Zhaojian He

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.

Theoretical study of subwavelength imaging by acoustic metamaterial slabs

We investigate theoretically subwavelength imaging by acoustic metamaterial slabs immersed in the liquid matrix. A near-field subwavelength image formed by evanescent waves is achieved by a designed metamaterial slab with negative mass density and positive modulus. A subwavelength real image is achieved by a designed metamaterial slab with simultaneously negative mass density and modulus. These results are expected to shed some light on designing novel devices of acoustic metamaterials.

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.

Subwavelength imaging of acoustic waves by a canalization mechanism in a two-dimensional phononic crystal

In this letter, the subwavelength imaging of acoustic waves is reported based on a mechanism that the evanescent modes of a source are canalized by the Bloch modes of a two-dimensional phononic crystal that served as the lens. The phononic crystal was designed to have a thickness that meets the condition of Fabry–Perot resonance in order to enhance wave transmission and hence to improve imaging performance. Numerical simulations demonstrated that for a point acoustic source an image as small as 0.16λ can be formed.

High refractive-index sonic material based on periodic subwavelength structure

We show that a steel plate with periodic array of subwavelength slits or holes can serve as a material of tunable refractive index for acoustic waves. The effective refractive index is inversely proportional to the filling factor of the slits or holes, which enables the realization of high refractive-index material for acoustic waves. An acoustic wave focusing lens based on this material is exemplified.

Graded negative index lens with designable focal length by phononic crystal

We realize a flat lens with graded negative refractive index by a two-dimensional phononic crystal. The index-grade is achieved by gradual modification of the filling fraction along the transverse direction to propagation. We demonstrate that this lens can realize the focusing and amplification of parallel incident acoustic waves. A formula for the refractive index profile is derived by which the lens with the desired focal length can be precisely designed and the aberrations of image can be effectively reduced. This designable lens is expected to have significance in applications such as coupling or integration with various types of acoustic devices.

Acoustic trapping of particle by a periodically structured stiff plate

We present a study on the acoustic radiation forces exerted on a cylindrical particle near the surface of a periodically structured brass plate. When resonance of the structured plate occurs, this configuration shows an interesting trapping effect, which essentially arises from the gradient force induced by gradient vortex velocity field near the surface. This artificial structure for providing a geometrically modulated trapping force may be of interest for acoustic manipulation and sorting in various complex mechanical systems.

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.

Negative-dynamic-mass response without localized resonance

We propose a new mechanism to achieve negative-dynamic-mass response. The system consists of a water-immersed soft solid plate patterned with periodical heavy gratings. The negative dynamic mass stems from collective excitations of the coupled acoustic surface waves in the thin plate, but not from local resonances as usual. A simple spring-mass model is further applied to understand this exotic phenomenon. Extensive applications of the new mechanism can be anticipated, such as in designing acoustic metamaterials with negative indices.