Third-order square-root topological insulators on decorated diamond sonic crystals.

The square-root operation can generate novel topological phases, whose nontrivial topological properties are inherited from the parent Hamiltonian. Here we report the acoustic realization of third-order square-root topological insulators by adding additional resonators between the site resonators of original diamond lattice. Due to the square-root operation, multiple acoustic localized modes appear in doubled bulk gaps. The bulk polarizations of the tight-binding models are employed to reveal the topological feature of the higher-order topological states. By tuning the coupling strength, we find the emergence of third-order topological corner states in doubled bulk gaps on tetrahedron-like and rhombohedron-like sonic crystals, respectively. The shape dependence of square-root corner states provides an extra degree of freedom for flexible manipulation on the sound localization. Furthermore, the robustness of the corner states in three-dimensional (3D) square-root topological insulator is well elucidated by introducing random disorders into the irrelevant bulk region of the proposed 3D lattices. This work extends square-root higher-order topological states into 3D system, and may find possible applications in selective acoustic sensors.

How I Speak Defines What I Do: Effects of the Functional Language Proficiency of Host Country Employees on Their Unethical Pro-organizational Behavior.

Functional language has been used in many multinational corporations (MNCs) as a way to overcome the problems caused by the coexistence of multiple languages in the workplace. The existing literature has explored the importance, adoption, and effectiveness of functional language. Yet, how functional language shapes host country employees' moral cognition and behavior is insufficiently researched. Guided by the Social Identity Theory, this manuscript shows that host country employees' functional language proficiency (i.e., English) enhances their unethical pro-organizational behavior through their linguistic group identification and moral disengagement. We tested our predictions using the data collected from 309 full-time host country employees through an online survey, and the results generally supported our hypotheses. The findings make contributions to both international management and language literature and organizational moral behavior literature.

Efficient realization of on-demand functional ultrasonic fields based on prolate spheroidal wave functions from sampling theorem.

The past decades have witnessed great efforts in the on-demand ultrasonic field design in which the time reversal technology was widely used in the whole-space acoustic hologram. In practice, the acoustic field of interest is usually bounded in a finite region with flexible distribution. Here, the use of prolate spheroidal wave functions to generate an arbitrary ultrasonic field in a finite region is proposed. The prolate spheroidal functions, which form a complete set of band limited functions and are orthogonal in the infinite and finite regions, can be efficiently reconstructed by the sampling theorem. To display the validation of the proposed method, two types of functional ultrasonic fields are numerically simulated. One type is the ultrasound standing wave field for which six nodes and two nodes are separately realized for two different types of standing waves in the limited range of (-2λ, 2λ). In addition, a composite standing wave field is stimulated with more complicated nodal distributions. The other type is the ultrasound focusing field, where three focal spots with the mainlobe sizes of λ, 0.5λ, and 0.35λ are demonstrated. It is worth noting that the nontrivial side lobes for super-oscillation focusing are designed to be about 3λ away from the central focal spot (the mainlobe size 0.35λ). This work has much significance in the applications of acoustic tweezing, ultrasonic imaging, and treatment.

Experimental realization of ultrasonic retroreflection tweezing via metagratings.

Acoustic tweezing and contactless object manipulation are significant and has become a research hotspot. Here, we present a prototype of retroreflection tweezer based on the metagratings, which can be used for implementing particle levitation and contactless movement. Different from previous tweezers, where the standing wave was typically generated from normal incidence, the retroreflection tweezer proposed here operates for the oblique incidence case that provides more degrees of freedom for real-time observation. We also give the exact relation between the metagrating geometries and retroreflection angles in a wide range based on the grating theory and optimization algorithm. Experiments are conducted to show the retroreflection at a preset angle on the designed metagrating, where levitation and contactless movement of particles are demonstrated.

An Acoustic Meta-Skin Insulator.

Acoustic metamaterials with artificial microstructures are attractive to realize intriguing functions, including efficient waveguiding, which requires large impedance mismatches to realize total side reflection with negligible transmission and absorption. While large impedance mismatch can be readily realized in an air environment, acoustic waveguiding in an underwater environment remains elusive due to insufficient impedance mismatch of state-of-the-art metamaterials. Here, a superhydrophobic acoustic metasurface of microstructured poly(vinylidene fluoride) membrane, referred to as a "meta-skin" insulator, which is able to confine acoustic waves in an all-angle and wide spectrum range due to tremendous impedance mismatch at stable air/water interfaces, viz., the Cassie-Baxter state is demonstrated. By utilizing the meta-skin insulator with broadband and high throughput, orbital-angular-momentum multiplexing at a high spectral efficiency and binary coding along large-angle bending channels for bit-error-free acoustic data transmission in an underwater environment are demonstrated. Very different from optical and/or electrical cable communications, acoustic waves can be simply and effectively coupled into remote meta-skin acoustic fibers from free space, which is technologically significant for long-haul and anti-interference communication. This work can enlighten many fluidic applications based on efficient waveguiding, such as in vivo ultrasound medical treatment and imaging.

Super-resolution acoustic image montage via a biaxial metamaterial lens.

Ever since the Victorian era, montage, the process of pictorial composition made by juxtaposing or superimposing photographs, has been a very popular post-editing imaging technique. Despite showing a strong power in demonstrating complex wave field effects, this technique has neither been fully explored in acoustic imaging nor been realized in real-time systems with the capability beyond diffraction limits. On the other hand, the recent prospect of metamaterials has shown their great potentials in super-resolution acoustic imaging. However, the miracle jigsaw of more advanced functional modulation of acoustic wave fields at deep subwavelength scale still remains elusive. Here we report the experimental implementation of super-resolution acoustic image montage through a judiciously designed biaxial metamaterial lens. Based on the non-diffraction birefringence in the biaxial metamaterials, we realized various montage functionalities such as duplication, composition, and decomposition of sound images with distinctive deep subwavelength features. Our work represents an important step in developing versatile functional acoustic metamaterial devices for imaging purposes, as it provides on-demand editing of sound field patterns beyond diffraction limits.

Ultrasonic super-oscillation wave-packets with an acoustic meta-lens.

The Schrödinger equation is a fundamental equation to describe the wave function of a quantum-mechanical system. The similar forms between the Schrödinger equation and the paraxial wave equation allow a paradigm shift from the quantum mechanics to classical fields, opening up a plethora of interesting phenomena including the optical super-oscillatory behavior. Here, we propose an ultrasonic meta-lens for generating super-oscillation acoustic wave-packets with different spatial momenta and then superimposing them to a diffraction-limit-broken spot, visually represented by the ring-shaped trapping of tiny particles. Moreover, based on the focused super-oscillation packets, we experimentally verify proof-of-concept super-resolution ultrasound imaging, opening up the arena of super-oscillation ultrasonics for advanced acoustic imaging, biomedical applications, and versatile far-field ultrasound control.

One-Way Localized Adiabatic Passage in an Acoustic System.

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.

Mirror-symmetry induced topological valley transport along programmable boundaries in a hexagonal sonic crystal.

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.

Topologically protected edge transport of sound in coupled cavities of a modified honeycomb lattice.

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.

Observation of low-loss broadband supermode propagation in coupled acoustic waveguide complex.

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.

Experimental demonstration of anomalous Floquet topological insulator for sound.

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.