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PUBLISHED ONLINE: 1 JUNE 2014 | DOI: http://www.nature.com/doifinder/10.1038/nmat3994
Web End =10.1038/NMAT3994
Guancong Ma1, Min Yang1, Songwen Xiao1, Zhiyu Yang1 and Ping Sheng1,2*
An impedance-matched surface has the property that an incident wave generates no reection. Here we demonstrate that by using a simple construction, an acoustically reecting surface can acquire hybrid resonances and becomes impedance-matched to airborne sound at tunable frequencies, such that no reection is generated. Each resonant cell of the metasurface is deep-subwavelength in all its spatial dimensions, with its thickness less than the peak absorption wavelength by two orders of magnitude. As there can be no transmission, the impedance-matched acoustic wave is hence either completely absorbed at one or multiple frequencies, or converted into other form(s) of energy, such as an electrical current. A high acousticelectrical energy conversion eciency of 23% is achieved.
Aperfect absorber of deep-subwavelength scale is of great scientific and engineering interest. It can act as the exact time-reversed counterpart of a point source1, with
important implications for time-reversal wave technology2,3.
Traditional means of acoustic absorption make use of porous and fibrous materials4 and gradient index materials, or employ perforated or micro-perforated panels57 with tuned cavity
depth behind the panels. They generally result in either imperfect impedance matching to the incoming wave, or very bulky structures with dimensions comparable to the wavelength. Space-coiling structures are potentially viable means to reduce the geometric dimensions810, but face the challenge of impedance mismatch to the background medium8,11. Active absorbers, on the other hand, require costly and sophisticated electrical designs12. Recently, it was shown that, for electromagnetic waves, structuring the interface between two dierent materials can lead to metasurfaces with diverse functionalities such as phase discontinuity, anomalous refraction/reflection, and polarization manipulation1316. In
particular, the coherent perfect absorber (CPA; ref. 17) was realized in optics by relying on phase matching (interference) of counter-propagating waves within a lossy material. Adaptation of the concept to acoustics, however, requires either acoustically thick materials18, or subwavelength but highly dissipative plates19 (such as metal for electromagnetic waves), neither of which is practical for low-frequency sound. In addition, all these mechanisms require a specific viscous property or an exact Q-factor value to attain optimal absorption performance, making them less robust and dicult to tune.
In this work, we advance...