On the validity of some new acoustic scattering approximations

Abstract

Two new approximations for predicting the elastic scattering of plane acoustic waves by a weak scatterer are proposed. The approximations have been obtained by drawing an analogy between acoustic and light scattering problems. The validity of these approximations has been examined numerically for the exactly soluble case of scattering by a homogeneous sphere. Results show that for small angle scattering the proposed approximations have a considerably larger domain of validity in comparison to the extensively used Born approximation.     

Light scattering studies of oleic acid under high pressure

Abstract

Elastic light scattering of oleic acid has been studied at room temperature and at pressures up to 0.5 GPa, where the strong scattering of radiation in the visible region occurs. For the application of pressure above 0.21 GPa the strong increase of the scattered light intensity has been observed some seconds after an application of pressure. More detailed analysis shows that Me theory is not sufficient to explain experimental results for size of Scattering particle greater than 18 μm.     

Monte Carlo simulation of photon transport in a randomly oriented sphere-cylinder scattering medium

A Monte Carlo simulation tool for simulating photon transport in a randomly oriented sphere-cylinder medium has been developed. The simulated medium represents a paper pulp suspension where the constituents are assumed to be mono-disperse micro-spheres, representing dispersed fiber fragments, and infinitely long, straight, randomly oriented cylinders representing fibers. The diameter of the micro-spheres is considered to be about the order of the wavelength and is described by Mie scattering theory. The fiber diameter is considerably larger than the wavelength and the photon scattering is therefore determined by an analytical solution of Maxwell’s equation for scattering at an infinitely long cylinder. By employing a Stokes–Mueller formalism, the software tracks the polarization of the light while propagating through the medium. The effects of varying volume concentrations and sizes of the scattering components on reflection, transmission and polarization of the incident light are investigated. It is shown that not only the size but also the shape of the particles has a big impact on the depolarization.     

微纳粒子光学散射分析

为实现利用光学方式对微纳尺度粒子性质的研究,探讨了亚微米线及亚微米球对光电磁波的散射效应.微纳米尺度粒子的光学散射,散射粒子尺寸与入射光波长尺寸可满足米氏(Mie)散射条件.利用Matlab数值模拟的方式,将分析结果以模拟图的形式清晰地展现出来.满足尺寸条件的层状粒子以及任意多个散射粒子存在时对电磁波的散射都可采用Mie散射分析方法,并且针对多粒子散射,分析了散射体位于不同位置时对散射造成的影响.通过分析光学散射光场相关的微分散射截面及近场散射电磁场分布,可得出散射光场随散射角度的变化趋势,以及散射光场受各类因素的影响,包括入射光偏振态、散射粒子尺寸、散射粒子结构及粒子构成层数、散射粒子数量等的影响,也包括一些隐含因素对散射光场的影响,如散射粒子与周围介质的相对折射率.本文的科学意义体现在:与入射光波长尺寸可比的亚微米尺度的粒子,可用作传感器,对于其位移的探测可通过光学方式来实现,而由于粒子本身特性对散射光的影响具有一定的参考价值,从而使通过光学方式对机械位移的读出具有更高准确度.研究结果对于光学方式探测亚微米线机械振动具有指导意义.     

Surface enhanced Raman scattering of light by ZnO nanostructures

Raman scattering (including nonresonant, resonant, and surface enhanced scattering) of light by optical and surface phonons of ZnO nanocrystals and nanorods has been investigated. It has been found that the nonresonant and resonant Raman scattering spectra of the nanostructures exhibit typical vibrational modes, E ₂(high) and A ₁(LO), respectively, which are allowed by the selection rules. The deposition of silver nanoclusters on the surface of nanostructures leads either to an abrupt increase in the intensity (by a factor of 10³) of Raman scattering of light by surface optical phonons or to the appearance of new surface modes, which indicates the observation of the phenomenon of surface enhanced Raman light scattering. It has been demonstrated that the frequencies of surface optical phonon modes of the studied nanostructures are in good agreement with the theoretical values obtained from calculations performed within the effective dielectric function model.     

Brillouin scattering studies of rare gas solids

Abstract

The scattering of light by elementar excitations in the matter is results in two phenomena, discriminated by the zero wavevector frequency of the excitation: if this frequency is zero, one deals with Brillouin scattering, and with Raman scattering in the other case. Brillouin scattering results from the interaction of light with thermal excitations (acoustic phonons in a crystal) of a material, or, from a classical point of view, with density waves. Contrary to Raman scattering, the selection rules allow always the observation of at least one mode. It is a powerful technic in the study of rare gases under pressure: at ambient temperature, rare gases crystallize in the face centered cubic structure (except helium which structure was recently found to be hexagonal) and are therefore Raman and infrared inactive.

Experimental results will be reviewed on rare gases and rare gas mixtures in the fluid phase, like He-Ne and He-H₂. These results will be discussed in relation with recent measurements of the frequency of global oscillations of Jupiter.     

Resonant Raman scattering in nanostructures with InGaAs/AlAs quantum dots

Raman scattering by optical phonons in In_xGa_{1 ? x} As/AlAs nanostructures with quantum dots has been studied experimentally for compositions corresponding to x = 0.3?1 under out-resonance conditions. Features due to scattering by GaAs-and InAs-like optical phonons in quantum dots have been detected, and the phonon frequencies have been determined as a function of the dot composition. With increasing excitation energy, a red shift is observed in the frequency of the GaAs-like phonon in quantum dots, which testifies to Raman scattering selective by the size of quantum dots. Under resonant conditions, multiphonon light scattering by optical and interface phonons is observed up to the third order, including overtones of the first-order phonons of InGaAs and AlAs materials and their combinations.     

Monte Carlo simulation of light scattering from size distributed sub-micron spherical CdS particles in a volume element

Simulation of polarized light scattering by spherical particles having modal radius of 180 nm is presented in this paper. A Monte Carlo method which is based on the Stokes-Mueller formalism developed in ANSI Standard C-language is used for simulation. Single scattering is considered in our program with monodispersed sub-micron sized spherical CdS particles. We have considered only θ dependent scattering as described by Mie theory for spherical CdS particles. The experiments for studying light scattering properties of these particles were conducted in a designed and developed laser based light scattering studies setup. The simulation results were compared with experimental results and theoretical results obtained purely from Mie theory. The closeness of agreement or disagreement between these results is discussed in this paper.     

On glancing scattering of light ions from tungsten single crystal surfaces

A computer calculation of light ion (200–800 keV protons and helium) scattering from a tungsten single crystal model with a (110)-type surface is presented. An axis (100) is considered. Thermal vibrations in 3 dimensions have been incorporated. The number, i.e. fraction, and spatial distribution of reflected ions for some angular scans are presented in an appendix. The importance of trajectory histories is demonstrated. 3 regimes of scattering behaviour as a function of the incidence angle ψ in to the string in the surface are found:

(i) for very small ψ_in surface-semi-proper-channelling of all ions;

(ii) for intermediate ψ_in mainly planar channelling and dechannelling resulting in a decreasing reflective index for increasing ψ _in;

(iii) axial channelling and transverse plane focussing are mainly of importance leading to a small reflective index, and surface semi-channelling like for (ii) occurs for very few ions.

The importance of the scattering potential is demonstrated and an expression is discussed.     

Photorefraction and anisotropic light scattering in LiNbO3-Fe crystals

By holographic recording in LiNbO₃-Fe and LiTaO₃-Fe crystals a new light scattering effect has been observed with an optical indicatrix along the optical axis. The kinetics of the light scattering depends on the intensity, wavelength as well as the polarization of the incident light The holographic volume grating is created by the interference of an incident light and light scattered by crystal inhomogeneities.     

Rayleigh scattering from a trapped bose condensate and the corresponding recoil atom velocity distribution

An approach has been developed that allows the Rayleigh scattering cross section to be calculated with allowance for the quantum character of motion of the center of mass of the trapped scattering particles. The shape of the line of light scattering from a Bose condensate in a parabolic trap has been studied. A shift of the scattering line center is equal to the recoil shift, while the line width depends on the chemical potential of the Bose gas and on the relaxation time of the velocity of the above-condensate recoil particles. A velocity distribution function in the beam of recoil atoms formed in the course of induced Rayleigh scattering is determined. It is shown that, under the typical experimental conditions, the characteristic width Δv/v of the recoil velocity distribution in this beam is on the order of 10^?3 at a velocity v on the order of several centimeters per second.     

Hyperfine interaction in amorphous ferromagnetic cobalt-phosphorous-alloys measured by inelastic scattering of neutrons

The hyperfine interaction at the cobalt nucleus in amorphous ferromagnetic cobalt-phosphorous-alloys CoP _x has been investigated by measuring the inelastic incoherent spin-flip scattering of neutrons. A neutron spectrometer of an extremely high energy resolution of 67 MHz full width at half maximum (FWHM) was used. It was found that the magnetic hyperfine splitting, in first order, is proportional to the magnetic moment of the cobalt atom and that there is a distribution of hyperfine fields of about 50 MHz around a mean value which depends on the phosphorous concentration.

     

Polarimetric weak-localization effect in scattering of natural light in the region of small phase angles

Results of laboratory measurements and interpretations of the polarimetric effect of weak localization (negative polarization) appearing under scattering of natural light by a dark ultradisperse surface (soot with albedo of 2.5%) are presented. The measurements were carried out in red light in a range of phase angles of 0.2°–4.2°. It was revealed that the soot, despite the low albedo, shows the polarimetric weak-localization effect inherent in bright surfaces such as the surface formed by smoked MgO. The results of measurements are interpreted using numerical simulation of multiple light scattering in a medium consisting of particles whose characteristics are close to those observed for soot. As the result of simulation, it was found that the scattering with the multiplicity exceeding two can give rise to a negative polarization branch, which becomes narrower with increasing scattering order. For the case of soot, four orders of scattering is sufficient to describe the observed polarimetric effect of weak localization with a necessary accuracy.     

Theory of Raman scattering in Europium-chalcogenides

The intensity of the spin assisted Raman scattering in the Eu-chalcogenides is calculated using the excited states which were used in the analysis of the optical absorption. Various mechanisms are examined for the Raman scattering. In these mechanisms, the cross effect of the 4f spin-orbit interaction and the exciton-phonon interaction is found to be the most important for the scattering intensity. The characteristics of the Raman scattering from this mechanism are as follows: When the frequency of the incident light is in the tail region of the absorption peak, the polarization of the scattered light is perpendicular to that of the incident light; when the light in the middle region of the absorption peak is applied, the polarization of the incident light is depolarized in the scattered light; the scattering intensity decreases when the spin fluctuation is suppressed by an application of magnetic field or by lowering temperature through the Curie temperature. These characteristics in the Raman scattering have been observed in the experiments.     

Four-wave polariton scattering of light in LiNbO3

Four-wave Stokes k-spectra for light scattering on polaritons in lithium niobate crystals with an Mg impurity are studied experimentally. The mechanisms for direct, cascade, coherent, and incoherent four-wave mixing of light are discussed in the course of interpreting the angular dependences of the scattered light intensity. It is shown that the dispersion of the real part of the polariton wave vector and the refractive index of the crystals at the polariton frequencies can be measured with an order of magnitude greater accuracy than by spontaneous three-wave polariton light scattering. A significant discrepancy is found between determinations of the polariton absorption coefficient from the angular spectra of three-wave scattering and four-wave scattering in terms of the model employed here. Zh. éksp. Teor. Fiz. 112, 441–452 (August 1997)     

Light scattering,flow birefringence and generalized hydrodynamics

The stress-optical coefficient ?, which is measured in flow birefringence experiments, is calculated using the generalized hydrodynamic equations which have been recently used to explain the k-dependent fine structure observed in depolarized (VH) light scattering. The resulting expressions allow a comparison of the strength of the birefiringence effect with that of the light scattering fine structure effect in a given liquid. It is seen that the combination of flow birefringence and VH scattering measurements yields the orientational structure factor g ⁽²⁾ of Ben-Reuven and Gershon. The theory is in reasonable agreement with experiment. The combination of our results with those of Raman and Krishnan yields a simple expression for the parameter δ which measures the strength of the stress-orientational effect. The dependence of the stress-optimal coefficient upon orientational order is briefly discussed.     

Study of quasi-two- and three-dimensional ordered porous structures by means of small-angle X-ray scattering in the grazing incidence geometry

The structure of surface layers of thin metal inverse opals has been studied first by the grazing-incidence small-angle X-ray scattering technique. Contributions of the form factor and structure factor to the small-angle diffraction pattern have been separated using a numerical model of the scattering process. The complementary use of the small-angle X-ray scattering and grazing-incidence small-angle X-ray scattering techniques has provided independent information about the bulk and surface properties of the samples and allowed a type of defect in the investigated structures to be determined. The measurement results have been verified by atomic force microscopy.

     

双峰分布的超细颗粒动态光散射信号模拟及精度分析

为获取双峰分布超细颗粒的动态光散射模拟信号,通过建立动态光散射随机过程的AR模型,利用修正的Levison Durbin递推算法确定模型参数和阶数的方法模拟光散射信号.分别对10 nm与90 nm,200 nm与1 000 nm双峰分布颗粒的动态光散射信号进行模拟,得到的模拟信号光强自相关函数与理论值吻合,用双指数法对颗粒粒径反演,相对误差小于3.55%.通过分析模型阶数、采样时间、采样频率、模拟数据长度等参数对模拟精度的影响,得出双峰分布颗粒光散射信号的模拟精度与各参数的关系:在低于阈值阶数时,模型阶数选择对精度影响大,模型阶数越高,信号模拟的精度越高,高于阈值阶数时,模型阶数选择对精度影响不大,可选阈值阶数模型模拟信号.选定一定的采样时间,采样频率越高,模拟数据长度越长,模拟精度越高.     

Imaging and multiple scattering through media containing optically active particles

Abstract

This paper examines the behaviour of polarized light scattered by a medium containing small chiral spheroidal particles. We show that for single scattering the observed phenomena of optical activity may be interpreted in terms of an averaged Mueller matrix and describe how the degree of polarization is affected by such a medium. The polarization properties of multiply scattered light by chiral particles are considered through the use of Monte Carlo simulations. It is shown that the effects of chirality under multiple scattering can be interpreted as an order-preserving influence in a disordered system and that this influence can, in principle, be exploited for the purposes of imaging.     

Depolarised Rayleigh light scattering in argon layer confined between graphite plains: MD simulation

The interaction-induced light scattering many-body correlation functions and their spectra in a thin argon layer located between two parallel graphite walls have been investigated by molecular dynamics simulation method. The calculations have been performed for three different distances between graphite plates. Our simulations show the increased intensity of the interaction-induced light scattering spectra at low frequencies for argon atoms in confined space, in comparison to the bulk (unconfined) sample. Moreover, we show a substantial dependence of the interaction-induced light scattering correlation functions of argon on the distances between graphite walls, that is, on the density of argon layer. The mean square displacement and related diffusion coefficient of argon atom in the confined space has been also investigated. Moreover, the structural feature of the thin layer has been studied by calculating the argon density profile, perpendicular to the graphite walls. An interesting observation is the development of a fluid phase in the innermost region of the confined argon layer.