Light-scattering by optically soft randomly oriented spheroids

In the framework of the Rayleigh–Gans–Debye approximation and anomalous diffraction approaches, the light scattering characteristics of randomly oriented spheroids have been investigated. It has been proved that the system of randomly oriented spheroids is equivalent to the system of polydisperse spherical particles that have the same values of volume and surface area as nonspherical particles. The power law size distribution meeting these requirements has been obtained.     

随机取向椭球粒子的吸收特性

用Ｔ矩阵方法计算了折射率虚部的范围在０．００１至０．１的几种椭球粒子随机取向时在几种等效尺度参数下的光散射与吸收特性，并与等效的球形粒子的光散射结果进行了比较。分析结果表明：椭球粒子的吸收特性与等效的球形粒子的吸收特性存在着差别，这种差别随粒子的形状、尺度和折射率而改变，考虑到目前气溶胶粒子复折射率虚部的测量精度，以等效的球体粒子处理非球形粒子的吸收不会带来显著的误差。     

Coherent transmission and angular structure of light scattering by monolayer films of polymer dispersed liquid crystals with inhomogeneous boundary conditions

We consider monolayer polymer films with oriented droplets of a nematic liquid crystal (LC). Relations for the coherent transmission coefficients of a layer of oriented ellipsoidal droplets and for the intensity of light scattered by monolayers of spherical and spheroidal droplets have been obtained. The amplitude-phase screen model and the interference approximation of the theory of multiple wave scattering have been used. To describe light scattering by an individual ellipsoidal droplet with inhomogeneous surface binding, we have developed an anomalous diffraction approximation. For monolayers of spherical LC droplets, the coherent scattering coefficients and the angular scattering structure have been analyzed. The internal structure of nematic droplets have been calculated by the relaxation method based on the solution of the minimization problem of the free energy volume density. We have studied basic regular features of light scattering by a monolayer with homogeneous and inhomogeneous boundary conditions at the LC-polymer interface. We show that, for films that contain droplets with inhomogeneous boundary conditions of the tangentially normal type, the angular structure of the scattered light is asymmetric with respect to the polar scattering angle.     

随机取向群聚椭球粒子光散射特性研究

利用离散偶极子近似方法,考虑单元粒子之间的电磁相互作用,数值计算了随机取向的不同尺度参数、不同纵横比的群聚椭球粒子的缪勒矩阵元素,给出了各个缪勒矩阵元素的角分布曲线,探讨了随机取向的群聚椭球粒子的尺度参数、纵横比、基本粒子相对位置对其缪勒矩阵元素的影响。并将随机取向群聚椭球粒子的光散射特性与单个等效球形粒子的数值结果进行了比较。结果表明,随机取向群聚椭球粒子的光散射特性与等效球形粒子的光散射特性存在很大差别,基本粒子的形状越偏离球形,这种差别就越大; 随机取向群聚椭球粒子中椭球粒子的纵横比和相对位置对整个群聚粒子的缪勒矩阵元素存在不同程度的影响,并且此影响随着粒子尺度参数的增大而变得更加显著。     

随机取向群聚椭球粒子光散射特性研究

利用离散偶极子近似方法,考虑单元粒子之间的电磁相互作用,数值计算了随机取向的不同尺度参数、不同纵横比的群聚椭球粒子的缪勒矩阵元素,给出了各个缪勒矩阵元素的角分布曲线,探讨了随机取向的群聚椭球粒子的尺度参数、纵横比、基本粒子相对位置对其缪勒矩阵元素的影响。并将随机取向群聚椭球粒子的光散射特性与单个等效球形粒子的数值结果进行了比较。结果表明,随机取向群聚椭球粒子的光散射特性与等效球形粒子的光散射特性存在很大差别,基本粒子的形状越偏离球形,这种差别就越大; 随机取向群聚椭球粒子中椭球粒子的纵横比和相对位置对整个群聚粒子的缪勒矩阵元素存在不同程度的影响,并且此影响随着粒子尺度参数的增大而变得更加显著。     

Study of dispersion and absorption of an ensemble of spherical particles inside an optical cavity and new possibilities of predicting the optical characteristics of biological media by intracavity laser spectroscopy

Optical characteristics, namely, dispersion and absorption spectra of an ensemble of spherical particles randomly oriented inside an optical cavity are investigated. The study is based on the self-consistent matching of new data from the inhomogeneous optical cavity with data from the scattering of an ensemble of spherical particles of different size, randomly oriented in free space. As a result, a new model, which self-consistently accounts for multiple scattering in the optical cavity, has been developed to predict absorption and dispersion of ensembles of spherical particles. This model is supposed to enhance potentiality of the intracavity method for plotting wavelength dependences of optical characteristics of media. A specific calculation of dispersion and absorption dependences on the wavelength shows that this method can be used for investigation of biological media consisting of spherical particles, in particular, erythrocyte suspension.     

随机取向烟尘团簇粒子的光学截面的数值计算

采用蒙特卡罗方法根据团簇—团簇凝聚（CCA）模型对由球形原始微粒凝聚而成的烟尘团簇粒子进行了模拟,利用离散偶极子近似（DDA）方法数值计算了不同原始微粒粒径和数目组成的随机取向的烟尘团簇粒子的总消光截面、吸收截面及散射截面等光学特性参数,研究了原始微粒粒径及数目对随机取向烟尘团簇粒子光学特性的影响。结果表明：当入射波长一定时,随机取向烟尘団簇粒子的光学特性主要取决于原始微粒的粒径和数目;烟尘团簇粒子对不同波段激光的吸收和散射存在差别,这种差别随原始微粒粒径及数目变化而变化。这一工作为研究电磁波在烟尘中的传输特性提供重要参考数据。     

Laboratory measurements of single light scattering by ensembles of randomly oriented small irregular particles in air. A review

In this paper we present an overview of light scattering experiments devoted to measure one or more elements of the scattering matrix as functions of the scattering angle of ensembles of randomly oriented small irregular particles in air. A summary of the most important findings in light scattering experiments on ensembles of randomly oriented particles in air is given. The particles of interest are relevant for studies of atmospheres of planets and satellites and also for other astronomical bodies and environments. Some applications of light scattering experiments are also presented.     

随机取向立方粒子光散射的数值分析

用离散偶极子近似法，计算了立方粒子随机取向时在几种等效尺度参数下的光散射特性，并与等效球形粒子的光散射特性进行了比较．结果表明：立方粒子和其他非球形粒子与其等效球形粒子光散射特性之间的差别大致相同，说明对于随机取向的非球形粒子的光散射问题，粒子的内在对称性和表面的突变不会带来明显的效果． 关键词：     

Light-scattering methods for modelling algal particles as a collection of coated and/or nonspherical scatterers

Ocean reflectance or ocean colour measurements are an important tool for oceanographic studies of phytoplankton dynamics. Theoretical models based on homogeneous, spherical particles underestimate algal backscattering and thus reflectance values. It is our understanding that more advanced light-scattering methods must be employed, both for refractive index retrieval (Mie, Aden–Kerker) with inverse models, and for backscattering calculations (Extended Boundary Condition Method, EBCM). The measured optical properties of a monospecific bloom of the marine brown tide pelagophyte Aureococcus anophagefferens are used to compare the effects of assuming various simulated particle geometries. Computational results from polydisperse, coated spherical particles show results that compare better to experimental reflectance values than calculations based on homogeneous spheres. No noticeable change in simulated reflectance values is observed when a randomly oriented coated spheroidal (rather than spherical) geometry is assumed for the particle population. Our results suggest that a layered spherical geometry, based on Aden–Kerker theory, can adequately reproduce experimentally determined light-scattering properties even supposedly shape-sensitive properties such as the backscattering coefficient.     

分形结构对随机取向烟尘团簇粒子光散射特性的影响

利用蒙特卡罗方法对不同分形维数和分形前向因子的随机取向烟尘团簇粒子的分形结构进行了仿真,采用离散偶极子近似（DDA）方法对随机取向烟尘团簇粒子的缪勒矩阵元进行了数值计算,并与球形粒子模型进行了比较,深入探讨了烟尘团簇粒子的分形维数和分形前因子对其散射特性的影响。研究表明,等效球形粒子的光散射特性与随机取向烟尘团簇粒子的光散射特性存在很大差别,并且此差别随着团簇粒子的分形维数以及分形前向因子的增大而减小;分形维数对表征团簇粒子散射特性的缪勒矩阵元的影响在一定散射角范围内均比较明显,分形前向因子对团簇粒子的缪勒矩阵元角分布的影响与分形维数的影响类似,不过其影响相对分形维数较弱。     

Scattering amplitude of absorbing and nonabsorbing spheroidal particles in the WKB approximation

This work is devoted to a theoretical study of scattering of light by absorbing and nonabsorbing oriented spheroidal particles in the Wentzel-Kramers-Brillouin (WKB) approximation. Within the framework of the scattering theory, we investigate the form factor and the scattering amplitude for this approximation. The Rayleigh-Gans-Debye theory (RGD), the diffraction approximation (DA), and the anomalous diffraction (AD) are treated as particular cases for nonabsorbing spheroids. To illustrate our formalism, we analyze some numerical examples.     

Anomalous diffraction of light with geometrical path statistics of rays and a Gaussian ray approximation

The anomalous-diffraction theory (ADT) of extinction of light by soft particles is shown to be determined by a statistical distribution of the geometrical paths of individual rays inside the particles. Light extinction depends on the mean and the mean-squared geometrical paths of the rays. Analytical formulas for optical efficiencies from a Gaussian distribution of the geometrical paths of rays are derived. This Gaussian ray approximation reduces to the exact ADT in the intermediate case of light scattering for an arbitrary soft particle and describes well the extinction of light from a system of randomly oriented and (or) polydisperse particles. The implications for probing of the sizes and shapes of particles by light extinction are discussed.     

The nanoparticle shape's effect on the light scattering cross-section

In the framework of kinetic approach we develop a theory for light scattering by ellipsoidal metallic nanoparticles whose dimensions are less than those of a free electron path. In this case, the surface of the particle starts to play a dominant role in electron scattering. When the size of the particle decreases below the free electron path at least in one direction, the optical conductivity becomes a tensor quantity, and the diagonal components of this tensor define the half-widths of the plasmon resonances peaks. Thus, the effect of the particles' shape both on the frequencies of plasmon resonances and on their half-widths is considered. Additionally, the expression for a cross-section of the light scattering by a collection of chaotically oriented spheroidal nanoparticles is obtained and averaged over different directions of particles in the collection. Our results highlight the plasmonic properties of metallic nanospheroids, notably, the spectrum of the light scattering has two peaks at the frequencies of plasmon resonances even if there is no preferential direction in the collection of particles.     

On the Mutual Excitation of Embedded Circular Chains of Small Plasmonic Particles

To solve the problems of the repeated scattering of electromagnetic waves at ensembles of dielectric particles taking into account their conductivity, the interaction between collective optical modes of 2D embedded chains of small spherical plasmonic particles, which are characterized by electric dipole coupling, is analytically described on the basis of the quasi-separable T-scattering operator approach. It is assumed that the polarization vector of the electric field of an incident wave is oriented perpendicular to the plane of particle chains. The mutual influence of currents is demonstrated using a regular triangle whose vertices and center of symmetry contain particles and two embedded square chains with eight particles located at their vertices.     

The comparison between the Mie theory and the Rayleigh approximation to calculate the EM scattering by partially charged sand

The Mie theory and Rayleigh approximation are two basic methods to study the EM scattering of uncharged spherical particle, and when the particle radius is much smaller than the incident wavelength, they are equivalent, but whether the Rayleigh approximation is still equivalent to Mie theory when we use them to calculate the EM scattering of small charged particle, there is still no any report published to discuss this problem. In this paper we make some comparisons between Mie theory and Rayleigh approximation to solve the EM scattering of partially electrification spherical particles. The results showed that the Mie theory would be more suitable to calculate the scattering of charged spherical particles.     

Non-Gaussian fluctuations of Stokes parameters in scattering by small particles

Calculations are presented for second-order fluctuations of Stokes parameters that result from singly scattered, coherent, polarized radiation by an ensemble of small spheroidal particles which are randomly oriented in space. The form of the second-order non-Gaussian enhancements are obtained and discussed, but attention is also given to terms describing amplitude fluctuations that are present in the Gaussian scattering regime. These terms provide a generalized Siegert relationship for polarized radiation. The possibility of using measurements of fluctuations to characterize the particle shape is considered.     

Light scattering by homogeneous and multilayer ellipsoids in the quasi-static approximation

The quasi-static approximation for homogeneous and multilayer ellipsoidal particles is considered in detail. Expressions for the elements of the amplitude scattering matrix, as well as for the absorption and scattering cross sections, are presented for an arbitrary orientation of the particles relative to the incident radiation. The accuracy and the domain of applicability of the approximation are investigated for the elements of the scattering matrix of absorbing particles. Comparison of some approximate and exact methods showed that the quasi-static approximation gives good results (it is substantially more preferable than the Rayleigh and Rayleigh-Gans approximations), if the ratio of the largest size of a particle (or the boundaries of the main layer) to the smallest one does not exceed ～3. A new rule of the effective medium theory for multilayer ellipsoidal particles is proposed, which adequately takes into account their structure.     

气溶胶凝聚粒子散射特性的数值计算

利用Cluster-cluster aggregation (CCA)模型,模拟了由相同数目球形原始微粒凝聚而成的四种随机取向气溶胶凝聚粒子．根据物质的电结构,将气溶胶凝聚粒子离散为一系列偶极子,结合离散偶极子近似方法,在获得每一个偶极子的电偶极矩之后,数值计算了气溶胶凝聚粒子散射强度的角分布,并分析了散射强度随入射光入射角度和气溶胶凝聚粒子尺寸参数变化的规律．结果显示：当散射角较小时,气溶胶凝聚粒子取向和入射光的入射角度对散射强度影响不大,当散射角增大时,散射强度则明显依赖于气溶胶凝聚粒子取向和入射光的入射角度;对于不同尺寸参数的气溶胶凝聚粒子,在同一角度入射情况下,随尺寸参数的增加,气溶胶凝聚粒子的散射主要集中于前向散射．     

Anomalous diffraction approximation method for retrieval of spherical and spheroidal particle size distributions in total light scattering

Using total light scattering technique to measure the particle size distribution has advantages of simplicity in measurement principle and convenience in the optical arrangement. However, the calculation of extinction efficiency based on Mie theory for a spherical particle is expensive in both time and resources. Thus, a simple but accurate approximation formula for the exact extinction efficiency may be very desirable. The accuracy and limitations of using the anomalous diffraction approximation (ADA) method for calculating the extinction efficiency of a spherical particle are investigated. Meanwhile, the monomodal and bimodal particle size distributions of spherical particles are retrieved using the genetic algorithm in the dependent model. Furthermore, the spheroidal model in the retrieval of non-spherical particle size distribution is also discussed, which verifies the non-sphericity has a significant effect on the retrieval of particle size distribution compared with the assumed homogeneous isotropic sphere. Both numerical computer simulations and experimental results illustrate that the ADA can be successfully applied to retrieve the particle size distributions for spherical and spheroidal particles with high stability even in the presence of random noise. The method has advantages of simplicity, rapidity, and suitability for in-line particle size measurement.