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Analyzing scattered wave to recognize object is of fundamental significance in wave physics. Recently-emerged deep learning technique achieved great success in interpreting wave field such as in ultrasound non-destructive testing and disease diagnosis, but conventionally need time-consuming computer postprocessing or bulky-sized diffractive elements. Here we theoretically propose and experimentally demonstrate a purely-passive and small-footprint meta-neural-network for real-time recognizing complicated objects by analyzing acoustic scattering. We prove meta-neural-network mimics a standard neural network despite its compactness, thanks to unique capability of its metamaterial unit-cells (dubbed meta-neurons) to produce deep-subwavelength phase shift as training parameters. The resulting device exhibits the “intelligence” to perform desired tasks with potential to overcome the current limitations, showcased by two distinctive examples of handwritten digit recognition and discerning misaligned orbital-angular-momentum vortices. Our mechanism opens the route to new metamaterial-based deep-learning paradigms and enable conceptual devices automatically analyzing signals, with far-reaching implications for acoustics and related fields., The authors present a passive meta-neural-network for real-time recognition of objects by analysis of acoustic scattering. It consists of unit cells termed meta-neurons, mimicking an analogous neural network for classical waves, and is shown to recognise handwritten digits and misaligned orbital-angular-momentum vortices.

The radiation of electromagnetic and mechanical waves depends not only on the intrinsic properties of the emitter but also on the surrounding environment. This principle has laid the foundation for the development of lasers, quantum optics, sonar, musical instruments and other fields related to wave–matter interaction. In the conventional wisdom, the environment is defined exclusively by its eigenstates, and an emitter radiates into and interacts with these eigenstates. Here we show experimentally that this scenario breaks down at a non-Hermitian degeneracy known as an exceptional point. We find a chirality-reversal phenomenon in a ring cavity where the radiation field reveals the missing dimension of the Hilbert space, known as the Jordan vector. This phenomenon demonstrates that the radiation field of an emitter can become fully decoupled from the eigenstates of its environment. The generality of this striking phenomenon in wave–matter interaction is experimentally confirmed in both electromagnetic and acoustic systems. Our finding transforms the fundamental understanding of light–matter interaction and wave–matter interaction in general, and enriches the intriguing physics of exceptional points. The modes of the radiation field generated from an emitter are usually determined by the eigenstates of the surrounding environment. However, this scenario breaks down in a non-Hermitian system, at the spectral degeneracy known as an exceptional point.

Making heat stand still Dissipative oscillating systems (waves) can be described mathematically in terms of non-Hermitian physics. When parity-time symmetric systems have dissipative components, the interplay between gain and loss can lead to unusual and exotic behavior. Li et al. show theoretically and demonstrate experimentally that such behavior need not be limited to wave systems. Looking at the diffusion of heat, they devised an experimental setup comprising two thermally coupled disks rotating in opposite directions. The thermal energy transported by each disk is strongly coupled to the disk rotating in the opposite direction, providing a return path for the heat wave. For a particular rotation rate, there is an exceptional point where thermal coupling and counterrotating motion balance, resulting in the thermal energy profile being stationary over time. Science , this issue p. 170

There is increasing interest in improving the freeze-thaw stability of emulsions by interfacial engineering in the food colloid field. The present work reported that for Pickering emulsions stabilized by heat-induced soy protein isolate (SPI) nanoparticles, their freeze-thaw stability could be well modulated by modifying the characteristics of the nanoparticles. The SPI nanoparticles with different characteristics (e.g., particle size, surface hydrophobicity) were obtained by heating the SPI solutions at initial concentrations (ci) of 0.25–6.0 wt% at 95 °C for 15 min. All the initial Pickering emulsions were formed at an oil fraction of 0.4 and protein concentration of 0.25 wt% in the aqueous phase, in the presence of 300 mM NaCl. In general, increasing the ci resulted in a progressive enhancement in freeze-thaw stability of the emulsions against creaming and coalescence, but accelerated the flocculation. The higher creaming stability at higher ci values seemed to be closely associated with the enhanced coalescence stability, as well as the strengthened gel-like network of these initial emulsions. There were close interplays between the freeze-thaw stability and the properties of the initial emulsions, or characteristics (e.g. particle size or surface hydrophobicity) of heat-induced SPI nanoparticles. The results confirmed that the SPI nanoparticles with larger sizes, higher surface hydrophobicity and more efficient packing at interface exhibit a greater potential to produce Pickering emulsions with higher freeze-thaw stability. The findings would be of relevance for understanding the freeze-thaw stability of Pickering emulsions stabilized by protein-based particles, as well as for the development of protein-stabilized emulsion formulations with a high freeze-thaw stability.

One-way sound-wave propagation has been a research hotspot in acoustics during the past decade, with the production of acoustic diode, acoustic topological insulator, and parity-time-symmetric acoustic metamaterial. Here, we investigate topological pumping for sound waves in a coupled cavity chain along selective paths driven by dynamic modulation. We first showcase the adiabatic passage of sound energy in a heterostructured Su-Schrieffer-Heeger cavity chain. By solving the Dirac-like governing equation, we calculate the eigenspectrum of this model and prove the existence of a topologically protected interface state and end states. From the eigenspectrum, if no level crossing exists, energy transfer of the sound wave (or acoustic topological pumping) is reversed under adiabatic modulation, leading to the release and return of sound energy for a specific topological state. However, when the level crossing occurs, we find that topological pumping between the interface state and end states can be selectively excited, rendering complete transfer of the sound energy between two topological states. Moreover, topological pumping of sound waves can exist along the asymmetric paths under driven modulation. Our work has promising applications for robust, selective, and efficient sound energy transfer in acoustic network systems.

Stimulated adiabatic passage has been extensively studied to achieve robust and selective population transfer in quantum systems. Recently, the quantum-classic analogy has been rapidly developing and can be considered responsible for the implementation of the adiabatic transfer of sound energy in cavity chain systems. In this article, we investigate the adiabatic transfer of sound energy between two topological end states in the Su-Schrieffer-Heeger (SSH) cavity chain, which can be considered to be the acoustic analog of the quantum chirped-pulse excitation. The topological adiabatic passage in SSH cavity chain has two categories. When the single-cavity resonance frequencies on the sublattices A and B in the SSH cavity chain do not switch their spectrum positions, the topologically protected adiabatic evolution results in the returning passage of the sound excited in one end cavity. When a level crossing with single-cavity resonance frequencies on the sublattices A and B exhibits switch in the frequency spectrum, acoustic energy is observed to be topologically pumped between the two end cavities of the SSH chain.

Stimulated adiabatic passage, as an efficient energy transfer in correlated systems, was intensively studied in condensed matter physics and quantum optics. Recently, a paradigm shift has been made to bring up quantum-classical analogs, giving rise to the acoustic version of stimulated Raman adiabatic passage and others. In this work, we focus on the adiabatic propagation of sound under topological protection, i.e., topological-pumping-based acoustic adiabatic propagation. We first study the topological pumping of sound in a two-state multicavity chain lattice, including the Su-Schrieffer-Heeger model, in which the acoustic energy is adiabatically transferred between the two topological end states under specific coupling interactions. Then, we consider the adiabatic propagation of sound via a dark mode in a three-state system. The three states correspond to two end states and one interface state in a heterostructured multicavity chain lattice, which is topologically protected. In a finite system, the three states strongly hybridize with the threefold energy degeneracy lifted. The hybridized states reside in the band gap of the systematic Hamiltonian spectrum in the topological pumping process, leading to a strong robustness against nonadiabatic perturbation, which allows for a faster and more efficient topological adiabatic propagation of acoustic waves.

Realizing direction-dependent energy responses greatly benefits the construction of switching and logic devices. This study proposes an archetype for an acoustic resonant-tunneling diode, made of mode-selective resonant metamaterial. Using this material, the authors experimentally demonstrate a broadband, high contrast ratio and single-mode unidirectional sound tunneling. This result is expected to impact control methodology in biomedical ultrasonography, acoustic communication, sound identification, and noise control.

Pouring position as the input heat source has great influence on the temperature field evolution. In this study, the Flow3D simulation software was applied to investigate the influence of pouring methods (with fixed or moving pouring channel) on AZ91 Mg alloy horizontal centrifugal casting (HCC) process. The simulation results show that the moving pouring channel method can effectively increase the cooling rate and formability of casting pipe. The casting experiment shows that an AZ91 Mg alloy casting pipe with homogeneous microstructure and clear contour was obtained by the moving pouring channel method, and the grain size of the casting pipe is significantly decreased. Meanwhile, serious macro-segregation appeared in the AZ91 casting pipe by the fixed pouring channel HCC process. Compared with the fixed pouring channel, the moving pouring channel can remarkably improve the ultimate tensile strength and elongation of the AZ91 HCC pipe from 142.2 MPa to 201.5 MPa and 6.2% to 6.7%, respectively.

We report on the lasing action in weakly modulated parity-time (PT) symmetric lattices. At the exceptional point in PT-symmetric lattices, the amplitude and phase of unidirectionally scattered waves are independently and flexibly determined by the PT modulations, thus, providing a distinctive route to construct a lasing mode in finite systems with weakly modulated structures. It is revealed that the resonant feedback mechanism plays a crucial role in building up the lasing action. Our finding would enrich the singularity dynamics in the weakly modulated PT systems.

Topological insulators continue to be a hot topic in research, particularly for their backscattering immunity. While previous works have restricted consideration to identical primitive unit cells, this work demonstrates an $a\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}d\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c$ valley topological insulator, with robust selection of ${C}_{3v}$ symmetric scatterers in adjacent sonic crystals. This study also presented a functional device, the valley-selective cross-waveguide beam splitter, in the proposed acoustic system. The ability to select scatterers found in this work may illuminate further possibilities for designing acoustic topological-insulator-based devices with flexible structures.

While a wave's spectrum generally can be manipulated by frequency-mixing methods or time-varying perturbations, as done in nonlinear optics, the low efficiency of acoustic nonlinearities makes this quite challenging for sound. Controlling the spectrum of sound is very desirable, though, especially for acoustic communication and voice encryption, where information is usually processed in the frequency domain. Thus the authors create synthetic gauge fields to generate Bloch oscillations of frequency in an acoustic waveguide, in which a time-varying bulk modulus can yield a frequency lattice. This allows $e.g.$ spectrum self-imaging, unidirectional transduction, and bandwidth engineering.

Focused ultrasound is important in clinical medical applications, to destroy tumor cells or treat a variety of neurological diseases, for example. The focused ultrasound field projected by a transducer array includes traditionally unavoidable side lobes, however, which may cause harmful heating in nontargeted regions. This work implements an active acoustic metasurface, comprising 16\ifmmode\times\else\texttimes\fi{}16 elements of subwavelength thickness, which can completely eliminate such side lobes. Experiments convincingly demonstrate that this active metasurface with programmable phase and amplitude distributions offers high performance in side-lobe-proof, steerable ultrasound focusing.

STIRAP (stimulated Raman adiabatic passage) is a powerful laser-based method, usually involving two photons, for efficient and selective transfer of populations between quantum states. A particularly interesting feature is the fact that the coupling between the initial and the final quantum states is via an intermediate state, even though the lifetime of the latter can be much shorter than the interaction time with the laser radiation. Nevertheless, spontaneous emission from the intermediate state is prevented by quantum interference. Maintaining the coherence between the initial and final state throughout the transfer process is crucial. STIRAP was initially developed with applications in chemical dynamics in mind. That is why the original paper of 1990 was published in The Journal of Chemical Physics. However, from about the year 2000, the unique capabilities of STIRAP and its robustness with respect to small variations in some experimental parameters stimulated many researchers to apply the scheme to a variety of other fields of physics. The successes of these efforts are documented in this collection of articles. In Part A the experimental success of STIRAP in manipulating or controlling molecules, photons, ions or even quantum systems in a solid-state environment is documented. After a brief introduction to the basic physics of STIRAP, the central role of the method in the formation of ultracold molecules is discussed, followed by a presentation of how precision experiments (measurement of the upper limit of the electric dipole moment of the electron or detecting the consequences of parity violation in chiral molecules) or chemical dynamics studies at ultralow temperatures benefit from STIRAP. Next comes the STIRAP-based control of photons in cavities followed by a group of three contributions which highlight the potential of the STIRAP concept in classical physics by presenting data on the transfer of waves (photonic, magnonic and phononic) between respective waveguides. The works on ions or ion strings discuss options for applications, e.g. in quantum information. Finally, the success of STIRAP in the controlled manipulation of quantum states in solid-state systems, which are usually hostile towards coherent processes, is presented, dealing with data storage in rare-earth ion doped crystals and in nitrogen vacancy (NV) centers or even in superconducting quantum circuits. The works on ions and those involving solid-state systems emphasize the relevance of the results for quantum information protocols. Part B deals with theoretical work, including further concepts relevant to quantum information or invoking STIRAP for the manipulation of matter waves. The subsequent articles discuss the experiments underway to demonstrate the potential of STIRAP for populating otherwise inaccessible high-lying Rydberg states of molecules, or controlling and cooling the translational motion of particles in a molecular beam or the polarization of angular-momentum states. The series of articles concludes with a more speculative application of STIRAP in nuclear physics, which, if suitable radiation fields become available, could lead to spectacular results., Journal of Physics B: Atomic, Molecular and Optical Physics, 52 (20), ISSN:1361-6455, ISSN:0368-3508, ISSN:0953-4075, ISSN:0022-3700

Recent advanced trackers, consisting of discriminative classification component and dedicated bounding box estimation, have achieved improved performance in the visual tracking community. The most essential factor for the development is the utilization of different Convolutional Neural Networks (CNNs), which significantly improves the model capacity via offline trained deep feature representations. Though powerful deep structures emphasize more semantic appearance through high dimensional latent variables, how to achieve effective feature adaptation in the online tracking stage has not been sufficiently considered yet. To this end, we argue the necessity of exploring hierarchical and complementary appearance descriptors from different convolutional layers to achieve online tracking adaptation. Therefore, in this paper, we propose an adaptive feature fusion mechanism, which can balance the detection granularities from shallow to deep convolutional layers. To be specific, the correlation between template and instance is employed to generate adaptive weights to achieve advanced saliency and discrimination. In addition, considering temporal appearance variation, the projection matrix for the multi-channel inputs is jointly updated with the correlation classifier to further enhance the robustness. The experimental results on four recent benchmarks, i.e., OTB-2015, VOT2018, LaSOT and TrackingNet, demonstrate the effectiveness and robustness of the proposed method, with superior performance compared to the state-of-the-art approaches.

Valley pseudo-spin and its associated interface wave transport in sonic crystals has attracted increasing attention from researchers for the potential manipulation of acoustic waves. The topological interface state, projected from a specific valley, is valley-locked, and, thus, renders robust reflection immunity against defects. In this work, we report on the experimental observation of the different parity generations of interface states at two distinct zigzag interfaces. By designing a “C”-shaped domain wall, we experimentally demonstrated the parity generation and selective excitation of interface valley-locked states. Benefiting from different parities of the interface states, one-way valley parity conversion was verified in sonic crystals without breaking the time reversal symmetry. Our findings contribute to the applications in noise control, acoustic communication, and logic processing for topological functional devices with unidirectional responses.

We have theoretically and experimentally demonstrated subwavelength acoustic energy harvesting via topological interface states in 1D Helmholtz resonator arrays. The system can produce two types of bandgaps in the subwavelength realm due to the resonance and band folding mechanism, respectively. Through the analyses of the Zak phase and eigenfield distributions, the topological properties of band-folding-induced gaps can be changed by tuning the lattice structural parameters, while those of resonant gaps maintain stability. When two different arrays are spliced together, topological interface states will appear in the band-folding-induced band gaps but do not exist in the resonant gaps. The experimental measurements agree well with the theoretical prediction. Our study expands the applicability of acoustic topological insulators in a compacted platform.

In this letter, we study the propagation of acoustic waves through a one-dimensional multilayer structure composed of a thin defect layer sandwiched by two phononic crystals. Two kinds of defect states will generate in band gaps and both of them cause unitary transmission. However, they have very unlike field distributions due to the different contrasted acoustic impedances between the defect layer and its neighboring layers. Spectral positions of transmission peaks can be exactly determined by the resonant phase condition. In a non-dissipative system, these resonant states correspond to single crossing point of two eigenvalues of scattering matrix. When gain and loss are introduced to judiciously construct an acoustic parity-time-symmetric lattice, the crossing point will split into a pair of exceptional points (EPs). Interestingly, the EPs correspond to unidirectional zero reflection that is very sensitive to the thickness of defect layer. Taking advantage of this virtue, a very sensitive acoustic sensor can be designed, which has potentially applications in ultrasonic inspection, noise detection, ultrasonic medicine, etc .

Optical antennas are utilized to link free-space optical radiation with localized energy, enabling varied applications such as beam focusing, freewheeling steering, biosensing, and optical communications. Here, we demonstrate a novel silicon nanoantenna array (SNA), comprising subwavelength silicon nanorods that direct light emission vertically. When the array of nanorods was positioned optimally, the efficiency of the upward light emission reached 46%. In addition, the rigorous design of the array of nanorods resulted in a well-confined profile of the vertically emitting beam. The SNA, therefore, provides a new approach for 3D optical interconnections between different decks of integrated photonics circuits.

Stimulated adiabatic passage utilizes radiation pulses to efficiently and selectively transfer population between quantum states, via an intermediate state that is normally decaying. In this Letter, we propose the analog of stimulated adiabatic passage in an acoustic system. It is realized with cavities that correlate through adiabatically time-varying couplings, where the cavities and time-varying couplings mimic discrete states and radiation pulses, respectively. With appropriate arrangements of coupling actions, an acoustic wave can be efficiently transferred from the initial excited cavity to the target cavity in the forward direction, immune to the intermediate dark cavity. On the other hand, for the backward propagation, the acoustic energy is perfectly localized in the intermediate dark cavity and completely dissipated. We analytically, numerically, and experimentally demonstrate such unidirectional sound localization and unveil the essential role of zero-eigenvalue eigenstates in the adiabatic passage process.

In this work, we investigate comprehensively the unidirectional transport of sound in coupled cavities of a modified honeycomb lattice. The results clearly show that a pair of topological states carrying opposite pseudospins can be constructed at the edge of truncated lattices of non-trivial band gaps, which is different from previous schemes where topological states with opposite pseudospins were constructed at the interface of modified honeycomb lattices of trivial and non-trivial band gaps. Our work paves the way to exploring unidirectional edge transport of sound with topological protection in closed systems that are beneficial in large-scale device integration and low-loss operation.

The fine manipulation of sound fields is critical in acoustics yet is restricted by the coupled amplitude and phase modulations in existing wave-steering metamaterials. Commonly, unavoidable losses make it difficult to control coupling, thereby limiting device performance. Here we show the possibility of tailoring the loss in metamaterials to realize fine control of sound in three-dimensional (3D) space. Quantitative studies on the parameter dependence of reflection amplitude and phase identify quasi-decoupled points in the structural parameter space, allowing arbitrary amplitude-phase combinations for reflected sound. We further demonstrate the significance of our approach for sound manipulation by producing self-bending beams, multifocal focusing, and a single-plane two-dimensional hologram, as well as a multi-plane 3D hologram with quality better than the previous phase-controlled approach. Our work provides a route for harnessing sound via engineering the loss, enabling promising device applications in acoustics and related fields., The formation of true holograms requires control of both amplitude and phase; however, acoustic metamaterials are generally limited to phase control only. Here, Zhu et al. tailor lossy metamaterials to independently control the amplitude and phase of acoustic wavefronts.

Topological elastic metamaterials offer insight into classic motion law and open up opportunities in quantum and classic information processing. Theoretical modeling and numerical simulation of elastic topological states have been reported, whereas the experimental observation remains relatively unexplored. Here we present an experimental observation and numerical simulation of tunable topological states in soft elastic metamaterials. The on-demand reversible switch in topological phase has been achieved by changing filling ratio, tension, and/or compression of the elastic metamaterials. By combining two elastic metamaterials with distinct topological invariants, we further demonstrate the formation and dynamic tunability of topological interface states by mechanical deformation, and the manipulation of elastic wave propagation. Moreover, we provide a topological phase diagram of elastic metamaterials under deformation. Our approach to dynamically control interface states in soft materials paves the way to various phononic systems involving thermal management and soft robotics requiring better use of energy., Here the authors present an experimental observation of topological states in soft elastic metamaterials. They show reversibility in topological phases by changing filling ratio, tension and/or compression, while also demonstrating tunability of topological interface states by mechanical deformation.

Optical interface states exhibit promising applications in nonlinear photonics, low-threshold lasing, and surface-wave assisted sensing. However, the further application of interface states in configurable optics is hindered by their limited tunability. Here, we demonstrate a new approach to generate dynamically tunable and angle-resolved interface states using graphene-embedded photonic crystal (GPC) heterostructure device. By combining the GPC structure design with in situ electric doping of graphene, a continuously tunable interface state can be obtained and its tuning range is as wide as the full bandgap. Moreover, the exhibited tunable interface states offer a possibility to study the correspondence between space and time characteristics of light, which is beyond normal incident conditions. Our strategy provides a new way to design configurable devices with tunable optical states for various advanced optical applications such as beam splitter and dynamically tunable laser.

By using optical phase modulators in a fiber-optical circuit, we theoretically and experimentally demonstrate large control over the spectrum of an impinging signal, which may evolve analogously to discrete diffraction in spatial waveguide arrays. The modulation phase acts as a photonic gauge potential in the frequency dimension, realizing efficient control of the central frequency and bandwidth of frequency combs. We experimentally achieve a 50 GHz frequency shift and threefold bandwidth expansion of an impinging comb, as well as the frequency analogue of various refraction phenomena, including negative refraction and perfect focusing in the frequency domain, both for discrete and continuous incident spectra. Our study paves a promising way towards versatile frequency management for optical communications and signal processing using time modulation schemes.

Inspired by the discovery of novel topological orders of condensed matters, topologically protected edge wave propagation in phononic crystals now is attracting great attention. In this paper, a new kind of solid phononic crystal based on the regular honeycomb lattice with circular rods connected by slender veins is designed. By changing the diameter of the two adjacent circular rods, topological transition has been realized. The vortex chirality of the bulk valley modes with opposite angular momentum is unveiled. Valley dependent edge states and the associated valley-protected backscattering suppression around Z-shape bend waveguide have been demonstrated. In our further work, a real device will be manufactured and tested to verify the results experimentally.

Exploring the concept of non-Hermitian Hamiltonians respecting parity-time symmetry with classical wave systems is of great interest as it enables the experimental investigation of parity-time-symmetric systems through the quantum-classical analogue. Here, we demonstrate unidirectional wave vector manipulation in two-dimensional space, with an all passive acoustic parity-time-symmetric metamaterials crystal. The metamaterials crystal is constructed through interleaving groove- and holey-structured acoustic metamaterials to provide an intrinsic parity-time-symmetric potential that is two-dimensionally extended and curved, which allows the flexible manipulation of unpaired wave vectors. At the transition point from the unbroken to broken parity-time symmetry phase, the unidirectional sound focusing effect (along with reflectionless acoustic transparency in the opposite direction) is experimentally realized over the spectrum. This demonstration confirms the capability of passive acoustic systems to carry the experimental studies on general parity-time symmetry physics and further reveals the unique functionalities enabled by the judiciously tailored unidirectional wave vectors in space.

We report the experimental realization of an acoustic Chern insulator (ACI), by using an angular-momentum-biased resonator array with the broken Lorentz reciprocity. High Q-factor resonance of the constituent rotors is leveraged to reduce the required rotation speed. ACI is a new topological acoustic system analogous to the electronic quantum Hall insulator, based on an effective magnetic field. Experimental results show that the ACI featured with a stable and uniform metafluid flow bias supports one-way nonreciprocal transport of sound at its edges, which is topologically immune to various types of defects. Our work opens up opportunities for exploring unique observable topological phases and developing topological-insulator-based nonreciprocal devices in acoustics.

A sound–structure interaction model is established to study the vibroacoustic characteristics of a semisubmerged cylindrical shell using the wave propagation approach (WPA). The fluid free surface effect is taken into account by satisfying the sound pressure release condition. Then, the far-field sound pressure is predicted with shell's vibration response using the stationary phase method. Modal coupling effect arises due to the presence of the fluid free surface. New approaches are proposed to handle this problem, i.e., diagonal coupling acoustic radiation model (DCARM) and column coupling acoustic radiation model (CCARM). New approaches are proved to be able to deal with the modal coupling problem efficiently with a good accuracy at a significantly reduced computational cost. Numerical results also indicate that the sound radiation characteristics of a semisubmerged cylindrical shell are quite different from those from the shell fully submerged in fluid. But the far-field sound pressure of a semisubmerged shell fluctuates around that from the shell ideally submerged in fluid. These new approaches can also be used to study the vibroacoustic problems of cylindrical shells partially coupled with fluid.

We investigate analytically, numerically, and experimentally the low-loss supermode propagation in a coupled acoustic waveguide complex within a broadband. The waveguide complex is implemented with air channels coupled via an ultrathin metafluid layer. We analytically derive the field distribution of incident sound needed for producing acoustic supermodes, and verify the periodically revival propagation in coupled waveguide systems numerically and experimentally. We find out that the supermode wavelength becomes longer for higher mode order or lower frequency. We have also demonstrated the robust propagation of supermodes in broadband. Our scheme can in principle be extended to three dimensions and the ultrasound regime with simplicity and may promote applications of high-fidelity signal transfer in complicated acoustic networks.

Zero index materials where sound propagates without phase variation, holds a great potential for wavefront and dispersion engineering. Recently explored electromagnetic double zero index metamaterials consist of periodic scatterers whose refractive index is significantly larger than that of the surrounding medium. This requirement is fundamentally challenging for airborne acoustics because the sound speed (inversely proportional to the refractive index) in air is among the slowest. Here, we report the first experimental realization of an impedance matched acoustic double zero refractive index metamaterial induced by a Dirac-like cone at the Brillouin zone centre. This is achieved in a two-dimensional waveguide with periodically varying air channel that modulates the effective phase velocity of a high-order waveguide mode. Using such a zero-index medium, we demonstrated acoustic wave collimation emitted from a point source. For the first time, we experimentally confirm the existence of the Dirac-like cone at the Brillouin zone centre., Impedance mismatch between acoustic metamaterials and a surrounding medium hinders efficient applications, especially for zero-index materials. Here, Dubois et al. utilize the Dirac-like dispersion in a double-zero-index material to overcome this problem and to collimate sound.

In this study, a Dirac cone is yielded at the corner of the Brillouin zone of a two-dimensional solid phononic crystal, which consists of a triangular array of hexagonal steel with a trilobal hole drilled in the center. With the rotation of the trilobal hole, the Dirac degeneracy switches between the deterministic form and accidental form. Every Dirac point is the critical state of a topological phase transition process, which occurs six times within an angular period of 120 ° . This structure offers a flexible way to achieve topological transitions and provides multiple routes to construct an interface that supports valley-dependent edge states between two topologically distinct PCs. The associated backscattering suppressed wave propagation along the multiple curved interface channels is also demonstrated. In addition, our design shows the robust propagation of topological interface states against the perturbation of the structure. This study could potentially be significant in the design of acoustic devices for practical applications, such as acoustic signal lossless transport and tunable multi-channel sound transmission.

Valley states, labeling the frequency extrema in momentum space, carry a new degree of freedom (valley pseudospin) for topological transport of sound in sonic crystals. Recently, the field of valley acoustics has become a hotspot due to its potentials in developing various topological-insulator-based devices. In most previous works, topological valley transport is implemented at the interfaces of two connected artificial crystals. With respect to the interface, the mirror symmetry of crystal structures supports either even-mode or odd-mode valley states. In this work, we propose a physical insight of transforming one hexagonal crystal into a virtual lattice by utilizing the mirror operation of rigid or soft boundaries, which greatly reduces the dimension of the acoustic structure and provides a possible way to implement the programmable routing of topological propagation. We investigate two cases that the rigid and soft boundaries are introduced either at the edge or inside a single hexagonal crystal. Our results clearly demonstrate the high-transmission valley transport along the folded boundaries, where reflection or scattering is prohibited at the sharp bending or corners due to topological protection. Three functional devices are exemplified, which are single-crystal-based topological delay-line filter, delay-line switcher and beam splitter. Our work reveals the inherent relation between the field symmetries of valley states and structural symmetries of sonic crystals. Programmable routing of topological sound transport through boundary engineering provides a platform for developing integrated and versatile topological-insulator-based devices.

Time-reversal invariant topological insulator is widely recognized as one of the fundamental discoveries in condensed matter physics, for which the most fascinating hallmark is perhaps a spin-based topological protection, the absence of scattering of conduction electrons with certain spins on matter surface. Recently, it has created a paradigm shift for topological insulators, from electronics to photonics, phononics and mechanics as well, bringing about not only involved new physics but also potential applications in robust wave transport. Despite the growing interests in topologically protected acoustic wave transport, T-invariant acoustic topological insulator has not yet been achieved. Here we report experimental demonstration of anomalous Floquet topological insulator for sound: a strongly coupled metamaterial ring lattice that supports one-way propagation of pseudo-spin-dependent edge states under T-symmetry. We also demonstrate the formation of pseudo-spin-dependent interface states due to lattice dislocations and investigate the properties of pass band and band gap states., Topological protected acoustic wave propagation has been predicted, but yet awaits for experimental demonstration. Here, Peng et al. report one-way propagation of pseudo-spin-dependent edge states for sound, analogous to Floquet topological insulator in solid state.

Acoustic waves are key to applications involving underwater communication and sonar, for example. Directional sensing of low-frequency (long-wavelength) sound is hampered, however, by the diffraction limit, which would necessitate inconveniently large (and probably immobile) detectors. The authors use the bounded metasurface surrounding a particle's rigid core to modulate acoustic pressures by an order of magnitude. Angle-dependent dipolar resonances allow direction-sensitive detection of plane waves, despite that the structures are far smaller than the wavelength.

The ability to slow down wave propagation in materials has attracted significant research interest. A successful solution will give rise to manageable enhanced wave–matter interaction, freewheeling phase engineering and spatial compression of wave signals. The existing methods are typically associated with constructing dispersive materials or structures with local resonators, thus resulting in unavoidable distortion of waveforms. Here we show that, with helical-structured acoustic metamaterials, it is now possible to implement dispersion-free sound deceleration. The helical-structured metamaterials present a non-dispersive high effective refractive index that is tunable through adjusting the helicity of structures, while the wavefront revolution plays a dominant role in reducing the group velocity. Finally, we numerically and experimentally demonstrate that the helical-structured metamaterials with designed inhomogeneous unit cells can turn a normally incident plane wave into a self-accelerating beam on the prescribed parabolic trajectory. The helical-structured metamaterials will have profound impact to applications in explorations of slow wave physics., There is great interest in slow wave propagation for a variety of applications. Here, Zhu et al. present a dispersion-free helical-structured metamaterial that implements acoustic wave deceleration at broad bandwidth and demonstrates specially designed phase modulation to incident sound through helicity tuning.

The topological sound transport, as an interesting phenomenon discovered in sonic crystals, has drawn tremendous interest in recent years. Here, in resonant acoustic systems, we demonstrate the existence of band inversion by slightly changing the lengths of tube resonators, which unveils the acoustic valley Hall phase transition characterized by the inverted valley Chern number. However, when the valley topological insulator is largely distorted, we can obtain flat-band-like edge states in the bandgaps with topological protection still existing. Those edge states can propagate along zigzag delay-lines with the backscatterings suppressed to a large amount. Our work provides a prototype of topological-insulator-based acoustic devices with the frequency-selecting functionality.

Topological manipulation of sound has recently been a hot spot in acoustics due to the fascinating property of defect immune transport. To the best of our knowledge, the studies on one-dimensional (1D) topological acoustic systems hitherto mainly focus on the case of the Su-Schrieffer-Heeger model. Here, we show that topologically protected bound states may also exist in 1D periodically modulated acoustic waveguide systems, viz., 1D Floquet topological insulators. The results show that tuning the coupling strength in a waveguide lattice could trigger topological phase transition, which gives rise to topologically protected interface states as we put together two waveguide lattices featured with different topological phases or winding numbers. However, for the combined lattice, input at the waveguides other than the interfacial ones will excite bulk states. We have further verified the robustness of interface bound states against the variation of coupling strengths between the two distinct waveguide lattices. This work extends the scope of topological acoustics and may promote potential applications for acoustic devices with topological functionalities.

The study for exotic topological effects of sound has attracted uprising interests in fundamental physics and practical applications. Based on the concept of valley pseudospin, we demonstrate the topological valley transport of plate-mode waves in a homogenous thin plate with periodic stubbed surface, where a deterministic two-fold Dirac degeneracy is form by two plate modes. We show that the topological property can be controlled by the height of stubs deposited on the plate. By adjusting the relative heights of adjacent stubs, the valley vortex chirality and band inversion are induced, giving rise to a phononic analog of valley Hall phase transition. We further numerically demonstrate the valley states of plate-mode waves with robust topological protection. Our results provide a new route to design unconventional elastic topological insulators and will significantly broaden its practical application in the engineering field.

Anomalous Floquet topological insulators (AFIs) for airborne sound have recently been realized in experiments. However, the implemented version suffers from significant loss and narrowband due to thermal viscosity and dispersive coupling strength between unit-cells. Here, we propose a solution for realizing low-loss and broadband acoustic AFI. We show that the loss after passing through one unit-cell can be less than 2% for the topological edge states. It is also theoretically unveiled that in the frequency range of nearly unitary coupling (∼0.97 from 4.8 kHz to 7.0 kHz in our case), around 84% corresponds to topological bands. Our proposal may promote the application of large-dimension acoustic topological devices.

We have constructed an interface which separates two different phononic crystals (PCs) with respectively effective negative density and negative bulk modulus through band inversion. Besides the eigenstates in weak localization stemming from the sign flipping of imaginary acoustic impedances at the interface, we observed an unusual type of strongly localized states at the band-inverting contact after a periodic lattice dislocation is purposely introduced. From the layered multiple scattering theory, we have uncovered that the underlying physics for these unique interface states in the hetero-structured PC are due to nontrivial constructive interferences of high-ordered Mie-scattered acoustic waves from the mismatched cylinders. The intriguing features include interface resonances of enormous quality factors (∼3×104) and chiral field patterns along the dislocation line. We envision potential applications of the work in slow sound trapping, notch filtering, and nonlinearity strengthening, etc.