Shasha Peng

Asymmetric acoustic gratings

The unidirectional transmission of acoustic waves is realized by a simple geometrically asymmetric steel grating structure. This exotic phenomenon stems from the one-way diffraction effect induced by the different periods of the slits on the both surfaces of the sample. And the frequency range of unidirectional transmission is simply determined by the structure periods. The experimental results agree well with the theoretical simulation. This remarkable effect is expected potential applications in ultrasonic devices, such as acoustic rectifiers and acoustic diodes.

Acoustic Transmission Enhancement through a Periodically Structured Stiff Plate without Any Opening

We report both experimentally and theoretically that enhanced acoustic transmission can occur in the subwavelength region through a thin but stiff structured plate without any opening. This exotic acoustic phenomenon is essentially distinct from the previous related studies originated from, either collectively or individually, the interaction of the incident wave with openings in previous structures. It is attributed to the structure-induced resonant excitation of the nonleaky Lamb modes that exist intrinsically in the uniform elastic plate. Our finding should have an impact on ultrasonic applications.We report an observation of the extraordinary high reflection of acoustic waves in water by thin epoxy plates partitioned by subwavelength cuts, whereas such plates without structure are acoustically-transparent as the acoustic properties of epoxy are close to water. It is demonstrated that this exotic phenomenon results from the resonant excitation of the local modes within the individual pieces derived by the cuts. The experiment agrees well with the theory.

Acoustic far-field focusing effect for two-dimensional graded negative refractive-index sonic crystals

Focusing effect is experimentally observed for acoustic plane wave normally incident onto a two-dimensional sonic crystal with gradient negative refractive index. The gradual refractive-index is achieved by gradual modification of the lattice spacing both along the transverse and longitudinal directions. It is found that the focal length is controllable by modulation of the lattice spacing. The experiment results are in excellent agreement with theoretical calculation by a multiple scattering theory method.

Acoustic wave transmission through a bull's eye structure

We study experimentally and theoretically acoustic transmission through a bull’s eye structure, consisting of a central hole with concentric grooves imprinted on both sides of a thin brass plate. At wavelength slightly larger than the groove periodicity, a transmission peak was observed for normally incident acoustic wave, with excellent collimation (only ±2° divergence) at far field. This phenomenon is a manifestation of the two-dimensional circular version of structure-factor induced resonant transmission. Theoretical predictions based on this mechanism are in good agreement with the experiments.

Experimental demonstration of directional acoustic radiation based on two-dimensional phononic crystal band edge states

The authors have experimentally studied the radiation of a point acoustic source placed inside a two-dimensional phononic crystal of square lattice. They show that a highly directional radiation with a half-power angular width of 6° can be achieved when operating at the band edge frequency for the phononic crystal. Such combination of a point source and a phononic crystal may serve as highly directional acoustic source in applications.

Making sound vortices by metasurfaces

Based on the Huygens-Fresnel principle, a metasurface structure is designed to generate a sound vortex beam in airborne environment. The metasurface is constructed by a thin planar plate perforated with a circular array of deep subwavelength resonators with desired phase and amplitude responses. The metasurface approach in making sound vortices is validated well by full-wave simulations and experimental measurements. Potential applications of such artificial spiral beams can be anticipated, as exemplified experimentally by the torque effect exerting on an absorbing disk.

Extraordinary acoustic reflection enhancement by acoustically transparent thin plates

We report an observation of the extraordinary high reflection of acoustic waves in water by thin epoxy plates partitioned by subwavelength cuts, whereas such plates without structure are acoustically transparent as the acoustic properties of epoxy are close to water. It is demonstrated that this exotic phenomenon results from the resonant excitation of the local modes within the individual pieces derived by the cuts. The experiment agrees well with the theory. Potential applications of such simple structures can be anticipated, e.g., in blocking sound.

Extraordinary acoustic shielding by a monolayer of periodical polymethyl methacrylate cylinders immersed in water

We study, both experimentally and numerically, the acoustic transmission through a monolayer of periodical polymethyl methacrylate cylinders immersed in water. Beyond our expectation, nearly-total reflection is observed for the system, consisting of two ingredients with low impedance contrast. Our investigation manifests that this extraordinary acoustic shielding mostly stems from the resonant excitation of the localized Stoneley surface waves in individual cylinders. Such local modes are rooted in the complicated coupling between the longitudinal and transverse waves and are unique in acoustic systems.

Acoustic collimating beams by negative refraction in two-dimensional phononic crystal

We report the realization of acoustic collimating beams by a two-dimensional phononic crystal, which has a negative effective refractive index of small magnitude. The radiation far field of a point source through the phononic crystal with a divergence angle of about 5° is obtained. The measured intensity field distribution in experiment further confirms this collimating property.

Experimental demonstration of surface acoustic waves in two-dimensional phononic crystals with fluid background

The surface acoustic waves of a two-dimensional phononic crystal in water are investigated experimentally. By measuring the field distribution on the surface of the phononic crystal, a surface acoustic mode is clearly demonstrated. By introducing a periodical corrugation onto the surface of the phononic crystal and by a pointlike stimulation on the surface, a collimated acoustic beam with a small divergent angle is observed, which results from the coupling of the point source to the surface modes. This experimental realization may be of significance in the applications of surface acoustic waves in phononic crystals.