Xin-ye Zou

Reflected wavefront manipulation based on ultrathin planar acoustic metasurfaces

The introduction of metasurfaces has renewed the Snells law and opened up new degrees of freedom to tailor the optical wavefront at will. Here, we theoretically demonstrate that the generalized Snells law can be achieved for reflected acoustic waves based on ultrathin planar acoustic metasurfaces. The metasurfaces are constructed with eight units of a solid structure to provide discrete phase shifts covering the full 2π span with steps of π/4 by coiling up the space. By careful selection of the phase profiles in the transverse direction of the metasurfaces, some fascinating wavefront engineering phenomena are demonstrated, such as anomalous reflections, conversion of propagating waves into surface waves, planar aberration-free lens and nondiffracting Bessel beam generated by planar acoustic axicon. Our results could open up a new avenue for acoustic wavefront engineering and manipulations.

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.

Acoustic focusing by coiling up space

We report the design of a gradient index acoustic lens by coiling up space, an entirely different, yet more direct approach compared with previous designs. The proposed model comprises a series of acoustic metamaterial units with curled channels. Acoustic waves propagate freely within the channels but their propagating phases can be delayed at will by adjusting the size of the units. The numerical results show that the designed acoustic metamaterial can mimic an acoustic gradient index lens with arbitrarily large refractive index and considerably high transmission efficiency. This may provide possibilities for the design and application of acoustic lenses.

Unidirectional acoustic transmission through a prism with near-zero refractive index

We propose a two-dimensional model of acoustic prism comprising metamaterials with near-zero refractive index to yield high efficiency unidirectional transmission and demonstrate an implementation by coiling up space. Due to the acoustic tunneling effect, the waveform is kept consistent between input and output waves, and the transmitted angle can be controlled by reshaping the prism, even in the presence of hard scatterer. A directional waveguide is also designed whose transmission property can be freely switched between all possible states. Our design may have potential for practical applications of acoustic one-way devices in various fields such as ultrasound imaging and treatment.

One-way mode transmission in one-dimensional phononic crystal plates

We investigate theoretically the band structures of one-dimensional phononic crystal (PC) plates with both antisymmetric and symmetric structures, and show how unidirectional transmission behavior can be obtained for either antisymmetric waves (A modes) or symmetric waves (S modes) by exploiting mode conversion and selection in the linear plate systems. The theoretical approach is illustrated for one PC plate example where unidirectional transmission behavior is obtained in certain frequency bands. Employing harmonic frequency analysis, we numerically demonstrate the one-way mode transmission for the PC plate with finite superlattice by calculating the steady-state displacement fields under A modes source (or S modes source) in forward and backward direction, respectively. The results show that the incident waves from A modes source (or S modes source) are transformed into S modes waves (or A modes waves) after passing through the superlattice in the forward direction and the Lamb wave rejections in the backward direction are striking with a power extinction ratio of more than 1000. The present structure can be easily extended to two-dimensional PC plate and efficiently encourage practical studies of experimental realization which is believed to have much significance for one-way Lamb wave mode transmission.

Extraordinary acoustic transmission through ultrathin acoustic metamaterials by coiling up space

We show that by coiling up space through curled channels, a two-dimensional ultrathin acoustic metamaterial can be constructed to support the extraordinary acoustic transmission. The exotic phenomenon intrinsically stems from Fabry-Perot resonances while the thickness of the structure is kept deep subwavelength. Broadband unity transmission is observed at a specific incident angle. It is also demonstrated that the coiling factor plays a dominant role in determining the property of transmission peak. Our results should contribute to the understanding of the underlying physics of structures by coiling up space and may potentially benefit their practical application.

A broadband acoustic omnidirectional absorber comprising positive-index materials

We propose a broadband acoustic omnidirectional absorber (AOA) made of acoustic metamaterials having a positive index, which guides an incident acoustic wave into a central cavity spirally without backscattering. Numerical simulations show that the AOA has an absorption cross section remarkably larger than its cavity within a broad band, which is of potential practical significance to various applications such as sound absorption and noise control. A possible scheme for the practical realization of the AOA is also briefly discussed.

Acoustic one-way open tunnel by using metasurface

We design and experimentally demonstrate an acoustic tunnel completely open for substances like fluids or other energy fluxes to exchange while allowing sound to pass only in one direction. This significant feature is based on a distinctive mechanism using metasurface pairs to yield asymmetric extraordinary reflections along opposite directions. Theoretical analysis is presented to analytically predict the trajectory of the wave. The experimental results agree well with the numerical results and the theoretical predictions. Our design may pave the way to more versatile acoustic one-way devices with potential applications in many scenarios like duct noise control and ultrasonic therapy.

Ultra-broadband absorption by acoustic metamaterials

We design and experimentally realize an ultra-broad band metamaterial-based acoustic absorption material. Unlike traditional acoustic absorbers, the designed device features a simple configuration unrestricted by the material type and does not require extra sound-absorbing materials, suggesting the potential to have simultaneously structural-stiffness and environmental-friendliness. Analytical analyses are provided to explain such distinct characteristics, which are revealed to stem from the localization and dissipation of waves with different frequencies at particular spatial positions. This is also demonstrated both numerically and experimentally. Our results may offer possible designs for various applications such as noise reduction and making underwater anechoic materials.

Broadband asymmetric acoustic transmission in a gradient-index structure

We propose a scheme of realizing broadband asymmetric acoustic transmission using gradient-index structure, and demonstrate an implementation utilizing phononic-crystal-based metamaterials which can realize an idealized model with desired parameters. The asymmetric transmission is valid within a remarkably broad frequency range. This phenomenon stems from the break of the geometric symmetry of wave trajectories along two opposite directions, essentially different from the mechanisms of previous designs. The numerical results agree well with the analytical predictions using acoustic ray theory. Our design may have potential applications in many fields such as ultrasonic therapy and noise control.