Within a time span of 15 years, acoustic metamaterials have emerged from academic curiosity to become an active field driven by scientific discoveries and diverse application potentials. This review traces the development of acoustic metamaterials from the initial findings of mass density and bulk modulus frequency dispersions in locally resonant structures to the diverse functionalities afforded by the perspective of negative constitutive parameter values, and their implications for acoustic wave behaviors. We survey the more recent developments, which include compact phase manipulation structures, superabsorption, and actively controllable metamaterials as well as the new directions on acoustic wave transport in moving fluid, elastic, and mechanical metamaterials, graphene-inspired metamaterials, and structures whose characteristics are best delineated by non-Hermitian Hamiltonians. Many of the novel acoustic metamaterial structures have transcended the original definition of metamaterials as arising from the collective manifestations of constituent resonating units, but they continue to extend wave manipulation functionalities beyond those found in nature.

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Acoustic metamaterials: From local resonances to broad horizons.

Conventional acoustic absorbers are used to have a structure with a thickness comparable to the working wavelength, resulting in major obstacles in real applications in low frequency range. We present a metasurface-based perfect absorber capable of achieving the total absorption of acoustic wave in an extremely low frequency region. The metasurface possessing a deep subwavelength thickness down to a feature size of ∼λ/223 is composed of a perforated plate and a coiled coplanar air chamber. Simulations based on fully coupled acoustic with thermodynamic equations and theoretical impedance analysis are utilized to reveal the underlying physics and the acoustic performances, showing an excellent agreement. Our realization should have an high impact on amount of applications due to the extremely thin thickness, easy fabrication, and high efficiency of the proposed structure.

Acoustic metasurface-based perfect absorber with deep subwavelength thickness

Control of sound waves has been inspired by recent advances in using tailored metamaterials to manipulate light. In this study an acoustic metasurface is analytically designed and experimentally realized, with the potential to mold reflected wavefronts into desired forms--even directing nondiffracting beams along complex and nonparaxial trajectories. This level of control may open new avenues for ultrasound imaging, surgical ablation, and other applications.

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Experimental realization of full control of reflected waves with subwavelength acoustic metasurfaces

Metasurfaces with subwavelength thickness have exhibited unconventional phenomena in ways that could not be mimicked by traditional materials. Here we report the analytical design and experimental realizations of acoustic metasurface with hitherto inaccessible functionality of manipulating the reflected waves arbitrarily. By suitably designing the phase shift profile covering 2 range induced by labyrinthine units, the metasurface can reflect acoustic waves in an unusual yet controllable manner. Anomalous reflection and ultrathin planar lens with adjustable focal point were both demonstrated with carefully designed metasurfaces. Remarkably, the free manipulation of phase shifts offers great flexibility in the design of non-paraxial or paraxial acoustic self-accelerating beams with arbitrary trajectories. With the extraordinary wave-steering ability, the metasurface should open exciting possibilities for designing compact acoustic components with versatile potential and may find a variety of applications ranging from ultrasound imaging to field caustic engineering.

/pdf/experimental-realizations-of-full-control-of-reflected-waves-2ei6f6rzh5.pdf

Experimental realizations of full control of reflected waves with subwavelength acoustic metasurfaces

The ability to slow down wave propagation in materials has attracted significant research interest. A successful solution will give rise to manageable enhanced wave-matter interaction, freewheeling phase engineering and spatial compression of wave signals. The existing methods are typically associated with constructing dispersive materials or structures with local resonators, thus resulting in unavoidable distortion of waveforms. Here we show that, with helical-structured acoustic metamaterials, it is now possible to implement dispersion-free sound deceleration. The helical-structured metamaterials present a non-dispersive high effective refractive index that is tunable through adjusting the helicity of structures, while the wavefront revolution plays a dominant role in reducing the group velocity. Finally, we numerically and experimentally demonstrate that the helical-structured metamaterials with designed inhomogeneous unit cells can turn a normally incident plane wave into a self-accelerating beam on the prescribed parabolic trajectory. The helical-structured metamaterials will have profound impact to applications in explorations of slow wave physics.

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Implementation of dispersion-free slow acoustic wave propagation and phase engineering with helical-structured metamaterials.

Acoustic lenses find applications in various areas ranging from ultrasound imaging to nondestructive testing. A compact-size and high-efficient planar acoustic lens is crucial to achieving miniaturization and integration, and should have deep implication for the acoustic field. However its realization remains challenging due to the trade-off between high refractive-index and impedance-mismatch. Here we have designed and experimentally realized the first ultrathin planar acoustic lens capable of steering the convergence of acoustic waves in three-dimensional space. A theoretical approach is developed to analytically describe the proposed metamaterial with hybrid labyrinthine units, which reveals the mechanism of coexistence of high refractive index and well-matched impedance. A hyperbolic gradient-index lens design is fabricated and characterized, which can enhance the acoustic energy by 15 dB at the focal point with very high transmission efficiency. Remarkably, the thickness of the lens is only approximately 1/6 of the operating wavelength. The lens can work within a certain frequency band for which the ratio between the bandwidth and the center frequency reaches 0.74. By tailoring the structure of the metamaterials, one can further reduce the thickness of the lens or even realize other acoustic functionalities, opening new opportunity for manipulation of low-frequency sounds with versatile potential.

https://cyberleninka.org/article/n/929561.pdf

Three-dimensional Ultrathin Planar Lenses by Acoustic Metamaterials

In this paper, a 28GHz Doherty power amplifier for 5G communication is presented in a 0.15-$\mu$m depletion mode (D-mode) Gallium Arsenide (GaAs) pHEMT from Win’s corporation. To simplify the impedance transformations both in output power back-off region and at saturation, the compensation line behind the main power amplifier is removed without degrading the back-off efficiency by choosing an appropriate output matching network. Electromagnetic (EM) simulated results show that the saturation output power and power added-efficiency (PAE) of the proposed Doherty power amplifier are better than 27dBm and 37% respectively in the frequency band of 27.5 GHz -28.5 GHz. The PAE at 6 dB output power back-off are obtained as 30.5% $\sim$31.3%, and the chip size is 3.2mm* 1.6mm.

Su Cheng

Papers

Three-dimensional Ultrathin Planar Lenses by Acoustic Metamaterials

A 28GHz Doherty Power Amplifier MMIC for 5G Millimeter Band Communication