Search

We theoretically report on an innovative and practical acoustic energy harvester based on a defected acoustic metamaterial (AMM) with piezoelectric material. The idea is to create suitable resonant defects in an AMM to confine the strain energy originating from an acoustic incidence. This scavenged energy is converted into electrical energy by attaching a structured piezoelectric material into the defect area of the AMM. We show an acoustic energy harvester based on a meta-structure capable of producing electrical power from an acoustic pressure. Numerical simulations are provided to analyze and elucidate the principles and the performances of the proposed system. A maximum output voltage of 1.3V and a power density of 0.54 lW/cm3 are obtained at a frequency of 2257.5 Hz. The proposed concept should have broad applications on energy harvesting as well as on low-frequency sound isolation, since this system acts as both acoustic insulator and energy harvester. [ABSTRACT FROM AUTHOR]

Conventional acoustic absorbers are used to have a structure with a thickness comparable to the working wavelength, resulting in major obstacles in real applications in low frequency range. We present a metasurface-based perfect absorber capable of achieving the total absorption of acoustic wave in an extremely low frequency region. The metasurface possessing a deep subwavelength thickness down to a feature size of ~λ=223 is composed of a perforated plate and a coiled coplanar air chamber. Simulations based on fully coupled acoustic with thermodynamic equations and theoretical impedance analysis are utilized to reveal the underlying physics and the acoustic performances, showing an excellent agreement. Our realization should have an high impact on amount of applications due to the extremely thin thickness, easy fabrication, and high efficiency of the proposed structure. [ABSTRACT FROM AUTHOR]

We report in this letter on a tunable solid acoustic metamaterial with negative elastic modulus by means of piezoelectric composite. The theoretical formulae for one-dimensional layer-stacked metamaterial embedding a piezoelectric material by means of external shunted inductors are presented. The acoustic band structure of the composite is calculated by the transfer matrix method. Results show that a band gap can be opened and tuned by the resonant behavior of the LC circuit. It is found further by the formulae that piezoelectric material with large piezoelectric constant and small elastic modulus will be beneficial for opening a wide band gap. The effective elastic constant of the system is also calculated by the unit-cell-boundary-averaging method. Result shows that the system behaves as an effective medium with a negative elastic modulus. This property is quite different from the typical solid metamaterial achieved by dispersing heavy inclusions coated with a soft layer into a matrix for which only the negative mass density can be obtained. [ABSTRACT FROM AUTHOR]

We report in this paper about the realization of domain inversion in z-cut lithium niobate by electron beam irradiation in order to perform phononic crystals. The fabrication of these phononic crystals on z-cut LiNbO3, which is, in our case, a periodic repetition of voids and LiNbO3, was achieved by domain inversion followed by wet etching, taking advantage of the large difference in etching rate between z+ and z- faces. A pertinent choice of irradiation conditions such as accelerating voltage, beam current, and charge density was determined and optimized. Two-dimensional structures at the micrometer scale were then realized on z-cut LiNbO3. We demonstrate the achievement of hexagons with diameters between 2 µm and 18 µm, with a very important depth close to 30 µm, which depends on the etching time and the hole size. The obtained structures, which exhibit a filling fraction varying from 1% to 64%, were characterized before etching by polarizing microscope to visualize the inverted domains. After HF etching, scanning electron microscopy was used to observe the obtained phoaonic structures. Taking into account the obtained filling fraction values and the size of created hexagons, the frequency band gap of these structures is expected at a range of 200 to 350 MHz. As expected in this frequency range, we have proven experimentally the existence of the phoaonic band gap on z-cut LiNbO3 by combination of a realized phononic crystal with a surface acoustic wave (SAW) device. [ABSTRACT FROM AUTHOR]

• Isotactic chiral phononic crystals for a low-frequency and broad bandgap. • Comparing the bandgap properties of the isotactic and syndiotactic chiral phononic crystals. • Illuminating an extremely simplified geometry of Thomson Scattering-induced bandgap by a practical example. • The numerical and experimental demonstration for the Thomson Scattering-induced continuous bandgap. Realizing a low-frequency and broad bandgap is extremely challenging because the lattice constant, equivalent stiffness, and equivalent density must be taken into account in practice. Despite the advances of inertial amplification which is one of the ideal strategies to address this issue, the design ideas are mainly limited to the classic geometries featured by mirror symmetry (syndiotacticity), confining then progress in this topic. Here, we report numerical and experimental investigation of a chiral phononic crystal without mirror symmetry (isotacticity) according to the analogous Thomson scattering. Numerical analysis shows that due to the lack of one degree of freedom in isotactic lattice (ISL) compared to syndiotactic lattice (SYL), the bandgap starting-frequency of the ISL is significantly lower than that the one of the SYL by up to 66%. The bandgap properties are further demonstrated by simulation and experimental measurements. This research reveals that the condition of having alternating arrangement orientations between neighboring atoms is not indispensable for this kind of bandgap, and a rational geometry satisfying the Thomson scattering-based bandgap conditions can further reduce the bandgap frequency without affecting the stiffness and lightweight of the structure. [Display omitted] [ABSTRACT FROM AUTHOR]

• Arbitrary wavefront modulation is realized utilizing the aperiodic design. • Full transmission of flexural wave in elastic metasurface is realized. • A semi-analytical inverse design method with a genetic algorithm is used to design metasurface. • The focusing efficiency can be increased by 19.35%. • The mechanism of the full transmission is revealed. Metasurfaces have attracted much attention due to their much more compact configurations and their abilities to modulate wavefronts of propagating waves by abruptly shifting their phase and/or amplitude. Most current metasurfaces are usually composed of subunits with periodic unit cells, which limits their design freedom for arbitrary phase and amplitude modulation. To overcome this limitation, in this paper, we propose an aperiodic elastic metasurface where arbitrary phase and amplitude modulation can be achieved. The aperiodic elastic metasurface is composed of pillared resonators with different heights and an analytical model-based genetic algorithm (GA) inverse design method is employed to determine the heights of the pillared resonators. The underlying mechanism of the phase/amplitude shift and mechanism of full transmission are revealed. The proposed aperiodic elastic metasurface can steer and focus flexural waves with full transmission and can steer flexural waves into different directions with different amplitudes. The inverse design method and the aperiodic design in this paper can inspire the design of other types of functional metasurfaces for fascinating applications. [Display omitted] [ABSTRACT FROM AUTHOR]

Releasing the dependence of the low-frequency and broad bandgaps on low stiffness and bulky masses has been an intractable bottleneck. Although inertial amplification is a terrific candidate for addressing the barrier, numerous extended ideas are limited to the classical models, thus, slowing or stalling the advances in this demand. To break this limit, we report a kind of planar chiral phononic crystal based on Thomson scattering. Different from the optimization effect produced in the locally resonant phononic crystals, the mirrored chirality can open a Thomson scattering-induced broad bandgap when the local resonant sub-structure is discarded. In addition, while simplifying the material components, we lower the starting frequency of the bandgap with a virtually unchanged width for the same lattice constant, stiffness and mass. Consequently, the starting frequency of the attenuation can be in the same order of magnitude as the local resonance bandgap, while the width is significantly better than the latter. Our proposal may open a new way to manipulate broadband elastic waves and validate that Thomson scattering is a promising alternative approach and mechanism for bandgap formation. [ABSTRACT FROM AUTHOR]

We propose a reconfigurable elastic metasurface for broadband plate wave control where the flexural-dominated plate mode is concerned. The structure consists of a plate with nuts and screws perpendicular to the plate. With the continuous changes of nut positions on the screws, the phase shift can cover 2π span with high transmittance over broadband frequency. The mechanism of the tunability of phase shift caused by the screw-in depth is discussed based on the dispersion theory of local resonant phononic crystals. Then, subwavelength elastic metasurfaces are designed based on the generalized Snell's law demonstrating numerically and experimentally abnormal refraction and wave focusing functionalities without remanufacturing the metasurface. Furthermore, we show asymmetric transmission with a structure containing two parallel metasurfaces. Different functionalities can be achieved by changing the screw-in depth at different frequencies. The proposed metasurface designs could provide a novel and feasible route to control flexural waves in broadband frequency range and take a critical step in the pragmatic development of elastic metasurfaces. [ABSTRACT FROM AUTHOR]

• Open low-frequency bandgaps induced by inertial amplification in a half-space. • Demonstrate experimentally the characteristics of bandgap of metamaterial plate. • Investigate broadband attenuation by using specific different unit cell designs. In the last two decades, seismic metamaterials have attracted significant attention of researchers because of the characteristic of the bandgap which can be used to control seismic surface waves. The inertial amplification mechanism has been used to design metamaterials capable of isolating elastic waves in rods, beams, and plates at low frequencies. In this work, we propose an alternative type of seismic metamaterial with a low-frequency bandgap induced by inertial amplification for isolating seismic surface waves. The characteristics of the bandgap induced by inertial amplification are experimentally demonstrated by using a metamaterial plate composed of 25 unit cells. The propagation of the flexural waves imaged by scanning laser Doppler vibrometer shows strong attenuation effects induced by the metamaterial plate in the bandgap. The broadband attenuation is investigated by using two kinds of unit cells. Finally, similar structures with inertial amplification are introduced to design the seismic metamaterials to isolate seismic surface waves at low frequencies. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Large-scale field experiments demonstrate the ultra-wide first bandgap. • Achieve a first bandgap with relative bandwidth of 1.17. • Study the characteristic of the first bandgap by using complex band structures. Seismic metamaterials (SMs) are expected to assist or replace traditional isolation systems owing to their strong attenuation of seismic waves. In this work, a one-dimensional inverted T-shaped SM (1D ITSM) composed of arrays of inverted T-shaped structures on a half space is proposed, which have an ultra-wide first bandgap (FBG) from 6.7 to 17.2 Hz. We find that the FBG is composed of two parts; part 1 with surface evanescent waves from 6.7 to 11.0 Hz and part 2 with no surface modes from 11.0 to 17.2 Hz by using the complex band structures. The propagation of seismic surface waves in the 1D ITSM is different in these two frequency ranges of the FBG. In part 1, the seismic surface waves are significantly attenuated in the 1D ITSM because of the surface evanescent waves, while in part 2, the surface waves are converted into bulk waves because surface waves cannot exist in the ITSM. Finally, the ultra-wide FBG is verified by using a kind of the two-dimensional ITSM in large-scale field experiments. [ABSTRACT FROM AUTHOR]

• Elastic metasurface (~0.495 λ) with constructive interference (PEMC) is proposed. • We reveal the phase shift of transmitted waves across the improved subunits. • Analytical model is established to predict phase and amplitude of transmitted waves. • Constructive and destructive interferences for two adjacent subunits are analyzed. • Deflecting and focusing functionality of the PEMC are experimentally verified. In engineering, plate structures are one type of the main load-bearing structures. However, most of the designed plate-like metamaterials/metasurfaces need to be heavily grooved or drilled on the original plate surface, which will inevitably cause some destruction to the strength and stiffness of the host plate structures. To overcome this weakness, we propose a new conceptual design of pillared elastic metasurface (~0.495 λ) to manipulate flexural waves in plates by considering constructive interference. The interference, which manifests itself as the coupling through out-of-plane vibration of the plate for the two adjacent subunits, is analyzed to reveal the physical mechanisms. In addition, we reveal the mechanism of the phase shift of transmitted waves across the sub-wavelength subunits and establish an analytical model for the multi-resonator subunits to accurately predict the phase shift and amplitude of the transmitted wave. We theoretically design and experimentally demonstrate the deflecting and focusing functionalities of the proposed elastic metasurface. Our design can provide a new route to broad applications of the constructive interference in elastic metamaterials/metasurfaces, which can be used to efficiently engineer arbitrary wave profiles. [ABSTRACT FROM AUTHOR]

Constructing a highly localized wave field by means of bound states in the continuum (BICs) promotes enhanced wave-matter interaction and offers approaches to high-sensitivity devices. Elastic waves can carry complex polarizations and thus differ from electromagnetic waves and other scalar mechanical waves in the formation of BICs, which is yet to be fully explored and exploited. Here, we report the investigation of local resonance modes supported by a Lamb waveguide side-branched with two pairs of resonant pillars and show the emergence of two groups of elastic BICs with different polarizations or symmetries. Particularly, the two groups of BICs exhibit distinct responses to external perturbations, based on which a label-free sensing scheme with enhanced-sensitivity is proposed. Our study reveals the rich properties of BICs arising from the complex wave dynamics in elastic media and demonstrates their unique functionality for sensing and detection.

We discuss the computation of the band structure of plate waves using the plane wave expansion (PWE) method. This method is generally used to formulate eigenvalue problems to compute dispersion diagrams for solid-solid phononic crystals. We show how the free surface boundary condition can be included implicitly in the form of the PWE solution, thus leading to an efficient eigenvalue problem. This generic method for wave dispersion is non-iterative and does not require an initial guess for the solution. Furthermore, surface acoustic wave velocities can be estimated from the slowest wave for large wave vectors. Examples for a single plate and a multilayer plate are given, and extension to piezoelectric materials is discussed.