Yangtao Ye

Anomalous refraction of airborne sound through ultrathin metasurfaces

Similar to their optic counterparts, acoustic components are anticipated to flexibly tailor the propagation of sound. However, the practical applications, e.g. for audible sound with large wavelengths, are frequently hampered by the issue of device thickness. Here we present an effective design of metasurface structures that can deflect the transmitted airborne sound in an anomalous way. This flat lens, made of spatially varied coiling-slit subunits, has a thickness of deep subwavelength. By elaborately optimizing its microstructures, the proposed lens exhibits high performance in steering sound wavefronts. Good agreement has been demonstrated experimentally by a sample around the frequency 2.55 kHz, incident with a Gaussian beam at normal or oblique incidence. This study may open new avenues for numerous daily life applications, such as controlling indoor sound effects by decorating rooms with light metasurface walls.

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.

Subwavelength imaging by a simple planar acoustic superlens

We experimentally realized a planar acoustic superlens by a simple structure consisting of periodically arrayed rigid slabs. Based on the excitation of guided modes and the moderate amplification of the evanescent waves, we have broken the diffraction limit and obtained a subwavelength image with a half-power beamwidth of 0.07λ. All the experimental results are in good agreement with the theoretical simulations via finite difference time domain method.

Broadband asymmetric acoustic transmission by a plate with quasi-periodic surface ridges

In this paper, an acoustic system with broadband asymmetric transmission is designed and fabricated, which consists of a water-immersed aluminum plate engraved with quasi-periodically-patterned ridges on single surface. It demonstrates that when the acoustic waves are launched into the system from the structured side, they can couple into the Lamb modes in the plate efficiently and attain a high transmission; on the contrary, when the waves are incident from the opposite flat side, the coupling is weak, and the transmission is low. Superior to systems with periodic patterning, this quasi-periodically-patterned system has a broad working frequency range due to the collective contributions from the multiple diffractions specific to the structure.

Transmission enhancement of acoustic waves through a thin hard plate embedded with elastic inclusions

We study the transmission response of acoustic waves through a water-immersed thin hard plate embedded with elastic inclusions. The transmission spectra show a striking peak at the subwavelength region, whose position is almost irrelevant with the incident angle. Our study states that the transmission enhancement stems from the resonant excitation of the localized bending mode in the inclusion. Different from the well-known Fabry-Perot effect, here the resonant frequency reduces rapidly as the sample is thinned down. The theory is further validated by experiment. Potential applications of the abnormal transmission effect produced by such thin plate structures can be anticipated.

Exotic acoustic transmission through hard plates perforated with quasiperiodic subwavelength apertures

In this work, we report the exotic acoustic transmission behavior through hard plates perforated with a quasiperiodic array of subwavelength apertures. The phenomenon is verified to be originated from the coherent diffraction induced by the long-range order of the system. The experimental results agree well with the numerical ones. We also present comparative studies on the periodical and random systems to reveal the intrinsic physics in a universal way.

Focusing of spoof surface-acoustic-waves by a gradient-index structure

We have investigated the focusing of the spoof surface acoustic waves (SSAWs) on a rigid surface engraved with array of cylindrical holes. To focus the SSAWs launched along x direction, the holes array, with a shape of rectangle as a whole, is fabricated to possess gradient index of refraction along both the +y and −y directions, which is achieved by varying linearly the radii of the holes along the corresponding directions. Good focusing performance has been demonstrated experimentally, which is in good agreement with the numerical simulations. This acoustic system is believed to have potential applications in acoustic field such as high resolution detecting.

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.

Transversal Anderson localization of sound in acoustic waveguide arrays

We present designs of one-dimensional acoustic waveguide arrays and investigate wave propagation inside. Under the condition of single identical waveguide mode and weak coupling, the acoustic wave motion in waveguide arrays can be modeled with a discrete mode-coupling theory. The coupling constants can be retrieved from simulations or experiments as the function of neighboring waveguide separations. Sound injected into periodic arrays gives rise to the discrete diffraction, exhibiting ballistic or extended transport in transversal direction. But sound injected into randomized waveguide arrays readily leads to Anderson localization transversally. The experimental results show good agreement with simulations and theoretical predictions.