Scattering of an Acoustic Field by Refraction–Density Inhomogeneities with a Small Wave Size and Solution of the Problem of Direct Scattering in an Inhomogeneous Medium

The paper considers acoustic wave scattering by inhomogeneities with a small wave size using the Green’s function apparatus, which makes it possible universally to take into account both the refraction and density components of an inhomogeneity. Estimates for the multipole components of a field scattered by a nonresonance inhomogeneity are presented. For an inhomogeneity with small dimensions, it suffices to consider only monopole and dipole scattering. These conclusions are confirmed by an analysis of the field scattered by a circular cylinder with a small wave radius. The results are used to numerically simulate a Lippmann–Schwinger equation. The form of the discretized matrix Green’s function for identical values of the spatial arguments is presented. This makes it possible to take into account multiple scattering processes within a discretization element with a small wave size. Its use automatically fulfills the relations between the phase and amplitude of secondary acoustic field sources.     

Application of the mutual-interaction method to a class of two-scatterer systems: I. Two discrete scatterers

In a recent paper we developed a formalism that fully accommodates the mutual interactions among scatterers separable by parallel planes. The total fields propagating away from these planes are the unknowns of a system of difference equations. Each scatterer is characterized by a scattering function that expresses the scattered wave amplitude as a function of the incident and scattered wavevectors for a unit-amplitude plane wave scattered from the object in isolation. This function can be derived completely from the scattered far field with the help of analytic continuation. For a two-scatterer system the mutual-interaction equations reduce to a single Fredholm integral equation of the second kind. It turns out that analytic solutions are tractable for those scattering functions that are Dirac deltas or a sum of products of separable functions of the incident and scattered wavevectors. Scattering functions for planes and isotropic scatterers, as well as electric and magnetic dipoles all possess this property and are considered. The exact scattering functions agree with results obtained by analytic continuation. This paper consists of two parts. Part I derives analytic solutions for two discrete scatterers (isotropic scatterers. electric dipoles, magnetic dipoles). Part II is devoted to scattering from an object (isotropic or dipole scatterer) near an interface separating two semi-infinite uniforn-media. Because the results in this paper are exact, the effects of near-field interactions can be assessed. The forms of the scattering solutions can be adapted to objects that are both radiating and scattering.     

Interference effects in the theory of multiphoton laser-simulated bremsstrahlung for electron wave packets that are broad in their momentum representation

The interference peculiarities of absorption and emission of external laser field quanta by a spatially narrow electron wave packet during its scattering at a potential center are considered. The angular and energy distributions for an electron due to scattering in a laser field are obtained and analyzed within the framework of the perturbation theory based on interaction with the potential center. A new mechanism of effective heating of plasma produced in a cluster under the effect of intense external laser field is found. Drastic changes in angular distribution due to the interference of incident and scattered parts of an electron wave packet are revealed.     

Der Einfluß der Dipolwechselwirkung auf die Magnetisierung dünner ferromagnetischer Schichten

The magnetization of thin films is calculated for low temperatures, taking into account the exchange interaction, an external magnetic field, and the dipole interaction. The calculations are performed within a quantized phenomenological spin wave theory. For thin enough films, within the temperature range considered, only the lowest spin wave band contributes to the decrease of the magnetization. The influence of the dipole interaction is as follows: The magnetization decreases less rapidly with growing temperature than predicted by calculations within the Heisenberg model; the decrease depends considerably on the angle between the magnetization and the film plane; even atT=0K there is a small increase of the magnetization with growing external field.     

考虑互耦的半波振子线阵辐射和散射方向图综合

对于半波振子阵列天线,可用感应电动势法算得其阻抗矩阵,依据等效电路原理计入单元间的互耦计算辐射场.将其推广至考虑互耦的散射场计算,基于此辐射场和散射场的计算公式,提出一种考虑互耦同时综合半波振子阵列天线辐射和散射方向图的新方法.运用粒子群优化算法,通过优化振子的间距,同时综合指定辐射和散射方向图.运用该方法,通过对中心工作频率处的辐射和散射特性以及在威胁角度给定频带内的散射特性优化,有效降低半波振子阵在中心工作频率处的辐射和散射方向图的副瓣电平,并减小其在威胁角度给定频带内的雷达散射截面.计算结果与FEKO仿真结果吻合良好,验证方法的正确性.     

Application of the mutual-interaction method to a class of two-scatterer systems: I. Two discrete scatterers

Abstract

In a recent paper we developed a formalism that fully accommodates the mutual interactions among scatterers separable by parallel planes. The total fields propagating away from these planes are the unknowns of a system of difference equations. Each scatterer is characterized by a scattering function that expresses the scattered wave amplitude as a function of the incident and scattered wavevectors for a unit-amplitude plane wave scattered from the object in isolation. This function can be derived completely from the scattered far field with the help of analytic continuation. For a two-scatterer system the mutual-interaction equations reduce to a single Fredholm integral equation of the second kind. It turns out that analytic solutions are tractable for those scattering functions that are Dirac deltas or a sum of products of separable functions of the incident and scattered wavevectors. Scattering functions for planes and isotropic scatterers, as well as electric and magnetic dipoles all possess this property and are considered. The exact scattering functions agree with results obtained by analytic continuation. This paper consists of two parts. Part I derives analytic solutions for two discrete scatterers (isotropic scatterers. electric dipoles, magnetic dipoles). Part II is devoted to scattering from an object (isotropic or dipole scatterer) near an interface separating two semi-infinite uniforn-media. Because the results in this paper are exact, the effects of near-field interactions can be assessed. The forms of the scattering solutions can be adapted to objects that are both radiating and scattering.     

Polar exit angle distributions of slow H<Superscript>+</Superscript> ions,related to the presence of a regular domain structure,calculated within the diffuse scattering approximation upon interaction of ions with the sample surface

The angular distributions of slow H ions scattered from the surface of a magnetic film with a stripe domain structure have been calculated. The calculations were performed within the models of specular and diffuse scattering upon collisions of ions with surface. It is shown that the magnetic fields of the stripe domain structure block the ions scattered at small exit angles. It is established that, within the more adequate model of diffuse scattering, the anisotropy of interaction of ions with the magnetic field of the stripe structure manifests itself in a wider angular range.     

Scattering of spatially inhomogeneous sound waves by a spherical particle

The problem of monopole, dipole, and rotational scattering of a spatially inhomogeneous time-harmonic sound field by an arbitrary spherical particle is solved for the cases of the medium being a viscous compressible liquid or an isotropic elastic medium. Equations for the spherical mean fields at the particle are obtained. These equations are used to derive the formulas for the scattered fields. Different limiting cases of particle behavior are considered. In particular, it is shown that the dipole scattering is determined by two components of particle oscillations, one of which corresponds to translational oscillatory motion and the other to oscillations of two antiphase monopoles. For these types of particle oscillations, a scattering matrix, which determines the scattering of an arbitrary field by a particle, is constructed. The matrix allows the formalization of the processes of multiple sound scattering by particles and is valid for any distances between the particles down to their contact.     

Symmetry and optical transition rule for low-dimensional semiconductor system with spin–orbit interaction and magnetic field

Starting from effective mass Hamiltonian, we systematically investigate the symmetry of low-dimensional structures with spin–orbit interaction and transverse magnetic field. The position-dependent potentials are assumed to be space symmetric, which is ever-present in theory and experiment research. By group theory, we analyze degeneracy in different cases. Spin–orbit interaction makes the transition between Zeeman sub-levels possible, which is originally forbidden within dipole approximation. However, a transition rule given in this paper for the first time shows that the transition between some levels is forbidden for space symmetric potentials.     

Electromagnetic field perturbation by an internal void in a conducting cylinder excited by a wire loop

A thin prolate spheroidal void in an infinite conducting circular cylinder is used to model an internal flaw in a wire rope. The rope is excited by an electric ring current which is a model for a thin solenoid or multi-turn wire loop. The anomalous external fields are computed from the induced electric and magnetic dipole moments of the void. For this type of excitation, the induced axial magnetic dipole moment is the dominant contributor to the scattered field. The results have application to nondestructive testing of wire ropes.     

Lamellar-like structures in ferrofluids placed in strong magnetic fields

We consider a ferrofluid system consisting of magnetic particles interacting with a magnetic dipole–dipole interaction. We study the strong magnetic field regime where all magnetic dipoles are completely polarized in the direction of the magnetic field. We introduce a lattice gas model that serves to describe space ordering phenomena in such systems. It is found that, within mean field theory, this model predicts a second order phase transition to a phase with inhomogeneous lamellar-like ordering below a certain critical temperature.     

Magnetic field tomography

Neutral atoms may be trapped via the interaction of their magnetic dipole moment with magnetic field gradients. One of the possible schemes is the cloverleaf trap. It is often desirable to have at hand a fast and precise technique for measuring the magnetic field distribution. We use for instantaneous imaging the equipotential lines of the magnetic field a diagnostic tool which is based on spatially resolved observation of the fluorescence emitted by a hot beam of sodium atoms crossing a thin slice of resonant laser light within the magnetic field region to be investigated. The inhomogeneous magnetic field spatially modulates the resonance condition between the Zeeman-shifted hyperfine sublevels and the laser light and therefore the amount of scattered photons. We apply this technique for mapping the field of our cloverleaf trap in three dimensions under various conditions. Received 20 March 2001 and Received in final form 12 May 2001     

Comparison of light scattering from self assembled array of nanoparticle chains with cylinders

We use a magnetic field to generate array of linear chains of magnetic nanoparticles in a magnetically polarizable nanofluid. The scattered patterns by these chains are described by electromagnetic wave scattering from an infinite cylinder. Comparison of the incident angle dependent scattered patterns from the linear chains of nanoparticles and macroscopic cylinders show a striking similarity. But, unlike the diffraction fringes of different orders observed on the cone of scattered light from macroscopic cylinder, the observed scattered cone from the field induced nanostructures is diffused. The observed optical patterns for micron sized cylinder are a simultaneous manifestation of both scattering and diffraction due to the interaction of light with cylindrical surfaces, whereas for nanoparticle chains it is mainly due to scattering. The diameters of the cylinders are precisely calculated from the best fit on the experimental diffraction intensity pattern. These results are important for better understanding of self assembled nanostructures for applications.     

Effect of electric field on light scattering by aqueous colloids of diamond and graphite

We present the results of our experimental investigation of light scattering by polydisperse colloids of diamond and graphite. The scattering is studied at a random orientation of particles and in an external radiofrequency electric field, which orients particles along the strength. The average dimensions of particles in both colloids are close to each other and comparable with the wavelength of the incident light. The shape of particles and the optical and electrooptical properties of diamond and graphite colloids are significantly different. We analyze the polarization components of scattered light energy when the light incident on the colloids is linearly polarized. We show that the quadrupole light scattering by isotropic diamond particles has the main effect on angular dependences of depolarization of scattered light. For light scattering by anisotropic graphite particles, the depolarization of scattered light is mainly determined by a particular feature of the dipole scattering of particles. It is shown that, in both colloids, the orientational order of particles considerably reduces the depolarization of light scattered by particles. We show that relative changes in the intensity and depolarization of scattered light, which depend on the scattering angle and polarization direction of light, as well as on the parameters of particles, can be used as a measure of electrooptical effects observed in colloids.     

Multipole resonance in the interaction of a spherical Ag nanoparticle with an emitting dipole

The effect of multipole resonance in the interaction between a spherical metallic nanoparticle （MNP） and an emitting dipole is studied with the Mie theory. The results show that the absorption peak of the MNP with respect to the field of the emitting dipole is blue-shifted with the decrease of the spacing between MNP and emitting dipole due to the enhanced multipole resonance. At a short distance, the enhanced multipole terms of scattering are not obvious compared with the dipole term. For the decay rate of the emitting dipole, multipole resonance brings about the enhancement of it largely at short spacing. For the radiative decay rate, the behavior is quite different. The dipole term is dominant at a short spacing, and the multipole term is dominant at a larger spacing.     

Theory of electromagnetic generation of acoustic waves in metals

The theory of the anomalous skin effect in metals is used in order to study the direct generation of ultrasound by electromagnetic radiation incident on a metal surface, in the absence and in the presence of a magnetic field. The non-monotonic behaviour of the acoustic flux observed experimentally, as a function of the magnetic field can be explained within the framework of the free-electron model assuming that the electrons are scattered diffusely from the metal surface. The reason for this behaviour is traced to the variation of the relative phases of the collision drag force on the bulk ions and that on the surface with increasing magnetic field. The relations between existing theories of direct generation of ultrasound have been investigated within both the free-electron approximation and the effective-mass theory. We conclude that, besides the collision drag force, the Lorentz force and the Bragg reaction force, an additional ‘deformation’ force acts upon the positive ions. We find that the assumption of electron diffuse surface scattering is the only mechanism that can account for the observations on the variety of materials investigated. We also analyse, within a simple effective-mass approximation, possible band structure effects on acoustic wave propagation in metals and on the helicon-phonon interaction.     

Coherent backscattering in nematic liquid crystals in an external magnetic field

We consider multiple light scattering in a nematic liquid crystal. Using the Monte Carlo method, we calculate for the first time the effect of a magnetic field on the shape of the peak of coherent backscattering taking into account the long-range action of fluctuations of the orientational order and anisotropy of the scattering length. For a small number of initial and final scattering events, we take into account the ordinary mode of light, which is weakly scattered in a nematic liquid crystal (NLC), whereas a strongly scattered extraordinary mode is taken into account for all scattering events. For simplicity, we use a single-constant approximation of the NLC elastic moduli. We show that the angular shape of the peak of coherent backscattering remains nearly unchanged, whereas the magnetic field and the scattering phase function vary by several orders of magnitude.     

Light scattering by closely packed clusters: Shielding of particles by each other in the near field

Equations of the theory of light scattering by clusters (aggregates) of spherical particles are analyzed, and peculiarities of interaction of scatterers in the near field are discussed. It is shown that one of the manifestations of the near field is a mutual shielding of particles. For simple clusters consisting of two identical spherical particles (bisphere), the mutual shielding leads to a decrease in the intensity of light scattered along the axis of the bisphere. If the bispheres are small as compared to the wavelength, shielding is caused by electrostatic interaction of charges induced in the particles by the external field. Calculations of the intensity of light scattered by randomly oriented clusters of spherical particles show that for the model ignoring the near field the intensity is significantly larger than for the model with the near field taken into account.     

Anomalous light scattering by small particles

Light scattering by a small spherical particle with a low dissipation rate is discussed based upon the Mie theory. It is shown that if close to the plasmon (polariton) resonance frequencies the radiative damping prevails over dissipative losses, sharp giant resonances with very unusual properties may be observed. In particular, the resonance extinction cross section increases with an increase in the order of the resonance (dipole, quadrupole, etc.); the characteristic values of electric and magnetic near fields for the scattered light are singular in the particle size, while energy circulation in the near field is rather complicated, so that the Poynting vector field includes singular points whose number, types, and positions are very sensitive to fine changes in the incident light frequency. The results may provide new opportunities for a giant, controlled, highly frequency-sensitive enhancement and variation of electromagnetic field at nanoscales.