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：     

Preparation and characterization of nano and micro particles of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) via emulsification/solvent evaporation and nanoprecipitation techniques

This paper describes the preparation and the characterization of micro and nanoparticles of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) via two different preparation techniques. The first technique is the emulsification/solvent evaporation while the second is the nanoprecipitation which is being reported for first time for the PHBV particles. The effect of various conditions of preparation on the size and the morphology of the produced particles was studied for both techniques. Field emission scanning electron microscopy, static light scattering and dynamic light scattering were used to analyse the size and the morphology of the particles. The particles produced using the emulsification/solvent evaporation were spherical porous micro and nanoparticles ranging from 300 up to 20 μm. The size, porosity and the particle size distribution were found to be controllable basically via choosing the surfactant and the polymer concentration when the emulsification technique was used. Choosing the appropriate anti-solvent and adjusting its polarity are crucial for getting spherical particles via nanoprecipitation. The micro and nanoparticles produced via both techniques are intended basically to be used for reinforcing biopolymeric matrices of PHBV and starches.     

Mie scattering efficiency of a large spherical particle embedded in an absorbing medium

The Mie scattering calculations are usually performed for non-absorbing spherical particles embedded in a non-absorbing medium. We consider a case of an absorbing sphere placed in an absorbing medium. We find, by numerical calculation for large size parameter of the order of 10⁴, that the scattering efficiency of a spherical particle in an absorbing medium approaches the reflectance of a plane surface at perpendicular incidence.     

The effect of multiple scattering in disperse media on polarization characteristics of scattered light

The effect of scattering of different multiplicity on polarization characteristics of scattered light is studied by the Monte Carlo computer simulation technique. The scattering multiplicity distribution versus the direction of scattering and dimensions of the scattering system is obtained for monodisperse systems of spherical particles of different size. The angular dependences of the elements of the light-scattering matrix (LSM) are calculated. It is shown that in a system of spherical particles, specific features of the LSM structure associated with multiple scattering have much in common with similar features of the LSM in systems of nonspherical particles under conditions of single scattering. The angular dependences of the degree of depolarization of the scattered light are studied.     

On features of the magnetic structure of spherical iron nanoparticles

The results of research into the behavior of the magnetic structure inherent to spherical ferromagnetic nanoparticles with radii of 5–30 nm are presented. The behavioral features are investigated in an external magnetic field by means of computer modeling. The hysteresis loops and formation of the vortex structure of magnetization are analyzed using particles with different sizes. The size effect of changes in the magnetization symmetry, which is analogous to phase transitions of the second kind, is established. The magnetic moments of spherical iron nanoparticles with radii of 5–30 nm are calculated. Calculations are performed by means of the Nmag micromagnetic simulation package.     

Light scattering by dielectric particles with axial symmetry: II

A new method is developed for calculating optical characteristics of axially symmetric particles. Electromagnetic fields are separated into two (axisymmetric and non-axisymmetric) parts. The light scattering problem is formed in the integral form and solved independently for each of the parts by using specially chosen scalar potentials. The potentials are expanded into series in spherical wave functions, and the expansion coefficients are calculated from solving the infinite systems of linear algebraic equations. The applicability of the proposed method for solving the problem of light scattering by Chebyshev particles, spheroids, and finite circular cylinders is briefly discussed, and some results of calculations performed for these particles are presented.     

Optical equivalence of isotropic ensembles of ellipsoidal particles in the Rayleigh-Gans-Debye and anomalous diffraction approximations and its consequences

Light scattering by isotropic ensembles of ellipsoidal particles is considered in the Rayleigh-Gans-Debye approximation. It is proved that randomly oriented ellipsoidal particles are optically equivalent to polydisperse randomly oriented spheroidal particles and polydisperse spherical particles. Density functions of the shape and size distributions for equivalent ensembles of spheroidal and spherical particles are presented. In the anomalous diffraction approximation, equivalent ensembles of particles are shown to also have equal extinction, scattering, and absorption coefficients. Consequences of optical equivalence are considered. The results are illustrated by numerical calculations of the angular dependence of the scattering phase function using the T-matrix method and the Mie theory.     

飞尘气溶胶粒子散射特性分析:对比实验和米散射方法

采用Mie散射理论计算了可见光波段等效球飞尘气溶胶粒子的Stokes散射矩阵,并与实验得到的空间随机取向的非球形飞尘气溶胶粒子结果进行了对比分析;由理论与实验方法得到的散射相函数,采用离散坐标法计算了两者的双向反射函数(BRDF),并对此结果进行了分析研究。结果表明:实验测量的非球形飞尘气溶胶粒子群的散射矩阵和基于球形粒子假设的Mie散射理论计算结果在大多数散射角上都不相同,但是不对称因子却大致相同;球形-非球形粒子群的BRDF随反射角的变化趋势基本一致,但是球形粒子群的BRDF曲线分布具有更大的波动趋势;随着光学厚度的增加,球形-非球形粒子群的BRDF曲线分布均趋于平坦,计算结果趋于一致。因此在飞尘气溶胶粒子散射特性研究中,当光学厚度较小时,用球形假设的方法会造成一定的误差,BRDF相对误差最大可以达到60%,需考虑粒子非球形特性造成的影响;而当光学厚度较大时,BRDF相对误差一般不会超过10%,采用球形假设的方法具有一定的适用性。     

In situ characterization of nanoparticles during growth by means of white light scattering

A simple method of characterization of suspensions of spherical nanoparticles with monotonically variable size is proposed. It allows for the in situ measurement of the particle size as well as spectral dependence of their refractive indices. The method requires three optical channels: one for the illumination of a suspension by white light and two for the measurements of the spectra of scattered light. Parameters of the particles are determined by fitting the measured temporal spectral surfaces by the calculated Mie scattering functions. The method is applied to the particles being grown in a low-pressure reactive plasma of a discharge in an acetylene-argon mixture.     

Polarization imaging of multiply-scattered radiation based on integral-vector Monte Carlo method

A new integral-vector Monte Carlo method (IVMCM) is developed to analyze the transfer of polarized radiation in 3D multiple scattering particle-laden media. The method is based on a “successive order of scattering series” expression of the integral formulation of the vector radiative transfer equation (VRTE) for application of efficient statistical tools to improve convergence of Monte Carlo calculations of integrals. After validation against reference results in plane-parallel layer backscattering configurations, the model is applied to a cubic container filled with uniformly distributed monodispersed particles and irradiated by a monochromatic narrow collimated beam. 2D lateral images of effective Mueller matrix elements are calculated in the case of spherical and fractal aggregate particles. Detailed analysis of multiple scattering regimes, which are very similar for unpolarized radiation transfer, allows identifying the sensitivity of polarization imaging to size and morphology.     

Preparation and Size Characterization of Silver Nanoparticles Produced by Femtosecond Laser Ablation in Water

Femtosecond laser ablation of silver plate placed in water is used to produce nanoparticle suspension. The method is easy to operate and the suspension is relatively stable. The optical properties and the size distribution of the suspension are studied with UV-vis absorption spectroscopy and dynamic light scattering, respectively. The shape of the nanoparticles is investigated by an atomic force microscope, which is near spherical. There are two kinds of nanoparticles, small particles with diameter about 35 nm, and large particles with diameter about 120 nm.     

Computer simulations for multiple scattering of light rays in systems of opaque particles

A new computer model for multiple light scattering in arbitrary systems of opaque diffusely scattering particles is considered. For ray tracing and scattering in such systems, the geometric optics approximation is used. Semi-infinite media and clusters with spherical and irregular shaped particles are investigated. The irregular particles are approximated with a discrete set of small triangular facets attached to each other. The particle surface is supposed to scatter by the Lambertian indicatrix. Scattering of the first six orders is considered, but the model can be effectively used for calculations of higher orders too. Phase-angle curves of scattering for media and clusters with different packing density are calculated. It is shown that the contributions of scattering orders rapidly diminish as the order grows even for non-absorbing particulate surfaces. Only the first scattering order shows the opposition effect and is rather sensitive to packing density. Higher orders do not show any features near zero phase angle. The contributions of high orders increase slightly, when the packing density increases. The form of particles is important mostly for the second scattering order. For clusters of particles both packing density and number of particles are important for phase function behavior. Clusters consisting of 100 particles show weak phase-angle dependences of high orders of scattering. These dependences become more prominent with increase of number of particles. Phase curves for spherical and cubic clusters are compared. It turns out that the influence of cluster shape is only a minor factor.     

Depolarized light scattering from interacting brownian spheres

The initial decay time of the spectrum and the integrated intensity of depolarized light scattered from a system of interacting spherical and optically isotropic colloidal particles as a function of scattering angle is calculated in the double scattering approximation. These quantities can be reduced in the short-time regime to the static structure factor of single scattering, as observed in polarized scattering for systems of intermediate concentrations of colloidal particles. The results are applied to charged polysterene spheres in solutions and it is shown by numerical calculations that the presence of interactions can modify the angular dependence of initial decay time and integrated intensity considerably.     

一种新型银溶胶的制备、表征及其SERS活性的研究

报道了用一种化学还原方法制备银溶胶的新方法。用紫外-可见光谱和透射电镜研究银纳米粒子的形成过程,粒子形状及粒径分布。结果表明,在反应初期形成球形和棒状两种形状的银纳米粒子,随着反应的进行,大部分的棒状粒子逐渐变为球形粒子,最终棒状粒子少于银纳米粒子总数的3%。因此,最终得到了一种形状均一的,平均粒径为17nm银纳米粒子。同时也用紫外-可见光谱研究了Cl-的加入对这种银纳米粒子光谱性质的影响,结果表明Cl-加速了银纳米粒子的聚集。这种银溶胶有着较高的SERS活性。     

Scattering by large bubbles: Comparisons between geometrical-optics theory and Debye series

The scattering patterns and optical properties of large spherical bubbles in water are calculated by both Geometrical-optics theory and Debye-series expansion of the Mie amplitudes. In the Geometrical-optics calculations, the wave-front spreading factor and phase information are all taken into consideration. A detailed comparison between the two methods is presented. It is found that scattering patterns calculated from the Geometrical-optics approach agree reasonably with those obtained by Debye series when the size parameter is larger than approximately 400. In the case of scattering properties, the Geometrical-optics method can produce reliable results with the Debye expansion for size parameters larger than approximately 50. The Geometrical-optics results without the phase interferences are also reliable, but it cannot display the oscillations and the ripple structure as a function of size parameter.     

The scattering spectra and color of disperse systems of weakly absorbing particles

The scattering spectra of weakly absorbing systems of spherical particles are calculated using various approximate and numerical methods. Comparative estimates are made for applicability of the single-scattering and mean-field approximations and the Monte Carlo computer simulation for different scattering multiplicities. The results of the calculations well agree with the experimental scattering spectra of the crystalline lens. Based on the spectral characteristics obtained, the chromaticity coordinates are calculated and regularities in variations of the color characteristics are analyzed as functions of the disperse system parameters.     

Absorption and Scattering of Light by Hybrid Metal/J-Aggregate Nanoparticles: Plasmon–Exciton Coupling and Size Effects

We report the results of our theoretical studies of the optical properties of hybrid nanoparticles consisting of the metal core covered with molecular J-aggregates. We evaluate the cross sections of absorption and scattering of light by such particles on the basis of the extended Mie theory for two concentric spheres with material dielectric functions that take into account the size effect associated with scattering of free electrons from the core/shell interface. We carry out our calculations in a wide range of light wavelengths and geometrical parameters of the composite system for silver and gold core and for a J-aggregate shell composed of different cyanine dyes. The results obtained demonstrate the quite different behavior of the extinction spectra of such particles caused by the different strengths of interaction between the Frenkel exciton and the dipolar or multipolar plasmons. We pay particular attention to the investigation of spectral peak positions associated with the eigenfrequencies of hybrid modes in the system and peak intensities as functions of reduced oscillator strength in the molecular J-band for various relationships between the core radius and shell thickness. This provides an efficient means for the explanation of the main features in the optical properties of metal/J-aggregate nanoparticles and can be used for an effective control of the plasmon–exciton coupling strength in such hybrid complexes.     

激光雷达回波信号的蒙特卡罗模拟研究:I.均匀薄雾的多重散射强度分布(英文)

本文简要报道了激光雷达回波信号及多重散射强度分布的蒙特卡罗模拟计算结果。激光雷达系统采用共轴设计 ,计算中采用具有一定厚度、均匀分布的球形薄雾粒子来模拟大气环境 ,研究了粒子尺寸对回波信号的影响 ,获得了不同粒子半径、不同接受视场角条件下多重散射信号强度分布的经验公式。以上两套经验公式具有良好的校正系数 ,可以直接用于激光雷达系统设计中多重散射效应的研究 ,节省大量的模拟计算时间 ,提高设计效率     

Calculation of the scattering indicatrix of systems of spheres based on the dimensionless formulation of the solution of the mie problem

Dimensionless formulas describing the field scattered by a single sphere are used to derive the scattering indicatrix of systems of spherical particles whose size parameter is less than unity. These formulas can greatly expedite the calculations when the required precision of the results is a few percent.