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 Snell's 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.

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.

We report the experimental demonstration of broadband negative refractive index obtained in a labyrinthine acoustic metamaterial structure. Two different approaches were employed to prove the metamaterial negative index nature: one-dimensional extractions of effective parameters from reflection and transmission measurements, and two-dimensional prism-based measurements that convincingly show the transmission angle corresponding to negative refraction. The transmission angles observed in the latter case also agree very well with the refractive index obtained in the one-dimensional measurements and numerical simulations. We expect this labyrinthine metamaterial to become the unit cell of choice for practical acoustic metamaterial devices that require broadband and significantly negative indexes of refraction.

Acoustic tweezers have recently raised great interest across many fields including biology, chemistry, engineering, and medicine, as they can perform contactless, label-free, biocompatible, and precise manipulation of particles and cells. Here, we present wave number–spiral acoustic tweezers, which are capable of dynamically reshaping surface acoustic wave (SAW) wavefields to various pressure distributions to facilitate dynamic and programmable particle/cell manipulation. SAWs propagating in multiple directions can be simultaneously and independently controlled by simply modulating the multitone excitation signals. This allows for dynamic reshaping of SAW wavefields to desired distributions, thus achieving programmable particle/cell manipulation. We experimentally demonstrated the multiple functions of wave number–spiral acoustic tweezers, among which are multiconfiguration patterning; parallel merging; pattern translation, transformation, and rotation; and dynamic translation of single microparticles along complex paths. This wave number–spiral design has the potential to revolutionize future acoustic tweezers development and advance many applications, including microscale assembly, bioprinting, and cell-cell interaction research.

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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.

An impedance matched surface is able, in principle, to totally absorb the incident sound and yield no reflection, and this is desired in many acoustic applications. Here we demonstrate a design of impedance matched sound absorbing surface with a simple construction. By coupling different 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.