Exploring the concept of non-Hermitian Hamiltonians respecting parity-time symmetry with classical wave systems is of great interest as it enables the experimental investigation of parity-time-symmetric systems through the quantum-classical analogue. Here, we demonstrate unidirectional wave vector manipulation in two-dimensional space, with an all passive acoustic parity-time-symmetric metamaterials crystal. The metamaterials crystal is constructed through interleaving groove- and holey-structured acoustic metamaterials to provide an intrinsic parity-time-symmetric potential that is two-dimensionally extended and curved, which allows the flexible manipulation of unpaired wave vectors. At the transition point from the unbroken to broken parity-time symmetry phase, the unidirectional sound focusing effect (along with reflectionless acoustic transparency in the opposite direction) is experimentally realized over the spectrum. This demonstration confirms the capability of passive acoustic systems to carry the experimental studies on general parity-time symmetry physics and further reveals the unique functionalities enabled by the judiciously tailored unidirectional wave vectors in space.

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Paper published as part of the special topic on Acoustic Metamaterials and Metasurfaces ARTICLES YOU MAY BE INTERESTED IN Acoustic metasurface-based perfect absorber with deep subwavelength thickness Applied Physics Letters 108, 063502 (2016);

In this work, we propose a simple scheme to realize an acoustic coherent perfect absorber (CPA) and laser modes by embedding a non-Hermitian dopant in a zero index metamaterial. When the dopant is filled with a loss medium at a specific level, the sample can absorb the incident waves completely. On the other hand, when the dopant is filled with a gain medium, the sample can act as a laser oscillator to boost the incident waves. The theoretical derivation based on the scattering matrix and the numerical simulation based on the finite element method are performed and both show good agreement with each other. We also discover that the CPA and laser modes are very sensitive and can be controlled by adjusting the structure parameters or the relative phase of the incident waves. Moreover, the case that asymmetric incidences have different beam widths is considered. We envision that our work may have potential applications in designing acoustic devices, such as absorbers, transducers, and receivers.

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We demonstrate a conformally mapped multifunctional acoustic metamaterial Mikaelian lens. Mikaelian lens is a gradient medium with a hyperbolic secant refractive index profile that can realize functions like beam self-focusing. Unlike the conventional design approaches, with a conformal transformation method, only isotropic material parameters with gradient refractive index profiles are required for the construction of such lens. To realize desired gradient index distribution, we carefully design a new type of cross-channel-shaped acoustic metamaterial, whose refractive index can be effectively modulated by simply changing the slit opening size. The distinct capabilities of the metamaterial Mikaelian lens in manipulating acoustic waves are experimentally verified with the fabricated sample. Simultaneous sound guiding and Talbot effects, which normally require respective geometrical and wave acoustic approximations, are observed in simulations and experiments. Furthermore, those effects of shaping acoustic wave propagations were validated within a relatively wide frequency range. Our study reveals how the conformal transformation method can help to bridge the ray acoustics with wave acoustics. It offers opportunities to the development of novel multifunctional acoustic devices for various applications, such as sound and particle manipulations.

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