多重散射方法研究轴对称空腔覆盖层的声学特性

发展了一种多重散射方法研究声学覆盖层的半数值半解析模型,分析了影响轴对称空腔结构声学性能的主要能量耗散机制。在球坐标条件下推导出轴对称空腔结构的位移和应力场基函数,通过对空腔表面基函数的数值积分,得到散射波和入射波之间的传输矩阵方程,结合分层介质声传播理论计算了周期性空腔结构覆盖层的反射、透射和吸声性能。研究结果表明;空腔共振是低频能量耗散的主要形式,边界条件对材料空腔结构的谐振特性影响很大,利用双空腔耦合共振可以拓宽材料的低频吸声频带;背衬对材料的高频吸声影响较小,材料的高频能量损耗取决于空腔的散射和波型转换特性。      

Photonic Scattering on a Trimer with <Emphasis Type="Italic">PT</Emphasis>-Symmetry

We analyze photonic scattering on a trimer with PT symmetry, where the photon propagates in an array of cavities. In the system, a passive and an active cavity couple to another cavity without loss and gain simultaneously, which is in the middle of the cavity array. Such three cavities can be regarded as the scattering target coupling to the unlimited number of cavities from two directions respectively. Our approach is to use the Schrödinger equation to calculate the reflection coefficient and transmission coefficient, and then let photonic transmission spreads from gain or loss cavity. It shows that the scattering of the photon is closely associated with the PT symmetric characteristic of the dynamics of the system. The atom’s modulation on the photonic transmission is also investigated. We found that the scattering center can behave as a photonic switch, which could be used potentially to control the photonic transmission.     

O型契伦柯夫器件慢波结构高频特性

理论推导了电磁波在半无限长直波导和均匀慢波结构交界面上的反射系数表达式,得到反射系数模值和相位随电磁波的纵向相移常数和慢波结构末端相位的变化关系。运用传输线理论以及反射系数的理论计算结果,得到了有限长慢波结构的纵向谐振条件,可以分析各种情况下有限长慢波结构的纵向谐振特性。计算了一种有限长慢波结构的纵向谐振频率,理论预测与数值仿真结果基本一致。对于一种非均匀慢波结构的数值计算结果表明,其纵向谐振模式对应的频率、场分布与相应的均匀慢波结构接近,因此仍可根据提出的纵向谐振条件对非均匀慢波结构进行分析。     

The layer multiple-scattering method for calculating transmission coefficients of 2D phononic crystals

We extend the layer multiple-scattering theory (LMST) to elastic waves propagating in two-dimensional (2D) periodical composites. The formalism to calculate the reflection and transmission coefficients for elastic waves through finite slabs is presented. In this spirit, the crystal is viewed as a sequence of identical monolayer which has one-dimensional (1D) periodicity along a given direction. The reflection and transmission coefficients for a multilayer slab can be obtained by a double-layer scheme through the calculation of the scattering matrix of a monolayer. To demonstrate the application of this formalism, we calculate transmission coefficients for systems consisting of pure solid components or mixing (solid and fluid) components. The validity of this method is checked by both band structure calculations and transmission measurement of ultrasonic experiment.     

Calculation methods for the transmission loss of single,double and triple partitions

Methods of calculating the transmission loss for single and double walls are presented. These methods are developed using a statistical energy analysis (SEA). In principle the methods are based on separate calculations of resonant and non-resonant transmission for frequencies less than the critical frequency, $\text{f}{}_{\mathrm{c}}$, of the panels and calculation of only resonant transmission for frequencies equal to, and greater than, $\text{f}{}_{\mathrm{c}}$.Comparisons between calculated and measured results show good agreement.A calculation method is also presented for the transmission loss of triple panels for frequencies greater than the cut-off frequency for the cavities. For frequencies less than the cut-off frequency for the smallest cavity depth it is shown that, in most cases, the effect of the middle panel is very slight.     

Reflection and transmission of phonons at a planar interface between two dissimilar solids

Two semi-infinite dissimilar crystals with, however, the same crystal structure and lattice parameter are in contact at a planar interface. Using a simple force constant model, restricted to near-neighbour interaction reflection and transmission coefficients for sound waves propagating along one solid are calculated. At low frequencies the reflection and transmission coefficients are determined solely by the force constants and the atomic masses in the two media. At high frequencies the transmission coefficient becomes small if the force constant at the interface between the two media is weak. Information of the local force constant in the interface region can be gained at least in principle, by measuring the reflection and transmission coefficients at high frequencies.     

Modal coupling in acoustic waveguides: planar discontinuities

A new derivation for a scattering matrix for reflection and transmission of higher order modes at the planar junction of two waveguides is presented. The derivation is extended to include finite junction wall impedance and offset waveguides. The resulting matrix equations are analyzed and the physical significance of the matrices is explained. As an example of the theory, analytical expressions for the coupling coefficients at a size change in a rectangular duct are developed and the resulting reflection and transmission coefficients are computed. The results should be of interest to the HVAC noise control community. The paper also shows the effects of modal truncation on the accuracy and convergence of the solution. It is shown that the proper selection of the ratio of the number of modes on either side of the discontinuity is related to the ratio of the characteristic sizes of the waveguides. Finally it is shown that at least one higher mode should be included for reasonable accuracy in the computation of plane wave reflection and transmission coefficients except at the very lowest frequencies.     

The scattering and transmission of elastic waves in quasi-two-dimensional planar waveguides with linear defect boundaries

The influence of linear defect boundaries on the transmission and scattering of elastic waves in quasi-two-dimensional wave-guides is studied using the matching method. A linear defect boundary separating two wave-guide crystalline lattice domains is characterised here by a linear chain of defect masses and by modified elastic constants in the boundary, different from their values in the bulk of the domains. In particular a square lattice is considered to model the domains of the two-dimensional planar wave-guide containing the linear defect. The reflection and transmission probabilities, and the total transmission probabilities are calculated numerically and presented for the scattering processes in a variety of cases. We show that the interaction between the localised modes introduced by the defect boundary and the propagating modes of the system leads to Fano resonances. These resonances shift to higher (lower) frequencies for smaller (larger) defect masses, and for the same mass as function of the angle of the incident wave. Other spectral features shown to exist are due to interference effects especially at oblique incidence and when modifying the boundary elastic constants. Received 8 November 1999 and Received in final form 14 January 2000     

Effects of intrasubband coupling on the scattering phases and density of states in a quantum wire

The properties of scattering phases and density of states in a quantum wire with an attractive scatterer are analyzed. We consider two bound states which couple to a scattering channel and give rise to two Fano resonances. It is shown that varying the parameters of the scatterer (such as its strength and position) produces significantly different effects on the phase behavior and density of states, depending on the subband they occur. These effects stem mainly from the difference between the coupling matrix elements of the two resonant levels with the propagating channel mode.     

平面超薄准直器件的参数依赖性

本文设计了一具有高效强指向性的平面超薄人工结构,当平面声波垂直入射到该结构时,若中心狭缝处产生的F-P(Fabry-Perot)共振和两侧类亥姆霍兹凹槽的共振频率耦合时,会产生一具有高度指向性的准直声束。通过对结构两侧的凹槽阵列的作用、凹槽周期D和凹槽数量r的探讨,发现凹槽阵列可将散射声波转换成声表面波,大大提高了透射效率,同时结构的共振频率、声透射率和准直效果和D、r的变化关系不大,说明对该结构准直效应产生影响的是F-P共振频率和类亥姆霍兹共振器共振频率,与布拉格散射无关。这一发现有助于在降低结构尺寸的同时仍保持结构的有效性,为后续指向性器件的应用提供了理论依据。     

Acoustic anechoic layers with singly periodic array of scatterers: Computational methods,absorption mechanisms,and optimal design

The acoustic properties of anechoic layers with a singly periodic array of cylindrical scatterers are investigated. A method combined plane wave expansion and finite element analysis is extended for out-of-plane incidence. The reflection characteristics of the anechoic layers with cavities and locally resonant scatterers are discussed. The backing is a steel plate followed by an air half space. Under this approximate zero transmission backing condition, the reflection reduction is induced by the absorption enhancement. The absorption mechanism is explained by the scattering/absorption cross section of the isolated scatterer. Three types of resonant modes which can induce efficient absorption are revealed. Due to the fact that the frequencies of the resonant modes are related to the size of the scatterers, anechoic layers with scatterers of mixed size can broaden the absorption band. A genetic optimization algorithm is adopted to design the anechoic layer with scatterers of mixed size at a desired frequency band from 2 kHz to l0 kHz for normal incidence, and the influence of the incident angle is also discussed.     

基于开口椭圆环的高效超宽带极化旋转超表面

电磁波的极化态在信号传输和灵敏度测量中有非常重要的应用价值. 本文设计、仿真并实验验证了微波频段基于开口椭圆环谐振器的极化旋转超表面. 理论上, 将多阶表面等离子谐振和高阻抗表面相结合, 解释了多谐振点、高效率极化旋转的物理机理. 数值上, 通过对结构参数的仿真分析, 给出了运用开口椭圆环结构设计多频段、超宽带高效极化旋转超表面的方法. 所设计和制作的超表面能够在相对带宽104.5%的频率范围内实现大于85%的极化旋转效率. 这些工作将为极化操控超表面的设计和应用提供重要帮助.     

Magnon scattering by a symmetric atomic well in free standing very thin magnetic films

A theoretical model is presented for the study of the scattering of magnons at an extended symmetric atomic well in very thin magnetic films. The thin film consists of three cubic atomic planes with ordered spins coupled by Heisenberg exchange, and the system is supported on a non-magnetic substrate, and considered otherwise free from magnetic interactions. The coherent transmission and reflection scattering coefficients are derived as elements of a Landauer type scattering matrix. Transmission and reflection scattering cross sections are hence calculated specifically, as a function of the varying local magnetic exchange on the inhomogeneous boundary. Detailed numerical results for the individual incident film magnons, and for the calculated overall magnon conductance, show characteristic transmission properties, with associated Fano resonances, depending on the magnetic boundary conditions and on the magnon incidence.     

Resonant Lattice Kerker Effect in Metasurfaces With Electric and Magnetic Optical Responses

To achieve efficient light control at subwavelength dimensions, plasmonic and all‐dielectric nanoparticles have been utilized both as a single element as well as in the arrays. Here we study 2D periodic nanoparticle arrays (metasurfaces) that support lattice resonances near the Rayleigh anomaly due to the electric dipole (ED) and magnetic dipole (MD) resonant coupling between the nanoparticles. Silicon and core‐shell particles are considered. Our investigations are carried out using two independent numerical techniques, namely, the finite‐element method and the method of coupled‐dipole equations based on the Green function approach. We numerically demonstrate that choosing of lattice periods independently in each mutual‐perpendicular direction, it is possible to achieve a full overlap between the ED‐lattice resonance and MD resonances of nanoparticles in certain spectral range and to realize the resonant lattice Kerker effect (resonant suppression of the backward scattering or reflection). At the effect conditions, the strong suppression of light reflectance in the structure is appeared due to destructive interference between electromagnetic waves scattered by ED and MD moments of every nanoparticle in the backward direction with respect to the incident light wave. Influence of the array size on the revealed reflectance and transmittance behavior is discussed. The resonant lattice Kerker effect based on the overlap of both ED and MD lattice resonances is also demonstrated.     

Structural, electronic, vibrational, and elastic properties of SWCNTs doped with B and N: an ab initio study

A microscopic model is developed in order to analyse the effects of dissipations on single-photon transport in a coupled cavity array where one of the cavities is coupled to a three-level atom and both cavities and the three-level atom are coupled to an external environment. By employing the quasi-boson approach, the single-photon transmission and reflection amplitudes are found exactly for the Ξ-type, V-type and Λ-type three-level atoms. We focus on the dissipation properties in the case of the Λ-type system. Comparing the dissipative case with the nodissipative one, it can be found that the dissipations of the cavities and the Λ-type three-level atom significantly affect the transmission amplitude of single-photon transport. Whether the atom is in tune with the resonant frequency of the cavity or not, incomplete reflection is mostly caused by atom dissipation near the middle dip of the single-photon transport spectrum, while reduced transmission appears to be mainly controlled by cavity dissipation. Dissipations broaden the line width of the single photon transport spectrum.     

Magnetoelastic Waves in Submicron Yttrium–Iron Garnet Films Manufactured by Means of Ion-Beam Sputtering onto Gadolinium–Gallium Garnet Substrates

A series of equidistant oscillations have been revealed in the transmission spectrum and dispersion law of Damon–Eshbach surface magnetostatic waves (SMSWs) propagating in submicron (200-nm) yttrium–iron garnet (YIG) films manufactured by means of ion-beam sputtering onto gadolinium–gallium garnet (GGG) substrates. These oscillations correspond to the excitation of magnetoelastic waves in the YIG–GGG structure at frequencies of resonant interaction between the surface magnetostatic waves and the elastic shear modes of the wave-guiding YIG–GGG structure. The obtained results show that the studied YIG films are characterized by an efficient magnetoelastic coupling between their spin and elastic subsystems and the matching of acoustic impedances at the YIG–GGG interface, thus providing the possibility to consider the ion-beam sputtering of YIG films onto GGG substrates as a promising technology for the creation of magnonic and straintronic devices.     

Electron transport through a quantum cavity

Electron transport through a quantum cavity coupled with two one-dimensional waveguides is studied using a generalized scattering matrix method. In a symmetric N-channel cavity model, we are able to obtain an exact solution that predicts the electron energies at which the transmission of electron waves become zero. We found that the zero of transmission is closely related to the longitudinal resonance through inter-channel scattering, in particular, to the resonance of the highest propagating mode inside the cavity. This model provides a simple way to calculate the electron transmission through a cavity which could be useful in quantum waveguide engineering.     

Numerical modeling of the reflection coefficients for the plane sound wave reflection from a layered elastic bottom

The matrix method and its numerical realization are considered in calculating the complex reflection coefficients and refraction indices of plane sound waves for geoacoustic models of the ocean bottom in the form of homogeneous elastic (liquid) absorbing layers overlying an elastic halfspace. In calculating the reflection coefficients at high frequencies or in the presence of a large numbers of sedimentary layers, a passage from the Thomson-Haskell matrix approach to the Dunkin-Thrower computational scheme is performed. The results of test calculations are presented. With the aim of developing resonance methods for the reconstruction of the parameters of layered elastic media, the behavior of the frequency-angular dependences of the reflection coefficient are studied for various geoacoustic bottom models. The structure of the angular and frequency resonances of the reflection coefficients is revealed. The dependence of the structure (the position, width, and amplitude) of two types of resonances on the parameters of the layered bottom is considered.     

Simple expressions of the reflection and transmission coefficients of fundamental Lamb waves by a rectangular notch

The scattering of Lamb waves by a two-dimensional rectangular notch is investigated for rapid inspection of defects in a structure. To derive the reflection and transmission coefficients of the scattered waves in a simple way, the scattering caused by the notch is analyzed through the composition of individual scattering processes. Linear equations corresponding to the reflection and transmission coefficients are constructed along with scattering graphs. For an illustration of the efficacy of the presented method, the scattering of fundamental symmetric and anti-symmetric modes are inspected according to the depth and width of a notch in a plate. Validity of these expressions is demonstrated by the comparison of the theoretical analysis results with those from the finite element analysis.