Electro-mechanical coupling wave propagating in a locally resonant piezoelectric/elastic phononic crystal nanobeam with surface effects

The model of a "spring-mass" resonator periodically attached to a piezoelectric/elastic phononic crystal(PC) nanobeam with surface effects is proposed, and the corresponding calculation method of the band structures is formulized and displayed by introducing the Euler beam theory and the surface piezoelectricity theory to the plane wave expansion(PWE) method. In order to reveal the unique wave propagation characteristics of such a model, the band structures of locally resonant(LR) elastic PC Euler nanobeams with and without resonators, the band structures of LR piezoelectric PC Euler nanobeams with and without resonators, as well as the band structures of LR elastic/piezoelectric PC Euler nanobeams with resonators attached on PZT-4, with resonators attached on epoxy, and without resonators are compared. The results demonstrate that adding resonators indeed plays an active role in opening and widening band gaps. Moreover, the influence rules of different parameters on the band gaps of LR elastic/piezoelectric PC Euler nanobeams with resonators attached on epoxy are discussed, which will play an active role in the further realization of active control of wave propagations.

Impact vibration properties of locally resonant fluid-conveying pipes

Fluid-conveying pipe systems are widely used in various equipments to transport matter and energy.Due to the fluid–structure interaction effect,the fluid acting on the pipe wall is easy to produce strong vibration and noise,which have a serious influence on the safety and concealment of the equipment.Based on the theory of phononic crystals,this paper studies the vibration transfer properties of a locally resonant(LR)pipe under the condition of fluid–structure interaction.The band structure and the vibration transfer properties of a finite periodic pipe are obtained by the transfer matrix method.Further,the different impact excitation and fluid–structure interaction effect on the frequency range of vibration attenuation properties of the LR pipe are mainly considered and calculated by the finite element model.The results show that the existence of a low-frequency vibration bandgap in the LR pipe can effectively suppress the vibration propagation under external impact and fluid impact excitation,and the vibration reduction frequency range is near the bandgap under the fluid–structure interaction effect.Finally,the pipe impact experiment was performed to verify the effective attenuation of the LR structure to the impact excitation,and to validate the finite element model.The research results provide a technical reference for the vibration control of the fluid-conveying pipe systems that need to consider blast load and fluid impact.

Numerical and Experimental Investigations on Tunable Low-frequency Locally Resonant Metamaterials

In this paper,a tunable locally resonant metamaterial is proposed for low-frequency band gaps.The local resonator composed of two pairs of folded slender beams and a proof mass is designed based on the theory of compliant mechanism.The design optimization on geometric parameters is carried out to fulfil the quasi-zero-stiffness property.The locally resonant metamaterial is formed by periodically arranged unit cells,and the transmittance of longitudinal wave is studied through three aspects:numerical predictions,finite element simulations and experimental tests.The variation trends revealed by these three methods match well with one another:the band gap moves to lower frequency and both its depth and width get smaller and smaller with the increase of pre-compression(Δ).The band gap overlays the frequency range of 73.10–92.38 Hz and 16.78–19.49 Hz atΔ=0mm andΔ=10mm,respectively,providing a wide range of tunability.Besides,the ultralow-frequency band gap can be achieved asΔapproaches 10 mm.This study may provide an avenue for achieving the tunable ultralow-frequency locally resonant band gap.

Two Kinds Equal Frequency Circuits to Achieve Locally Resonant Band Gap of a Circular Plate Attached Alternately by Piezoelectric Unimorphs

A circular thin plate is proposed for vibration attenuation,which is attached alternately by annular piezoelectric unimorphs with resonant shunt circuits.Two kinds of equal frequency resonant shunt circuits are designed to achieve an integrated locally resonant(LR)band gap(BG) with a much smaller transmission factor:(1) the structure is arrayed periodically while the resonant shunt circuits are aperiodic;(2) the resonant shunt circuits are periodic while the structure is aperiodic.The transmission factor curve is calculated,which is validated by the finite element method.Dependences of the LR BG performance upon the geometric and electric parameters are also analyzed.

Layered metastructure containing freely-designed local resonators for wave attenuation

Combining periodic layered structure with three-dimensional cylindrical local resonators,a hybrid metastructure with improved wave isolation ability was designed and investigated through theoretical and numerical approaches.The metastructure is composed of periodic rubber layers and concrete layers embedded with three-dimensional resonators,which can be freely designed with multi local resonant frequencies to attenuate vibrations at required frequencies and widen the attenuation bandgap.The metastructure can also effectively attenuate seismic responses.Compared with layered rubber-based structures,the metastructure has more excellent wave attenuation effects with greater attenuation and wider bandgap.

Electro-mechanical coupling properties of band gaps in an with periodically attached “spring-mass” resonators

The model of a locally resonant (LR) epoxy/PZT-4 phononic crystal (PC)nanobeam with “spring-mass” resonators periodically attached to epoxy is proposed. The corresponding band structures are calculated by coupling Euler beam theory, nonlocal piezoelectricity theory and plane wave expansion (PWE) method. Three complete band gaps with the widest total width less than 10GHz can be formed in the proposed nanobeam by comprehensively comparing the band structures of three kinds of LR PC nanobeams with resonators attached or not. Furthermore, influencing rules of the coupling fields between electricity and mechanics,“spring-mass” resonator, nonlocal effect and different geometric parameters on the first three band gaps are discussed and summarized. All the investigations are expected to be applied to realize the active control of vibration in the region of ultrahigh frequency.

Improving the UV-light stability of silicon heterojunction solar cells through plasmon-enhanced luminescence downshifting of YVO_(4):Eu^(3+),Bi^(3+)nanophosphors decorated with Ag nanoparticles

The ultraviolet(UV)light stability of silicon heterojunction(SHJ)solar cells should be addressed before large-scale production and applications.Introducing downshifting(DS)nanophosphors on top of solar cells that can convert UV light to visible light may reduce UV-induced degradation(UVID)without sacrificing the power conversion efficiency(PCE).Herein,a novel composite DS nanomaterial composed of YVO_(4):Eu^(3+),Bi^(3+)nanoparticles(NPs)and AgNPs was synthesized and introduced onto the incident light side of industrial SHJ solar cells to achieve UV shielding.The YVO_(4):Eu^(3+),Bi^(3+)NPs and Ag NPs were synthesized via a sol-gel method and a wet chemical reduction method,respectively.Then,a composite structure of the YVO_(4):Eu^(3+),Bi^(3+)NPs decorated with Ag NPs was synthesized by an ultrasonic method.The emission intensities of the YVO_(4):Eu^(3+),Bi^(3+)nanophosphors were significantly enhanced upon decoration with an appropriate amount of~20 nm Ag NPs due to the localized surface plasmon resonance(LSPR)effect.Upon the introduction of LSPR-enhanced downshifting,the SHJ solar cells exhibited an~0.54%relative decrease in PCE degradation under UV irradiation with a cumulative dose of 45 k W h compared to their counterparts,suggesting excellent potential for application in UV-light stability enhancement of solar cells or modules.

Self-Supporting Nanoporous Copper Film with High Porosity and Broadband Light Absorption for Efficient Solar Steam Generation

Solar steam generation(SSG)is a potential technology for freshwater production,which is expected to address the global water shortage problem.Some noble metals with good photothermal conversion performance have received wide concerns in SSG,while high cost limits their practical applications for water purification.Herein,a self-supporting nanoporous copper(NP-Cu)film was fabricated by one-step dealloying of a specially designed Al_(98)Cu_(2)precursor with a dilute solid solution structure.In-situ and ex-situ characterizations were performed to reveal the phase and microstructure evolutions during dealloying.The NP-Cu film shows a unique three-dimensional bicontinuous ligament-channel structure with high porosity(94.8%),multi scale-channels and nanoscale ligaments(24.2±4.4nm),leading to its strong broadband absorption over the 200–2500 nm wavelength More importantly,the NP-Cu film exhibits excellent SSG performance with high evaporation rate,superior efficiency and good stability.The strong desalination ability of NP-Cu also manifests its potential applications in seawater desalination.The related mechanism has been rationalized based upon the nanoporous network,localized surface plasmon resonance effect and hydrophilicity.

Plasmonic CsPbBr_(3)–Au nanocomposite for excitation wavelength dependent photocatalytic CO_(2) reduction

The optoelectronic performance of CsPbBr_3 nanocrystal(NC) has been dramatically limited by the severe charge carrier recombination and its narrow light absorption range,which are anticipated to be resolved via coupling with plasmonic Au nanoparticle(NP).In view of this,CsPbBr_3-Au nanocomposite is fabricated and further employed as a concept model to study the electronic interaction between perovskite NC and Au NP for the first time.It has been found that the excitation-wavelength dependent carrier transfer behavior exists in CsPbBr_3-Au nanocomposite.Upon illumination with visible light(λ>420 nm),photo-generated electrons in CsPbBr_3 can inject into Au with an electron injection rate and efficiency of 2.84×10~9 s^(-1) and 78%,respectively.The boosted charge separation is further translated into a 3.2-fold enhancement in CO_2 photocatalytic reduction activity compared with pristine CsPbBr_3.On the other hand,when solely exciting Au NP with longer wavelength light(λ>580 nm),the localized surface plasmon resonance(LSPR) induced hot electrons in Au NPs can transfer to CsPbBr_3 NC and further participate in photocatalytic reaction towards CO_2 reduction.The present study provides new insights into preparing plasmonic nanostructure to enhance the performance of perovskite based optoelectronic devices.

Photocurrent improvement of an ultra-thin silicon solar cell using the localized surface plasmonic effect of clustering nanoparticles

The cluster-shaped plasmonic nanostructures are used to manage the incident light inside an ultra-thin silicon solar cell.Here we simulate spherical,conical,pyramidal,and cylindrical nanoparticles in a form of a cluster at the rear side of a thin silicon cell,using the finite difference time domain(FDTD)method.By calculating the optical absorption and hence the photocurrent,it is shown that the clustering of nanoparticles significantly improves them.The photocurrent enhancement is the result of the plasmonic effects of clustering the nanoparticles.For comparison,first a cell with a single nanoparticle at the rear side is evaluated.Then four smaller nanoparticles are put around it to make a cluster.The photocurrents of 20.478 mA/cm2,23.186 mA/cm2,21.427 mA/cm2,and 21.243 mA/cm2 are obtained for the cells using clustering conical,spherical,pyramidal,cylindrical NPs at the backside,respectively.These values are 13.987 mA/cm2,16.901 mA/cm2,16.507 mA/cm2,17.926 mA/cm2 for the cell with one conical,spherical,pyramidal,cylindrical NPs at the backside,respectively.Therefore,clustering can significantly improve the photocurrents.Finally,the distribution of the electric field and the generation rate for the proposed structures are calculated.

The Experimental and Theoretical Research of "Local Resonance" in Ultrasonic Honing System

In 1982, Professor Fang Guoliang found the "Non full resonance" phenomenon in a tool system while he use the thin-long tool ultrasonically machining deep-small hole. He called it as "local resonance". Also this "Non full resonance" phenomenon was discovered in the ultrasonic drilling and the ultrasonic honing system later. To its mechanism, professor Fang thought that the coupling of long-thin tool bar and driving system is weak, so the tool bar can vibrate independently, but the quantitative relation between the coupling factor and diameter ratio is not made certain. Then several theories come forth to interpret it but still haven’t a common conclusion. Through the systematic experimental and theoretical research, this paper reveals that the "local resonance" phenomenon of ultrasonic honing system has the same essence with the "local resonance" phenomenon in deep hole machining system: when the section area ratio of tool bar and driving system is small enough, some resonance frequencies of combined system are close to the resonance frequencies of "fixed-free" state tool bar, the combined system is still resonant. According to the given depth of hole and structure size, we can use the transfer matrix deduced in this paper to design flexible bar and oilstone seat not only satisfying mechanical structure size but also achieving enough magnitude. It greatly simplified the design. This new method can be named as "local resonance" design method for ultrasonic honing system. The experiment, deduction and design method have a certain common meaning to the study and design of other ultrasonic system.

Propagation Characteristics of Flexural Wave in One-Dimensional Phononic Crystals Based on Lattice Dynamics Model

In this paper, we establish discrete flexural lattice chain models of Bragg and locally resonant phononic crystals by setting mass defect atoms and local resonant elements on the flexural lattice chain. The bandgap characteristics of flexural wave in phononic crystals are studied by establishing the governing equations of the model. The results from models show that with the change of the mass ratio of defective atoms to normal atoms, the bandgap of the flexural wave produced by Bragg scattering shows a certain rule. When the local resonant bandgap and Bragg scattering bandgap are close to each other, the two bandgaps will be coupled to form a wider flexural wave bandgap. The effect of axial strain on bending wave propagation is only the shift of bandgap position. The effect of material damping on the propagation of a bending wave is only energy dissipation at high frequency. In addition, we use finite element simulation to calculate the bandgap of flexural wave in phononic crystals with mass defects, and the results are consistent with lattice chain model. This shows that lattice chain model can effectively guide the bandgap design of phononic crystals. This comprehensive study may help to elucidate the rule of bandgap generation of flexural wave in one-dimensional phononic crystals.

Fano Resonance Enabled Infrared Nano-Imaging of Local Strain in Bilayer Graphene

Detection of local strain at the nanometer scale with high sensitivity remains challenging.Here we report near-field infrared nano-imaging of local strains in bilayer graphene by probing strain-induced shifts of phonon frequency.As a non-polar crystal,intrinsic bilayer graphene possesses little infrared response at its transverse optical phonon frequency.The reported optical detection of local strain is enabled by applying a vertical electrical field that breaks the symmetry of the two graphene layers and introduces finite electrical dipole moment to graphene phonon.The activated phonon further interacts with continuum electronic transitions,and generates a strong Fano resonance.The resulted Fano resonance features a very sharp near-field infrared scattering peak,which leads to an extraordinary sensitivity of-0.002%for the strain detection.Our results demonstrate the first nano-scale near-field Fano resonance,provide a new way to probe local strains with high sensitivity in non-polar crystals,and open exciting possibilities for studying strain-induced rich phenomena.

Bandgaps and vibration isolation of local resonance sandwich-like plate with simply supported overhanging beam

The concept of local resonance phononic crystals proposed in recent years provides a new chance for theoretical and technical breakthroughs in the structural vibration reduction.In this paper,a novel sandwich-like plate model with local resonator to acquire specific low-frequency bandgaps is proposed.The core layer of the present local resonator is composed by the simply supported overhanging beam,linear spring and mass block,and well connected with the upper and lower surface panels.The simply supported overhanging beam is free at right end,and an additional linear spring is added at the left end.The wave equation is established based on the Hamilton principle,and the bending wave bandgap is further obtained.The theoretical results are verified by the COMSOL finite element software.The bandgaps and vibration characteristics of the local resonance sandwich-like plate are studied in detail.The factors which could have effects on the bandgap characteristics,such as the structural damping,mass of vibrator,position of vibrator,bending stiffness of the beam,and the boundary conditions of the sandwich-like plates,are analyzed.The result shows that the stopband is determined by the natural frequency of the resonator,the mass ratio of the resonator,and the surface panel.It shows that the width of bandgap is greatly affected by the damping ratio of the resonator.Finally,it can also be found that the boundary conditions can affect the isolation efficiency.

A multi-band and polarization-independent perfect absorber based on Dirac semimetals circles and semi-ellipses array

We design a four-band terahertz metamaterial absorber that relied on the block Dirac semi-metal(BDS).It is composed of a Dirac material layer,a gold reflecting layer,and a photonic crystal slab(PCS)medium layer.This structure achieved perfect absorption of over 97%at 4.06 THz,6.15 THz,and 8.16 THz.The high absorption can be explained by the localized surface plasmon resonance(LSPR).And this conclusion can be proved by the detailed design of the surface structure.Moreover,the resonant frequency of the device can be dynamically tuned by changing the Fermi energy of the BDS.Due to the advantages such as high absorption,adjustable resonance,and anti-interference of incident angle and polarization mode,the Dirac semi-metal perfect absorber(DSPA)has great potential value in fields such as biochemical sensing,information communication,and nondestructive detection.

Optical absorption tunability and local electric field distribution of gold-dielectric-silver three-layered cylindrical nanotube

The effects of inner nanowire radius,shell thickness,the dielectric functions of middle layer and surrounding medium on localized surface plasmon resonance(LSPR)of gold-dielectric-silver nanotube are studied based on the quasi-static approximation.Theoretical calculation results show that LSPR of gold-dielectric-silver nanotube and LSPR numbers can be well optimized by adjusting its geometrical parameters.The longer wavelength of |ω--> mode takes place a distinct red-shift with increasing the inner nanowire radius and the thickness of middle dielectric layer,while a blue-shift with increasing outer nanotube thickness.The physical mechanisms are explained based on the plasmon hybridization theory,induced charges and phase retardation.In addition,the effects of middle dielectric function and surrounding medium on LSPR,and the local electric field factor are also reported.Our study provides the potential applications of gold-dielectricsilver nanotube in biological tissues,sensor and related regions.

Multi-frequency focusing of microjets generated by polygonal prisms

We systematically investigate the power distribution characteristics of microjets generated by prismatic scatterers with different shapes at sub-THz region(λ = 8.57 mm). Among these prismatic scatterers, the hexagonal-type one shows better focusing feature than the others. Aiming at the hexagonal-type one, we propose a double-layer scatterer composed of a Teflon hexagonal prism as an outer layer and a semiconductor cuboid as an inner layer. Aiming at the double-layer scatterer, we further study the effects of refractive index, size, and shape of the inner cuboid on microjet’s features. The study allows us to present an optimized double-layer scatterer, which has a side length λ/2(λ) and a refractive index 2.0(1.4) for the inner(outer) layer. We show that the optimized scatterer can produce an ultra-strong, ultra-narrow microjet with a power enhancement of;0 and a full width at half maximum(FWHM) of;0.26λ, and the microjet is just located at the output face. The microjet keeps compact within the distance range of λ from the output face. These features and effects are explained from the viewpoint of ray optics theory. According to the optimized double-layer scatterer, we further study the multi-frequency focusing features of the microjets, and find that the microjet remains good features at harmonic frequencies 2f_(0) and 3f_(0). In addition, we investigate the effect of an Au sphere presence in the center of the microjet on the power distribution. The results show that a spherical dark spot with a size similar to that of the Au sphere emerges in the area where the Au sphere is placed. The feature can be used to measure the size of a metallic particle.

Subwavelength asymmetric Au–VO_2 nanodisk dimer for switchable directional scattering

We propose an asymmetric Au–VO_2 nanodisk dimer for realizing a switchable directional scattering. Specifically, the directional scattering can be triggered on/off through controlling the phase transition of the VO_2 nanodisk from metallic to semiconductor state. More strikingly, an obvious directional scattering with the directivity of ～40 dB is achieved under the metallic state of VO_2 nanodisk. This tunable directional scattering is further explained with an interference model where the Au and VO_2 nanodisks are treated as two weakly interacting electric dipoles. The phase transition controlled scattering patterns of asymmetric Au–VO_2 nanodisk dimer are then well interpreted from the phase difference between these two dipoles.

Optical interaction between one-dimensional fiber photonic crystal microcavity and gold nanorod

Localized surface plasmon resonance(LSPR) has demonstrated its promising capability for biochemical sensing and surface-enhanced spectroscopy applications. However, harnessing LSPR for remote sensing and spectroscopy applications remains a challenge due to the difficulty in realizing a configuration compatible with the current optical communication system. Here, we propose and theoretically investigate a hybrid plasmonic-photonic device comprised of a single gold nanorod and an optical fiber-based one-dimensional photonic crystal microcavity, which can be integrated with the optical communication system without insertion loss. The line width of the LSPR, as a crucial indicator that determines the performances for various applications, is narrowed by the cavity-plasmon coupling in our device. Our device provides a promising alternative to exploit the LSPR for high-performance remote sensing and spectroscopy applications.

Controlled plasmon-enhanced fluorescence by spherical microcavity

A surrounding electromagnetic environment can engineer spontaneous emissions from quantum emitters through the Purcell effect.For instance,a plasmonic antenna can efficiently confine an electromagnetic field and enhance the fluorescent process.In this study,we demonstrate that a photonic microcavity can modulate plasmon-enhanced fluorescence by engineering the local electromagnetic environment.Consequently,we constructed a plasmon-enhanced emitter(PE-emitter),which comprised a nanorod and a nanodiamond,using the nanomanipulation technique.Furthermore,we controlled a polystyrene sphere approaching the PE-emitter and investigated in situ the associated fluorescent spectrum and lifetime.The emission of PE-emitter can be enhanced resonantly at the photonic modes as compared to that within the free spectral range.The spectral shape modulated by photonic modes is independent of the separation between the PS sphere and PEemitter.The band integral of the fluorescence decay rate can be enhanced or suppressed after the PS sphere couples to the PE-emitters,depending on the coupling strength between the plasmonic antenna and the photonic cavity.These findings can be utilized in sensing and imaging applications.