Observation of Coherent Perfect Acoustic Absorption at an Exceptional Point.

Non-Hermitian systems have recently shown new possibilities to manipulate wave scattering by exploiting loss, yet coherent perfect absorption at an exceptional point (CPA EP) remains elusive in acoustics. Here, we demonstrate it based on a two-channel waveguide with compact lossy resonators. We realize imbalanced losses crucial for CPA EP by using active components to independently modulate the non-Hermiticity.

ℤ-Classified Topological Phases and Bound States in the Continuum Induced by Multiple Orbitals.

ℤ-classified higher-order topological insulators (HOTIs) with chiral-symmetric higher-order topological phases protected by multipole chiral numbers (MCNs) have attracted extensive interest recently. However, how to design artificial ℤ-classified HOTIs with multiple topological phases remains an unresolved issue. Here, multiorbital degrees of freedom are introduced to acoustic crystals and the various methods of topological phase transitions are achieved for the orbital ℤ-classified HOTIs.

Local-Nonlinearity-Enabled Deep Subdiffraction Control of Acoustic Waves.

Diffraction sets a natural limit for the spatial resolution of acoustic wave fields, hindering the generation and recording of object details and manipulation of sound at subwavelength scales. We propose to overcome this physical limit by utilizing nonlinear acoustics. Our findings indicate that, contrary to the commonly utilized cumulative nonlinear effect, it is in fact the local nonlinear effect that is crucial in achieving subdiffraction control of acoustic waves.

Remote Water-to-Air Eavesdropping with a Phase-Engineered Impedance Matching Metasurface.

Efficiently receiving underwater sound remotely from air is a long-standing challenge in acoustics hindered by the large impedance mismatch at the water-air interface. Here, a phase-engineered water-air impedance matching metasurface is proposed and experimentally demonstrated for remote and efficient water-to-air eavesdropping. The judiciously designed metasurface with near-unity transmission efficiency, long monitoring distance, and high mechanical stiffness is capable of making the water-air interface acoustically transparent and, at the same time, freewheelingly patterning the transmitted wavefront.

Efficient and High-Purity Sound Frequency Conversion with a Passive Linear Metasurface.

Despite the significance for wave physics and potential applications, high-efficiency frequency conversion of low-frequency waves cannot be achieved with conventional nonlinearity-based mechanisms with poor mode purity, conversion efficiency, and real-time reconfigurability of the generated harmonic waves in both optics and acoustics. Rotational Doppler effect provides an intuitive paradigm to shifting the frequency in a linear system which, however, needs a spiral-phase change upon the wave propagation. Here a rotating passive linear vortex metasurface is numerically and experimentally presented with close-to-unity mode purity (>93%) and high conversion efficiency (>65%) in audible sound frequency as low as 3000 Hz.

Metamaterial-based real-time communication with high information density by multipath twisting of acoustic wave.

Speeding up the transmission of information carried by waves is of fundamental interest for wave physics, with pivotal significance for underwater communications. To overcome the current limitations in information transfer capacity, here we propose and experimentally validate a mechanism using multipath sound twisting to realize real-time high-capacity communication free of signal-processing or sensor-scanning. The undesired channel crosstalk, conventionally reduced via time-consuming postprocessing, is virtually suppressed by using a metamaterial layer as purely-passive demultiplexer with high spatial selectivity.

Experimental Realization of Weyl Exceptional Rings in a Synthetic Three-Dimensional Non-Hermitian Phononic Crystal.

Weyl points-topological monopoles of quantized Berry flux-are predicted to spread to Weyl exceptional rings in the presence of non-Hermiticity. Here, we use a one-dimensional Aubry-Andre-Harper model to construct a Weyl semimetal in a three-dimensional parameter space comprising one reciprocal dimension and two synthetic dimensions. The inclusion of non-Hermiticity in the form of gain and loss produces a synthetic Weyl exceptional ring (SWER).

Tunable acoustic metasurface based on tunable piezoelectric composite structure.

Due to the potential engineering needs, the passive tunable metasurfaces with a high performance equivalent to the active phased array is worthy of research. Here, a passive ultrathin metasurface unit composed of a piezoelectric composite structure (PCS) connected to an external capacitor, which can modulate the phase of the transmitted acoustic waves at a deep subwavelength scale only by controlling the external capacitor but without changing the structure, is proposed. Then, a tunable acoustic metasurface composed of 20 identical PCSs is introduced to realize three acoustic functions, beam steering, beam focusing, and tweezer-like beam generating, just by changing the external capacitors.

Helical Higher-Order Topological States in an Acoustic Crystalline Insulator.

The topological states in quantum Hall insulators and quantum spin Hall insulators that emerge helical are considered nondissipative. However, in crystalline systems without spin-orbit couplings, the existing higher-order topological states are considered not helical, and the energy suffers from dissipation during propagation. In this work, by introducing the intrinsic pseudospin degree of freedom, we theoretically and experimentally present the existence of the helical higher-order topological states in the C_{6}-symmetric topological crystalline insulators based on the acoustic samples.

Generation of Non-aliased Two-dimensional Acoustic Vortex with Enclosed Metasurface.

Two-dimensional (2D) acoustic vortex allows new physics and applications different from three-dimensional counterparts, yet existing mechanisms usually have to rely on active array composed of transducers which may result in complexity, high cost and, in particular, undesired spatial aliasing effect. We propose to generate 2D acoustic vortex inside an enclosed metasurface illuminated by axisymmetric wave carrying no orbital angular momentum. We derive the criterion on unit size for eliminating spatial aliasing effect which is challenging for conventional active approaches and design a membrane-based metasurface to implement our mechanism.

Ultrathin Acoustic Parity-Time Symmetric Metasurface Cloak.

Invisibility or unhearability cloaks have been made possible by using metamaterials enabling light or sound to flow around obstacle without the trace of reflections or shadows. Metamaterials are known for being flexible building units that can mimic a host of unusual and extreme material responses, which are essential when engineering artificial material properties to realize a coordinate transforming cloak. Bending and stretching the coordinate grid in space require stringent material parameters; therefore, small inaccuracies and inevitable material losses become sources for unwanted scattering that are decremental to the desired effect.

Ultrathin Planar Metasurface-based Acoustic Energy Harvester with Deep Subwavelength Thickness and Mechanical Rigidity.

Despite the growing attentions dedicated to the harvesting of acoustic energy that is a clean and renewable yet usually wasted energy source, the long wavelength of airborne sound still poses fundamental limits on the miniaturization of harvester devices and hinders practical applications. Here we present an ultrathin and planar acoustic energy harvester with rigidity. We propose a distinctive metasurface-based mechanism that reduces the effective wavelength to produce extraordinarily strong local energy within deep-subwavelength dimension and enable high-efficiently harvesting energy of incident airborne sound with considerably long wavelength.

Illusion for Airborne Sound Source by a Closed Layer with Subwavelength Thickness.

The past decade witnesses considerable efforts to design acoustic illusion cloak that produces the desired scattered field for a specific object illuminated by an external field. Yet the possibility of generating acoustic illusion directly for a sound source still remains unexplored despite the great fundamental and practical significance, and previous transformation acoustics-based designs need to have bulky sizes in terms of working wavelength. Here we propose to produce arbitrary illusion for an airborne sound source with no need to resort to coordinate transformation method.

Twisted Acoustics: Metasurface-Enabled Multiplexing and Demultiplexing.

Metasurfaces are used to enable acoustic orbital angular momentum (a-OAM)-based multiplexing in real-time, postprocess-free, and sensor-scanning-free fashions to improve the bandwidth of acoustic communication, with intrinsic compatibility and expandability to cooperate with other multiplexing schemes. The metasurface-based communication relying on encoding information onto twisted beams is numerically and experimentally demonstrated by realizing real-time picture transfer, which differs from existing static data transfer by encoding data onto OAM states. With the advantages of real-time transmission, passive and instantaneous data decoding, vanishingly low loss, compact size, and high transmitting accuracy, the study of a-OAM-based information transfer with metasurfaces offers new route to boost the capacity of acoustic communication and great potential to profoundly advance relevant fields.

Convert Acoustic Resonances to Orbital Angular Momentum.

We use acoustic resonances in a planar layer of half-wavelength thickness to twist wave vectors of an in-coming plane wave into a spiral phase dislocation of an outgoing vortex beam with orbital angular momentum (OAM). The mechanism is numerically and experimentally demonstrated by producing an airborne Bessel-like vortex beam. Our acoustic resonance-based OAM production differs from existing means for OAM production by enormous phased spiral sources or by elaborate spiral profiles.

Broadband non-reciprocal transmission of sound with invariant frequency.

We design and experimentally demonstrate a broadband yet compact acoustic diode (AD) by using an acoustic nonlinear material and a pair of gain and lossy materials. Due to the capabilities of maintaining the original frequency and high forward transmission while blocking backscattered wave, our design is closer to the desired features of a perfect AD and is promising to play the essential diode-like role in realistic acoustic systems, such as ultrasound imaging, noise control and nondestructive testing. Furthermore, our design enables improving the sensitivity and the robustness of device simultaneously by tailoring an individual structural parameter.

Dispersionless Manipulation of Reflected Acoustic Wavefront by Subwavelength Corrugated Surface.

Free controls of optic/acoustic waves for bending, focusing or steering the energy of wavefronts are highly desirable in many practical scenarios. However, the dispersive nature of the existing metamaterials/metasurfaces for wavefront manipulation necessarily results in limited bandwidth. Here, we propose the concept of dispersionless wavefront manipulation and report a theoretical, numerical and experimental work on the design of a reflective surface capable of controlling the acoustic wavefront arbitrarily without bandwidth limitation.