation ultimately radiates backward. Here, we propose a gradient metasurface, a rigid surface structured with gradient perforation along the wave propagation direction, in which the inherent thermal and viscous losses inside the holes are considered. We show that the gradually diminished group velocity of the structure-induced surface acoustic waves (SSAWs) supported by the metasurface becomes anomalous at the trapping position, induced by the existence of the inherent losses, which implies that the system's absorption reaches its maximum. Together with the progressively increased attenuation of the SSAWs along the gradient direction, reflectionless spatial-spectral modulation and sound enhancement are achieved in simulation. Such phenomenon, which we call as absorptive trapped rainbow, results from the balanced interplay among the local resonance inside individual holes, the mutual coupling of adjacent unit cells, and the inherent losses due to thermal conductivity and viscosity. This study deepens the understanding of the SSAWs propagation at a lossy metasurface and may contribute to the practical design of acoustic devices for high performance sensing and filtering.
Publisher: American Institute of Physics
Journal: Journal of applied physics 
ISSN: 0021-8979
EISSN: 1089-7550
DOI: 10.1063/1.4997631
Rights: © 2017 Author(s).
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in T Liu, S Liang, F Chen, and J Zhu, J. Appl. Phys. 123, 091702 (2018) and may be found at https://doi.org/10.1063/1.4997631.
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