Wenqi Wang

Wavefront modulation and subwavelength diffractive acoustics with an acoustic metasurface

Metasurfaces are a family of novel wavefront-shaping devices with planar profile and subwavelength thickness. Acoustic metasurfaces with ultralow profile yet extraordinary wave manipulating properties would be highly desirable for improving the performance of many acoustic wave-based applications. However, designing acoustic metasurfaces with similar functionality to their electromagnetic counterparts remains challenging with traditional metamaterial design approaches. Here we present a design and realization of an acoustic metasurface based on tapered labyrinthine metamaterials. The demonstrated metasurface can not only steer an acoustic beam as expected from the generalized Snells law, but also exhibits various unique properties such as conversion from propagating wave to surface mode, extraordinary beam-steering and apparent negative refraction through higher-order diffraction. Such designer acoustic metasurfaces provide a new design methodology for acoustic signal modulation devices and may be useful for applications such as acoustic imaging, beam steering, ultrasound lens design and acoustic surface wave-based applications.

Broadband Acoustic Hyperbolic Metamaterial.

In this Letter, we report on the design and experimental characterization of a broadband acoustic hyperbolic metamaterial. The proposed metamaterial consists of multiple arrays of clamped thin plates facing the y direction and is shown to yield opposite signs of effective density in the x and y directions below a certain cutoff frequency, therefore, yielding a hyperbolic dispersion. Partial focusing and subwavelength imaging are experimentally demonstrated at frequencies between 1.0 and 2.5 kHz. The proposed metamaterial could open up new possibilities for acoustic wave manipulation and may find usage in medical imaging and nondestructive testing.

Design and demonstration of broadband thin planar diffractive acoustic lenses

We present here two diffractive acoustic lenses with subwavelength thickness, planar profile, and broad operation bandwidth. Tapered labyrinthine unit cells with their inherently broadband effective material properties are exploited in our design. Both the measured and the simulated results are showcased to demonstrate the lensing effect over more than 40% of the central frequency. The focusing of a propagating Gaussian modulated sinusoidal pulse is also demonstrated. This work paves the way for designing diffractive acoustic lenses and more generalized phase engineering diffractive elements with labyrinthine acoustic metamaterials.

Tunable Asymmetric Transmission via Lossy Acoustic Metasurfaces

In this study, we show that robust and tunable acoustic asymmetric transmission can be achieved through gradient-index metasurfaces by harnessing judiciously tailored losses. We theoretically prove that the asymmetric wave behavior stems from loss-induced suppression of high order diffraction. We further experimentally demonstrate this novel phenomenon. Our findings could provide new routes to broaden applications for lossy acoustic metamaterials and metasurfaces.

Acoustic Holographic Rendering with Two-dimensional Metamaterial-based Passive Phased Array

Acoustic holographic rendering in complete analogy with optical holography are useful for various applications, ranging from multi-focal lensing, multiplexed sensing and synthesizing three-dimensional complex sound fields. Conventional approaches rely on a large number of active transducers and phase shifting circuits. In this paper we show that by using passive metamaterials as subwavelength pixels, holographic rendering can be achieved without cumbersome circuitry and with only a single transducer, thus significantly reducing system complexity. Such metamaterial-based holograms can serve as versatile platforms for various advanced acoustic wave manipulation and signal modulation, leading to new possibilities in acoustic sensing, energy deposition and medical diagnostic imaging.

A low‐frequency near‐field interferometric‐TOA 3‐D Lightning Mapping Array

We report on the development of an easily deployable LF near-field interferometric-time of arrival (TOA) 3-D Lightning Mapping Array applied to imaging of entire lightning flashes. An interferometric cross-correlation technique is applied in our system to compute windowed two-sensor time differences with submicrosecond time resolution before TOA is used for source location. Compared to previously reported LF lightning location systems, our system captures many more LF sources. This is due mainly to the improved mapping of continuous lightning processes by using this type of hybrid interferometry/TOA processing method. We show with five station measurements that the array detects and maps different lightning processes, such as stepped and dart leaders, during both in-cloud and cloud-to-ground flashes. Lightning images mapped by our LF system are remarkably similar to those created by VHF mapping systems, which may suggest some special links between LF and VHF emission during lightning processes.

Subwavelength diffractive acoustics and wavefront manipulation with a reflective acoustic metasurface

Acoustic metasurfaces provide useful wavefront shaping capabilities, such as beam steering, acoustic focusing, and asymmetric transmission, in a compact structure. Most acoustic metasurfaces described in the literature are transmissive devices and focus their performance on steering sound beam of the fundamental diffractive order. In addition, the range of incident angles studied is usually below the critical incidence predicted by generalized Snells law of reflection. In this work, we comprehensively analyze the wave interaction with a generic periodic phase-modulating structure in order to predict the behavior of all diffractive orders, especially for cases beyond critical incidence. Under the guidance of the presented analysis, a broadband reflective metasurface is designed based on an expanded library of labyrinthine acoustic metamaterials. Various local and nonlocal wavefront shaping properties are experimentally demonstrated, and enhanced absorption of higher order diffractive waves is experimentally shown for the first time. The proposed methodology provides an accurate approach for predicting practical diffracted wave behaviors and opens a new perspective for the study of acoustic periodic structures. The designed metasurface extends the functionalities of acoustic metasurfaces and paves the way for the design of thin planar reflective structures for broadband acoustic wave manipulation and extraordinary absorption.

Anisotropic acoustic metafluid for underwater operation

The paper presents a method to design and characterize mechanically robust solid acoustic metamaterials suitable for operation in dense fluids such as water. These structures, also called metafluids, behave acoustically as inertial fluids characterized by anisotropic mass densities and isotropic bulk modulus. The method is illustrated through the design and experimental characterization of a metafluid consisting of perforated steel plates held together by rubber coated magnetic spacers. The spacers are very effective at reducing the effective shear modulus of the structure, and therefore effective at minimizing the ensuing coupling between the shear and pressure waves inside the solid effective medium. Inertial anisotropy together with fluid-like acoustic behavior are key properties that bring transformation acoustics in dense fluids closer to reality.

A broadband acoustic absorber based on phase-modulating reflective metasurfaces

Most high performance acoustic absorbers exhibit outstanding absorption capabilities at resonant frequencies, thus their bandwidth are significantly limited. Acoustic metasurfaces enabled unprecedented wave manipulation with their planar profiles, subwavelength thicknesses, and large degree of design freedom. However, studies have been primarily focused on extraordinary wavefront shaping functionalities, and little explorations has been made on manipulating transmitted/reflected power of diffracted beams. By carefully tailoring the design and arrangement of acoustic metamaterial cells, enhanced absorption can be achieved for a broad range of frequencies. Building on our previous works on transmissive and reflective metasurfaces with exotic properties, including anomalous refraction, surface mode conversion, and extraordinary beam-steering (Xie et al. Nat. Commun. 2014), we present in this work a broadband acoustic absorber built with an expanded library of labyrinthine acoustic metamaterials. We demonstra...

A sound absorbing metasurface with coupled resonators

An impedance matched surface is able, in principle, to totally absorb the incident sound and yield no reaction, and this is desired in many acoustic applications. Here, we demonstrate a new design of impedance matched sound absorbing surface with a simple construction. By coupling di�erent resonators and generating a hybrid resonance mode, we designed and fabricated a metasurface that is impedance-matched to airborne sound at tunable frequencies with subwavelength scale unit cells. With careful design of the coupled resonators, over 99% energy absorption at central frequency of 511 Hz with a 50% absorption bandwidth of 140 Hz is achieved experimentally. The proposed design can be easily fabricated and is mechanically stable. The proposed metasurface can be used in many sound absorption applications such as loudspeaker design and architectural acoustics.