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Rainbow trapping is a significant strategy for capturing waves of different frequencies at different spatial positions. It offers significant potential for multi-frequency on-chip nanophotonic devices in the era of big data and has attracted much interest recently, and topological rainbow trapping that is immune to disorder has been observed in optics and acoustics. Reported here is the topological rainbow trapping of elastic waves using gradient valley phononic crystal plates (PCPs) with a hexagonal lattice, based on valley edge states that are localized at the boundary between two PCPs with opposite valley topological phases. The gradient valley PCPs are achieved by gradually adjusting the height of the scatterers or the thickness of the PCPs. The topological rainbow trapping of elastic waves is shown by applying vertical vibration to the studied gradient PCPs, which are simple in structure and easily fabricated, providing a good solution for manipulating and trapping elastic waves. [ABSTRACT FROM AUTHOR]

Anomalies in reflection from periodic structures, well known in optics, result from either the transition from specular reflection to diffraction of light (Rayleigh's anomaly) or by coupling to the leaky surface mode supported by the structure (Wood's anomaly). Experimental observation and detailed analysis of both kinds of anomalies are presented here in the case of antisymmetric Lamb wave (A0) incidence at the 1D periodic structure located along the edge of the elastic plate. A structure with a strong phononic effect has been chosen, which enables the separation of both effects in terms of frequency. Another distinct feature of elastic waves in the periodic structure considered in this work is the existence of several localized modes, which leads to the multiple Wood's anomalies. Here, we report the experimental observation of double Wood's anomaly in addition to the Rayleigh's type of anomaly. [ABSTRACT FROM AUTHOR]

Static superlattices that do not break time-reversal symmetry can support robust topologically protected elastic waves with non-zero amplitude in the forward propagating direction but zero amplitude in the opposite direction. We form a prototypical acoustic wave device by sandwiching a finite superlattice that supports one-way propagating waves between input and detector layers. Compared to conventional elastic waves, topologically protected waves provide a significant benefit for reducing the return loss of the prototypical device. Superlattices supporting topologically protected acoustic waves provide attractive and disruptive solutions for designing the next-generation of low-loss acoustic wave devices for telecommunication or sensing. [ABSTRACT FROM AUTHOR]

In this paper, a kind of broadband elastic wave coding metasurfaces is proposed. The metastructure unit designed by topology optimization is used as logical unit "1," and the pure plate is set as logical unit "0." The phase difference of the transmitted longitudinal wave between two units is introduced as the objective function in optimization. The transmittance of the designed unit is kept above 60%, and the transmitted phase difference between the two coding units is approximately π in the broadband frequency range. In addition, a set of achromatic focusing coding units is designed topologically to achieve customized dispersion. By coding the logical units, the designed metasurface can control the elastic wave between frequencies of 20.4 and 23.6 kHz, and wave performances, such as beam focusing and splitting, are achieved. The experimental results of the designed metasurface are in good agreement with numerical simulations. Therefore, our design has potential applications in the fields of engineering such as nondestructive testing. [ABSTRACT FROM AUTHOR]

Skin effect is one of the intriguing phenomena exhibited by non-Hermitian wave systems. It reflects the localization of the modes at the boundaries of the structure. We demonstrated the skin effect for elastic waves propagating in a non-Hermitian phononic plate containing piezoelectric components in their unit cells. The latter behave as sensors and actuators by using the direct and inverse piezoelectric effects. The demonstration is based on the calculation of the complex non-reciprocal dispersion curves and their analysis for any direction of the wavevector in the two-dimensional space. Therefore, localization phenomena at different boundaries and corners of a finite square structure are presented. Furthermore, by applying different levels of non-Hermiticity in different parts of a square structure, it is shown that the localized features can appear at different positions and with various shapes. These localized phenomena can be reconfigured by acting on the non-Hermiticity parameters. Our results provided a feedback control strategy to introduce the non-Hermitian skin effect in two-dimensional elastic systems for potential applications, such as vibration control, energy harvesting, and sensing. [ABSTRACT FROM AUTHOR]

We report a topology-transformable resonator with two distinct stable states, one kinematically determinate and endowed with nearly rigid-body motion at low-frequencies, and the other accompanied by a floppy pseudo zero-energy mode capable of showing low-frequency local resonance. Through a combination of numerical simulations and experiments, we unveil the role of contact-induced topological transformation, a phenomenon that empowers the resonator with negative dynamic effective mass. We demonstrate that the bistable resonator can be embedded into elastic metamaterials to enable in situ switch of local resonance, allowing on-demand augmentation and attenuation of elastic wave propagation within a prescribed regime of frequency. [ABSTRACT FROM AUTHOR]

Architected metamaterials have emerged as a central topic in materials science and mechanics, thanks to the rapid development of additive manufacturing techniques, which have enabled artificial materials with outstanding mechanical properties. This Letter seeks to investigate the elastodynamic behavior of octet truss lattices as an important type of architected metamaterials for high effective strength and vibration shielding. We design, fabricate, and experimentally characterize three types of octet truss structures, including two homogenous structures with either thin or thick struts and one hybrid structure with alternating strut thickness. High elastic wave transmission rate is observed for the lattice with thick struts, while strong vibration mitigation is captured from the homogenous octet truss structure with thin struts as well as the hybrid octet truss lattice, though the underlying mechanisms for attenuation are fundamentally different (viscoelasticity induced dampening vs bandgaps). Compressional tests are also conducted to evaluate the effective stiffness of the three lattices. This study could open an avenue toward multifunctional architected metamaterials for vibration shielding with high mechanical strength. [ABSTRACT FROM AUTHOR]

Materials with grain contacts or partially closed cracks exhibit anomalous elastic behavior: hysteresis in quasi-static experiments and slow dynamics in fast dynamic ones. Albeit the behavior in the two cases (which correspond to very different strain ranges) appears different, it should stem from the same physics and, thus, could be modeled by a universal equation of state. We propose a modification of the standard acoustoelastic theory, introducing the concept of conditioning induced non-equilibrium strain, which allows us to predict the evolution of elastic wave velocity in both quasi-static and dynamic ranges, including the velocity anisotropy induced by external uniaxial loading. [ABSTRACT FROM AUTHOR]

Nanomechanics, nanoacoustics, and nanophononics refer to the engineering of acoustic phonons and elastic waves at the nanoscale and their interactions with other excitations, such as magnons, electrons, and photons. This engineering enables the manipulation and control of solid-state properties that depend on the relative positions of atoms in a lattice. The access to advanced nanofabrication and novel characterization techniques enabled a fast development of the fields over the last decade. The applications of nanophononics include thermal management, ultrafast data processing, simulation, sensing, and the development of quantum technologies. In this review, we cover some of the milestones and breakthroughs and identify promising pathways of these emerging fields. [ABSTRACT FROM AUTHOR]

What makes elastic waves different from other waves is the existence of various wave modes and coupling between these modes. Accordingly, the manipulation of elastic waves suffers from many limitations due to mode coupling, which is an inherent complex physical property of elastic waves. In this study, we propose fluid-like elastic metasurfaces that act as an acoustic (fluid) surface to perfectly eliminate mode coupling. Because longitudinal and shear waves are decoupled, only reflected longitudinal (or shear) waves exist when longitudinal (or shear) waves are incident. Using a strip-type unit cell, elastic metasurfaces mimicking acoustic hard-wall and soft-wall were designed and realized. In addition, numerical analysis and experiments were conducted to prove the validity of the designed unit cells. This study presents a more versatile metasurface by solving the mode coupling of solid elastic waves. In addition, two types of designed fluid-like metasurfaces are expected to be utilized in further studies considering the opposite phase shift characteristic. [ABSTRACT FROM AUTHOR]

We report on time-domain thermoreflectance measurements of cross-plane thermal conductivity of In0.63Ga0.37As/In0.37Al0.63As superlattices with interface densities ranging from 0.0374 to 2.19 nm−1 in the temperature range 80–295 K. The measurements are complemented by a three-dimensional finite-difference time-domain solution to the elastic wave equation, in which the rms roughness and correlation length at heterointerfaces are varied, and the parameters yielding best agreement with experiment are determined using machine learning. Both experimental measurements and simulations demonstrate the existence of a minimum in the cross-plane thermal conductivity as a function of interface density, which is evidence of a crossover from incoherent to coherent phonon transport as the interface density increases. This minimum persists with increasing temperature, indicating the continued dominance of the temperature-independent interface and alloy-disorder scattering over the temperature-dependent three-phonon scattering in thermal transport through III–V alloy superlattices. [ABSTRACT FROM AUTHOR]

Dissipation alone can produce counterintuitive topological wave transport that is otherwise absent in a non-dissipative system. This work demonstrates the influence of dissipation on degeneracies that arise in the context of elastic wave transport. The conditions on the parameters necessary to observe non-Hermitian degeneracies in the Bloch spectrum are precisely derived. It will be shown—contrary to the expectation from singularity theory of a linear eigenvalue problem—that a proportionally damped system with commutative damping does not exhibit non-Hermitian degeneracies. The necessity of a non-commutative and non-proportional dissipation model to observe non-Hermitian degeneracies (or exceptional points) is emphasized. Non-proportional dissipation is used to induce a non-Hermitian degeneracy in a local resonance sub-Bragg bandgap of a linear chain, without using negative damping. While Bloch waves are chosen to illustrate the influence of dissipation, the results readily extend to waves in non-periodic media as well as other wave and vibration transport problems. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate the capability of architected plates, with a frame-like cellular structure, to inhibit the propagation of elastic flexural waves. By leveraging the octet topology as a unit cell to design the tested prototypes, a broad and easy-to-tune bandgap is experimentally generated. The experimental outcomes are supported by extensive numerical analyses based on 3D solid elements. Drawing from the underlying dynamic properties of the octet cell, we numerically propose a tailorable design with enhanced filtering capabilities. We transform the geometry of the original unit cell by applying a uniaxial scaling factor that, by breaking the in-plane symmetry of the structure, yields independently tuned struts and consequently multiple tunable bandgaps within the same cell. Our findings expand the spectrum of available numerical analyses on the octet lattice, taking it a significant step closer to its physical implementation. The ability of the octet lattice to control the propagation of flexural vibrations is significant within various applications in the mechanical and civil engineering domains, and we note such frame-like designs could lead to advancements in energy harvesting and vibration protection devices (e.g., lightweight and resonance-tunable absorbers). [ABSTRACT FROM AUTHOR]

Weyl points (WPs) have been experimentally observed in optics and air-borne acoustic crystals. Although elastic Weyl physics has been theoretically studied, there is no experimental evidence to date. In this paper, WPs in the elastic wave field are investigated and realized in a synthetic three-dimensional (3D) space by one-dimensional (1D) phononic crystals with acoustic black hole structures. The synthetic 3D space constitutes one physical dimension and two geometrical parameters. Results show that the topological edge states and interface states can be directly observed. Furthermore, some WPs are experimentally visualized in an elastic wave system and the measured data are in good agreement with numerical predictions. This opens a new pathway for manipulating 1D elastic waves in an extraordinary way. [ABSTRACT FROM AUTHOR]

Klein tunneling is an intriguing phenomenon in quantum mechanics, which refers to the unity transmission of particles through a higher energy barrier without hindrance in the case of normal incidence. The introduction of phononic crystals enables the direct observation of this phenomenon from a different point, giving rise to a broader application prospect. In this paper, we aim at realizing the Klein tunneling for elastic plate waves. To this end, we combine two kinds of Y-shaped phononic crystals with different Dirac point energies to structure a sandwich-like plate, which supports the propagation of Lamb waves while artificially introducing a potential barrier. Under such a configuration, perfect wave transmission regardless of the barrier in a wide frequency range can be realized, which also enables the numerical prediction and experimental observation of the Klein tunneling phenomenon in Lamb waves. The results of this work provide a direction and reference for the design and applications of phononic crystal devices, such as the chip-scale elastic waveguides. [ABSTRACT FROM AUTHOR]

In last two decades, it has been theoretically and experimentally demonstrated that seismic metamaterials are capable of isolating seismic surface waves. Inertial amplification mechanisms with small mass have been proposed to design metamaterials to isolate elastic waves in rods, beams, and plates at low frequencies. In this Letter, we propose an alternative type of seismic metamaterial providing an ultra-low-frequency bandgap induced by inertial amplification. A unique kind of inertially amplified metamaterial is first conceived and designed. Its bandgap characteristics for flexural waves are then numerically and experimentally demonstrated. Finally, the embedded inertial amplification mechanism is introduced on a soil substrate to design a seismic metamaterial capable of strongly attenuating seismic surface waves around a frequency of 4 Hz. This work provides a promising alternative way to conceive seismic metamaterials to steer and control surface waves. [ABSTRACT FROM AUTHOR]

Through a combination of analytical, numerical, and experimental methods, we study a three-dimensional metamaterial with the ability to attenuate both airborne sound and mechanical vibrations, simultaneously, and in all directions. In addition, due to the auxetic nature of the design (i.e., having a negative Poisson's ratio), the metamaterial can shrink (or expand) in a relatively uniform manner, without buckling. We utilize an external load to cause a systematic shape change in the metamaterial and tune the attenuation frequency bands. The presented design principles can be utilized in many applications related to acoustic and elastic wave manipulation as well as acoustic devices. [ABSTRACT FROM AUTHOR]

The Willis material model, coupling kinetic energy with a potential one, is shown to equip an extraordinary capacity in characterizing complex acoustic and elastic wave phenomena of metamaterials. This model has been further extended to active systems via breaking the symmetry between two coupling coefficients, leading to odd or non-reciprocal Willis material models [Quan et al., Nat. Commun. 12(1), 2615 (2021)]. In this work, through a 2D homogenous non-reciprocal acoustic Willis material (NRAWM), we demonstrate that the bulk local mode, referred to as skin effect in non-Hermitian systems, can survive on boundary of NRAWMs under proper conditions. The direction of the localization is closely related to the intrinsic direction embedded in the NRAWMs, and the localization is robust and topologically protected. To validate the prediction, a 2D discrete lattice made of non-local active acoustic scatterers is proposed and then homogenized as a NRAWM based on the retrieval method. The far-field radiation patterns of the local modes for both the 2D discrete lattice and the homogenized 2D NRAWM are evaluated, and they are in good agreement with each other. This work paves the way to design and explore the rich wave phenomena in non-Hermitian acoustic systems. [ABSTRACT FROM AUTHOR]

Non-Hermitian systems have gained a great deal of interest in various wave problems due their ability of exhibiting unprecedented phenomena such as invisibility, cloaking, enhanced sensing, or the skin effect. The latter manifests itself by the localization of all bulk modes in a specific frequency range at a given boundary, with an unconventional bulk-boundary correspondence. In this work, we propose to realize the skin effect for flexural waves in a non-Hermitian piezoelectric phononic beam with feedback control between a sensor and an actuator in each unit cell. By implementing a non-Hermitian parameter, effective gain and loss can be achieved in the phononic beam characterized by complex eigen frequencies, and non-reciprocal pass bands are obtained. We highlight that the split point separating the gain and loss areas can occur not only at the edges of the Brillouin zones but also inside the same Brillouin zone. We further analyze the influence of the geometric and non-Hermitian parameters on the complex dispersions and the split point. The topology of the complex bands is characterized by the winding number, which supports the skin effect together with the non-reciprocity. The localization degree of the skin mode manifested by the enhanced beam's vibration energy at one boundary is related to the strength of the non-reciprocity, and the skin mode can be always excited regardless of the source position. Our results provide a potential platform to introduce non-Hermiticity into phononic or metamaterial systems with novel functions for elastic waves such as topological insulators, vibration attenuation or amplification, and energy harvesting. [ABSTRACT FROM AUTHOR]

Metasurfaces exhibiting spatially asymmetric inner structures have been shown to host unidirectional scattering effects, benefiting areas where directional control of waves is desired. In this work, we propose a non-Hermitian planar elastic metasurface to achieve unidirectional focusing of flexural waves. The unit cells are constructed by piezoelectric disks and metallic blocks that are asymmetrically loaded. A tunable material loss is then introduced by negative capacitance shunting. By suitably engineering the induced loss profile, a series of unit cells are designed, which can individually access the exceptional points manifested by unidirectional zero reflection. We then construct a planar metasurface by tuning the reflected phase to ensure constructive interference at one side of the metasurface. Unidirectional focusing of the incident waves is demonstrated, where the reflected wave energy is focused from one direction, and zero reflection is observed in the other direction. The proposed metasurface enriches the flexibility in asymmetric elastic wave manipulation as the loss and the reflected phase can be tailored independently in each unit cell. [ABSTRACT FROM AUTHOR]

Nonreciprocal mechanical devices are of great interest for directional elastic wave manipulation. In this Letter, we introduce a design of a compact low-frequency nonreciprocal metamaterial for flexural waves, whose dimension is less than 1 / 3 of the operating wavelength. This structure is made of two well-placed coil-cantilever-magnet resonators, where the electromagnetic forces can be temporally modulated, which enables time varying of the effective stiffness of the resonators. A phase shift is introduced between the stiffness modulations of these two resonators, which breaks the time-reversal symmetry and enables nonreciprocal wave propagation at the resonance frequency of the structure. A semi-analytical method based on harmonic wave decomposition is developed to describe the system, leading to results that match well with numerical predictions from a finite element method. We also experimentally demonstrate nonreciprocal flexural wave propagation with good agreement with the predictions made. Our system could inspire the design of compact nonreciprocal devices for flexural waves. [ABSTRACT FROM AUTHOR]

Recently, concepts from topological physics have been used to achieve exceptional elastic wave transport phenomena in 3D mechanical systems. Although interesting, the previously studied 3D topological elastic structures are fixed after fabrication and thus cannot adapt to changes in the external environment or operating conditions. This lack of reconfigurability limits potential performance and functionality. In this Letter, we advance the state of the art by proposing a programmable 3D topological metamaterial that can be tuned adaptively via carefully designed metastability. A metastable switching methodology is developed that enables the synthesis of multiple unique topological states in a 3D mechanical structure. By taking advantage of the adaptivity of the proposed metamaterial, the path of 2D topological surface states is on-demand controlled, a bilayer-locked topological state is discovered, and 3D elastic wave networks with advanced filtering and splitting capabilities are realized. The findings presented in this Letter offer insight into how topology can be used to control the flow of energy in 3D elastic structures. Furthermore, the advanced functionalities of the proposed metamaterial could be harnessed to create intelligent and robust devices for various purposes, such as mechanologic, vibration mitigation, energy harvesting, and remote sensing. [ABSTRACT FROM AUTHOR]

For systems that require complete metallic enclosures (e.g., containment buildings for nuclear reactors), it is impossible to access interior sensors and equipment using standard electromagnetic techniques. A viable way to communicate and supply power through metallic barriers is the use of elastic waves and ultrasonic transducers, introducing several design challenges that must be addressed. Specifically, the use of multiple communication channels on the same enclosure introduces an additional mechanism for signal crosstalk between channels: guided waves propagating in the barrier between channels. This work numerically and experimentally investigates a machined phononic crystal to block MHz Lamb wave propagation between ultrasonic communication channels, greatly reducing wave propagation and the resulting crosstalk voltage. Blind grooves are machined into one or both sides of a metallic barrier to introduce a periodic unit cell, greatly altering the guided wave dispersion in the barrier. Numerical simulations are used to determine a set of groove geometries for testing, and experiments were performed to characterize the wave-blocking performance of each design. The best-performing design was tested using piezoelectric transducers bonded to the barrier, showing a 14.4 dB reduction in crosstalk voltage. The proposed periodic grooving method is a promising technique for completely isolating ultrasonic power/data transfer systems operating in a narrow frequency range. [ABSTRACT FROM AUTHOR]

Metamaterials, together with transformation optics, have been exploited to study the effects of curved spacetime predicted by Einstein's general theory of relativity in the laboratory. However, the stringent requirements in metamaterial fabrications and the narrow operating frequency make their realistic implementations challenging. In this work, we propose and demonstrate experimentally a paradigm to mimic the gravitational lensing effect using flexural waves in an elastic plate without using metamaterials. The needed refractive index field is fulfilled just by slightly varying the local thickness of plates. The generated focusing effect has a broad band from 5 to 100 kHz and shows robustness against changes on structural shapes and thicknesses of the lens. This work provides insight to study nontrivial gravitational phenomena analogically using conventional solid structures and may suggest a possible route to regulate elastic waves arbitrarily. [ABSTRACT FROM AUTHOR]

Rapid advances have been made in the properties of metastructures, including phononic crystals, metamaterials, and metasurfaces, with respect to their robustness, high power density, and broadband behavior. This presents unique advantages for energy harvesting devices to meet the demands for technology upgrades in various applied science and engineering applications such as wireless sensors, MEMS, and wearable devices. We provide a systematic overview of the mechanisms underlying the above properties, considering different configurations of wave localization in elastic metastructures for energy harvesting purposes. The energy localization can result from cavity modes, topologically protected edge/cavity modes, chiral edge mode, focusing effects, among others. Finally, we give the prospects of energy harvesting metastructures such as the introduction of nonlinear design, time-modulation, machine learning inverse design, and nonlocal effects. [ABSTRACT FROM AUTHOR]

We studied the formation and attenuation of GHz elastic waves in free-standing nanoporous gold films by MeV ultrafast electron diffraction and finite element simulations. By tracing the evolution of the high frequency acoustic waves in time domain, we found that the bicontinuous nanoporous structure in nanoporous gold films results in three-dimensionally acoustic response with low coherence, leading to fast attenuation of the elastic waves in comparison with solid gold films. The morphologically dominated dynamics indicates the nanoporosity plays an important role in the high-frequency acoustic energy relaxation, which shines a light on the applications of dealloyed nanoporous materials in nanodevices and sensors as GHz and THz acoustic filters and dampers. [ABSTRACT FROM AUTHOR]

Space folding techniques based on non-monotonic transforms lead to a new class of 2D isotropic cloaks with a constant negative shear modulus and a spatially varying negative density for antiplane elastic waves. We consider an external cloak consisting of a core with positive shear modulus and density, and a shell with simultaneously negative shear modulus and density. Such a core–shell resonant system creates a virtual folded region outside the shell. To handle such negative physical parameters in the time-domain, a two-step strategy is used: (i) assuming resonant (Drude and Lorentz-types) effective parameters in the frequency-domain; (ii) returning to the time-domain by applying the formalism of the auxiliary fields. We numerically show that, at the designed central frequency, scattering of a cylindrical antiplane elastic wave incident upon a finite set of small clamped obstacles is drastically reduced after a lapse of time, when they are placed in the close neighborhood of the external cloak. However, cloaking efficiency decreases when the source is placed in the intense near field of the external cloak and moreover at short times, cloaking breaks down. [ABSTRACT FROM AUTHOR]

Topological insulators have attracted intensive attention due to their robust properties of path defect immunity, with diverse applications in electromagnetic, acoustic, and elastic systems. The recent development of elastic topological insulators (ETIs), based on artificially structured phononic crystals, has injected new momentum into the manipulation of elastic waves. Earlier ETIs with unreconfigurable geometry and narrow frequency bandgaps hinder the exploration and design of adaptable devices. In this work, a tunable phononic crystal plate with Y-shaped prisms is designed to support valley transport of elastic waves, based on the analogy of the quantum valley Hall effect. By rotating the prisms to reconstruct the configuration, the mirror symmetry is broken to open a new bandgap. Based on this characteristic, we design an interface between two ETIs with different symmetry-broken geometries, which supports topologically protected edge states. We further design a reconfigurable device for elastic wave channel switching and beam splitting and demonstrate it both numerically and experimentally. In addition, in order to meet the requirement of the wide frequency range, the genetic algorithm is adopted to optimize the geometry so as to achieve the broadband valley transportation of elastic waves. The results obtained in this paper can promote the practical applications of tunable broadband elastic wave transmission. [ABSTRACT FROM AUTHOR]

Spin current phenomena are at the heart of the active research field of spintronics that aims to develop new perspectives for emerging information technologies. In recent years, several groups reported experiments in which spin currents are used to excite coherent magnetization dynamics in magnetic nanostructures. Here, we show experimentally two effects of the large spin current generated by the giant spin Hall effect in a platinum strip with nanoscopic silver particles adjacent to a film of the insulating ferrimagnet yttrium iron garnet (YIG). The first, demonstrated by ferromagnetic resonance (FMR) experiments, is the dramatic reduction of the magnon damping measured by the FMR linewidth due to the spin torque produced by the spin current. The second, observed by Brillouin light scattering (BLS), is the excitation of quasi-particles in the YIG film with frequencies that do not vary with the applied magnetic field. We interpret the BLS signal as due to phonons excited by the magnonic spin current injected into the YIG film, in a process that is the Onsager reciprocal of the spin pumping by coherent elastic waves. [ABSTRACT FROM AUTHOR]

The ability to confine light to subwavelength scales has led to exciting developments in fields ranging from sensing to single molecule chemistry. In this paper, we demonstrate how arrays of annular hole resonators can be used to confine surface acoustic waves (SAWs) to regions of the propagating surface that are much smaller than the wavelength of the SAWs. These microscopic elastic waves are used in devices for signal processing and sensing and, increasingly, in areas such as quantum information and microfluidics. This work potentially transforms the ability to enhance interactions with SAWs and could pave the way for advances that mirror those in nano-photonics. [ABSTRACT FROM AUTHOR]

Achieving total mode conversion from longitudinal to shear waves for a broad incident angle has been a big scientific challenge in elastic fields, which was impossible to be achieved in classical elastic wave theory. In this paper, we propose and realize a refractive transmodal elastic metasurface that can convert an incident longitudinal wave to a shear wave for a broad incident angle. Here, the total mode conversion is achieved via a sufficiently large phase gradient, while the full transmission is achieved with the impedance-matched single-layered metasurface. Numerical and experimental investigations show that the proposed metasurface can provide almost total mode conversion for a broad incident angle from −20.4 ° to 22.3 °. We expect that the proposed refractive transmodal metasurface can be applied in various ultrasonic systems. [ABSTRACT FROM AUTHOR]

We present an evaluation of attenuation of elastic waves in the GHz range, based on the decay of thickness-stretch resonances of plates. We measured the elastic response of micro-scale aluminum plates, using a laser-ultrasound technique. The thermo-elastic laser excitation provides significant coupling into thickness-stretch modes in the frequency range above 1.5 GHz. To suppress interference from other resonant and transient modes, we used an excitation spot size well above the plate thickness and applied signal processing in the time and frequency domain. We adapted existing theory on the decay of thickness-shear modes to apply for thickness-stretch modes, validated the derived theory with simulations, and applied it to experiments. A set of samples with different thicknesses in the range of 1.1–2.15 μm allowed us to obtain damping values in the corresponding frequency range of 1.5–3 GHz. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate that a rainbow-based metamaterial, created by a graded array of resonant rods attached to an elastic beam, operates as a mechanical delay-line by slowing down surface elastic waves to take advantage of wave interaction with resonance. Experiments demonstrate that the rainbow effect reduces the amplitude of the propagating wave in the host structure. At the same time, it dramatically increases both the period of interaction between the waves and the resonators and the wavefield amplitude in the rod endowed with the harvester. Increased energy is thus fed into the resonators over time: we show the enhanced energy harvesting capabilities of this system. [ABSTRACT FROM AUTHOR]

Mechanical waves exhibit complex propagations and waveform transitions on the interfaces of different media. By attaching an ultra-thin layer including local resonators on a solid panel, we demonstrate that this ultra-thin layer contains two control paths for solid and air, manipulating elastic and sound waves on the solid–air interface, respectively. Owing to simultaneously breaking the spatial symmetry of the solid panel and adjusting the boundary impedance of the solid–air interface, the proposed ultra-thin layer combines three operating states that are unique to each of the existing mechanical wave metamaterials. Specifically, we observe the "sound state" for intensely reflecting sound waves at the local anti-resonance, the "elastic state" for completely preventing elastic waves at the local resonance, and the "transition state" for totally converting elastic waves into sound waves at the entire monopole-type resonance. Hence, we denote such an ultra-thin layer as the "hybrid metamaterial." Our work may broaden the way of designing multifunctional materials and devices for manipulating mechanical waves. [ABSTRACT FROM AUTHOR]

Superfocusing of acoustic and elastic waves is generally achieved by the combination of negative refraction and the enhancement of the evanescent waves. Here, we numerically and experimentally demonstrate the bifunctionality of a superlens that can simultaneously focus acoustic and flexural waves beyond the diffraction limit. The designed structure is composed of a two-dimensional arrangement of pillars that act as rigid scatterers for the sound waves and as resonant scatterers for the flexural waves. The band structure presents modes with negative dispersion bands allowing negative refraction for both types of waves within the frequency range of 6.9–7.4 kHz, which is induced by the Bragg scattering effect. Edge modes that enhance the evanescent waves through resonant coupling appear around 7.2 kHz for the flexural and sound wave. The simultaneous superlensing is then observed at this frequency. Our finding will enlighten multiphysical and multifunctional wave manipulations and could have pragmatic applications involving multiwave devices. [ABSTRACT FROM AUTHOR]

Preventing elastic waves from traveling down thin structures is a subject of great interest from the point of view of both physics and applications. It represents a problem—mirrored by the case of light in waveguides—that has broad implications. To completely prohibit sound waves in a given frequency range in rods, for example, all axially propagating acoustic eigenmodes must exhibit strong damping. Here, we demonstrate experimentally and by simulation a metamaterial rod made from a single material that can simultaneously shut out all elastic-wave polarizations, namely longitudinal, flexural, and torsional modes, in a band in the sub-kHz range. We first bond five acrylic building blocks together to make a subwavelength resonator and then fix an array of these inside an acrylic tube to form a cylindrical metarod that inhibits sound transmission in the metamaterial bandgap frequency range. Applications include vibration control and earthquake mitigation. [ABSTRACT FROM AUTHOR]

Complex polarizations of elastic waves allow mode conversions between two solids, making wave mode separation extremely difficult even for a narrow frequency range with resonant metamaterials. In this Letter, a non-resonant elastic metasurface design is proposed to achieve a perfect splitting of longitudinal and shear waves in space. The key to this broadband design is a singly polarized solid with engineered static elastic tensor, which provides a tool to tune the polarization through non-resonant microstructure design. Both full-wave simulations and experiments are conducted to validate the mode splitting function of the designed metasurface. Potential applications such as elastic wave shifting and selective wave mode focusing are also demonstrated. This research provides an alternative route to design broadband and compact metasurfaces for elastic wave communications, biomedical diagnosis, and wave-based damage evaluations. [ABSTRACT FROM AUTHOR]

Our early work showed that the evolution of the twin boundary pattern exhibits an avalanche behavior upon external loading of ferroelastic materials [Salje et al., Phys. Rev. B 83, 104109 (2011)]. The distribution of "jerks" (singularities of potential energy change) was found to follow a power law distribution below a Vogel–Fulcher temperature, mainly related to the movement of kinks in domain boundaries. We use molecular dynamics simulations to study the nucleation, scattering, and annihilation of phonons that are generated by the nucleation and propagation of such kinks. The interaction and scattering of phonons are correlated over a short time period and gradually become uncorrelated before annihilation at large temperature intervals. The movement and interaction of phonons show avalanche behavior. The probability of finding energy jerks follows a power law with exponents around 2.5–3. The distribution of waiting times between jerks also follows a power law. At temperatures above the Vogel–Fulcher temperature, scattering with thermal phonons becomes predominant and no phononic avalanches were observed. [ABSTRACT FROM AUTHOR]

Accomplishing low frequency and wide bandgaps with a single phase metamaterial remains a challenging task. In this work, we propose a single phase acoustic metamaterial having periodically arranged divergent shaped star units and performed numerical simulation to obtain the bandgaps. The simulation results demonstrate that the metamaterial exhibits an uninterrupted and extended bandgap that lies in the low frequency regime. We fabricated the structure based on numerical investigations, and using the experimental technique, elastic wave transmission characteristics have been analyzed. Experimental observations are in good agreement with numerically calculated bandgaps. Moreover, we perform a parametric study on the bandgaps with geometrical parameters like the concave angle and length of divergent ribs of the unit cell. [ABSTRACT FROM AUTHOR]

Developing a tunable phononic crystal (PC) based on relatively simple technology is challenging. Herein, we report a thermally controlled two-dimensional PC which consists of a honeycomb array of thermosensitive perovskite ferroelectric scatterers. The inversion symmetry of the PC is broken, and the topological properties of the band structure are readily tuned by heating the scatterers in a well-controlled manner to form a temperature gradient in the unit cell. Numerical simulation is used to demonstrate the one-way propagation of the edge state along the interface between two PCs with different topological properties and robustness of the edge states against defects. The propagation of the interfacial wave can be tuned extensively by varying the temperature in a few unit cells of the honeycomb lattice array. The present study could be potentially significant in designing tunable, efficient, and multifunctional acoustic devices. [ABSTRACT FROM AUTHOR]

We propose and analyze theoretically a class of energy-efficient magnetoelastic devices for analog signal processing. The signals are carried by transverse acoustic waves while the bias magnetic field controls their scattering from a magnetoelastic slab. By tuning the bias field, one can alter the resonant frequency at which the propagating acoustic waves hybridize with the magnetic modes, and thereby control transmission and reflection coefficients of the acoustic waves. The scattering coefficients exhibit Breit-Wigner/Fano resonant behavior akin to inelastic scattering in atomic and nuclear physics. Employing oblique incidence geometry, one can effectively enhance the strength of magnetoelastic coupling, and thus countermand the magnetic losses due to the Gilbert damping. We apply our theory to discuss potential benefits and issues in realistic systems and suggest routes to enhance the performance of the proposed devices. [ABSTRACT FROM AUTHOR]

For actual realization of full-power longitudinal-to-transverse (transverse-to-longitudinal) mode-converting transmission, we propose a single-phase "monolayer" anisotropic metamaterial that is easy to fabricate with a dimension of 4–5 times the target wavelength. Full-power mode conversion can be critically useful in ultrasonic applications, but this has not been experimentally validated because a metamaterial consisting of subwavelength unit cells is difficult to fabricate. Here, we achieve full-power mode-converting transmission with a monolayer, and its effective material properties should satisfy the required theoretical conditions. While an S-parameter based method can be employed to retrieve the effective material properties, an elaborate method should be developed to estimate its effective size because the monolayer metamaterial directly interfaces with the background medium. To this end, we propose a T-matrix based method to estimate its size. Finally, ultrasonic experiments performed with the designed monolayer confirmed the realization of nearly full-power mode conversion. [ABSTRACT FROM AUTHOR]

A metamaterial rod with resonators containing negative-stiffness (NS) mechanisms is proposed for generating very low-frequency bandgaps. The underlying principle employs the NS mechanism (a pair of mutual repelling permanent magnet rings) to partially or totally neutralize the stiffness of the positive-stiffness element (two coil springs) of the resonator and thus to achieve an ultralow, even zero, stiffness, which enables a significant shift of the bandgap from a high frequency to a low one. Experiments on the restoring force feature of the resonator and the bandgap of the metamaterial rod are carried out, which provide sufficient evidence to validate the proposed concept for substantially lowering bandgaps in locally resonant metamaterials. This study opens a potential path to manipulate elastic waves within a very low-frequency range. [ABSTRACT FROM AUTHOR]

We study the Bloch oscillation of elastic waves in a chain composed of hollow elliptical cylinders (HECs). These HECs are 3D-printed in different wall thicknesses and are arranged to form a graded chain. We find that the frequency band structure of this lattice can be manipulated in a way to create a narrow strip of transmission range sandwiched between slanted stop bands. This results in the trapping of elastic waves at a specific location of the chain, which depends on the input frequency of the propagating elastic waves. This elastic Bloch oscillation in a tailorable 3D-printed system enables the control of energy localization in solids, potentially leading to engineering applications for vibration filtering, energy harvesting, and structural health monitoring. [ABSTRACT FROM AUTHOR]

This letter presents the concept of the Total Internal Reflection metasurface (TIR-MS) which supports the realization of structure-embedded subwavelength acoustic shields for elastic waves propagating in thin waveguides. The proposed metasurface design exploits extreme phase gradients, implemented via locally resonant elements, in order to achieve operating conditions that are largely beyond the critical angle. Such artificial discontinuity is capable of producing complete reflection of the incoming waves regardless of the specific angle of incidence. From a practical perspective, the TIR-MS behaves as a sound hard barrier that is impenetrable to long-wavelength modes at a selected frequency. The TIR metasurface concept is first conceived for a flat interface embedded in a rectangular waveguide and designed to block longitudinal S0-type guided modes. Then, it is extended to circular plates in order to show how enclosed areas can be effectively shielded by incoming waves. Given the same underlying physics, an equivalent dynamic behavior was also numerically and experimentally illustrated for flexural A0-type guided modes. This study shows numerical and experimental evidence that, when the metasurface is excited at the target frequency, significant vibration isolation can be achieved in the presence of waves having any arbitrary angle of incidence. These results open interesting paths to achieve vibration isolation and energy filtering in certain prototypical structures of interest for practical engineering applications. [ABSTRACT FROM AUTHOR]

Traditional asymmetric transmission of elastic waves is mostly based on mode conversion, which presents a real challenge to get pure-mode elastic waves. In this letter, without the aid of mode conversion, we present an innovative concept of asymmetric flexural wave transmission within a wide frequency band, based on dual-layer elastic wave gradient metasurfaces. According to the generalized Snell's law, we theoretically and numerically design and experimentally demonstrate the asymmetric flexural wave transmission by tuning the supercell lengths of dual-layer metasurfaces. The experimental results confirm that the proposed design provides a wide effective frequency band feature, which agrees well with the theoretical analysis and predictions. Our concept offers the flexibility to control the wave energy flow, opening the route to pragmatic applications in many fields, such as ultrasonic detection, energy harvesting, and vibration control. [ABSTRACT FROM AUTHOR]

We propose a method to simultaneously determine the thickness of an isotropic plate together with the longitudinal and shear elastic wave velocities of its material. The method requires knowledge of the frequencies of two zero-group-velocity Lamb modes and one respective wavenumber. These quantities are defined by the Rayleigh-Lamb equations, which we use in an inverse problem to obtain the properties of the plate. Experimentally, the frequencies of zero-group-velocity points can be obtained at high precision by measuring the elastic response spectrum of a plate, using laser-ultrasound techniques. By shaping the excitation laser spot with a spatial light modulator, we extend this to enable measurements of the corresponding wavenumber. The introduced method is demonstrated for a homogeneous tungsten and an aluminium plate. [ABSTRACT FROM AUTHOR]

We have investigated optically excited magnetoelastic waves by phase-resolved spin-wave tomography (PSWaT). PSWaT reconstructs the dispersion relation of spin waves together with their phase information by using time-resolved magneto-optical imaging for spin-wave propagation followed by an analysis based on the convolution theorem and a complex Fourier transform. In PSWaT spectra for a Bi-doped garnet film, we found a 180°-phase shift of magnetoelastic waves at around the crossing of the dispersion relations of spin and elastic waves. The result is explained by a coupling between spin waves and elastic waves through the magnetoelastic interaction. We also propose an efficient way for the phase manipulation of magnetoelastic waves by rotating the orientation of magnetization less than 10°. [ABSTRACT FROM AUTHOR]

This letter reports designs of adaptive metasurfaces capable of modulating incoming wave fronts of elastic waves through electromechanical-tuning of their cells. The proposed elastic metasurfaces are composed of arrayed piezoelectric units with individually connected negative capacitance elements that are online tunable. By adjusting the negative capacitances properly, accurately formed, discontinuous phase profiles along the elastic metasurfaces can be achieved. Subsequently, anomalous refraction with various angles can be realized on the transmitted lowest asymmetric mode Lamb wave. Moreover, designs to facilitate planar focal lenses and source illusion devices can also be accomplished. The proposed flexible and versatile strategy to manipulate elastic waves has potential applications ranging from structural fault detection to vibration/noise control. [ABSTRACT FROM AUTHOR]

We explore a phononic crystal Luneburg lens through design, fabrication, and analysis for omnidirectional elastic wave focusing and enhanced energy harvesting both numerically and experimentally. The proposed lens is formed using hexagonal unit cells with blind holes of different diameters, which are determined according to the Luneburg lens refractive index distribution obtained by finite-element simulations of the lowest asymmetric mode Lamb wave band structure. Wave simulations are performed numerically under plane wave excitation from a line source, and focusing is observed at the opposite border of the lens with respect to the incident wave direction. Numerically simulated elastic wave focusing results are validated through a set of experiments. Omnidirectionality is demonstrated by testing the lens under plane wave excitation for different angles of incidence. With piezoelectric energy harvesters located at the boundary of the phononic crystal Luneburg lens, more than an order of magnitude larger power output can be extracted as compared to the baseline case of energy harvesting without the lens under the same plane wave excitation. [ABSTRACT FROM AUTHOR]