Derivatives of light scattering properties of a nonspherical particle computed with the T-matrix method

Based on the T-matrix formalism, we analytically calculate derivatives of light scattering quantities by a nonspherical particle with respect to its microphysical parameters. Illustrative computations are performed for a spheroid, and the results agree with those obtained by finite differencing. The proposed formalism also predicts correctly derivatives for a sphere obtained by linearized Lorenz-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。     

基于平均质量的悬浮颗粒物的质量密度算法

基于米氏(Mie)散射理论得到了粒子计数器测量球形颗粒物质量密度的计算公式。考虑非球形颗粒,从颗粒群粒度分布概念出发,提出了统计意义上的平均质量概念,推导了非球形颗粒物质量密度的理论公式。运用理论公式证明了粒子计数器测量非球形颗粒物质量密度计算公式的合理性,进而给出了基于平均质量的悬浮颗粒物的质量密度算法,该算法只需对两个系数进行标定。实验表明,该算法的质量密度计算值与实际值十分吻合,两者拟合直线的斜率为0.9713,相关系数为0.9998。该算法为实现粒子计数器在线测量悬浮颗粒物的质量密度提供了一种可行途径。     

非球形气溶胶粒子散射相函数经验公式

散射相函数是研究电磁波传输特性的重要参数，直接影响电磁波传输方程的简化程度和解的精度。基于电磁散射与辐射传输中的基本理论，对非球形粒子散射相函数的经验公式进行了研究。为了很好的模拟非球形粒子的后向散射峰值，提高辐射传输方程的简化程度和解的精度，提出了一种新的相函数经验公式。分析新的相函数对非球形粒子的适用性，以单个沙尘性气溶胶为例，计算了不同形状粒子的Henyey-Greenstein*相函数和新的相函数随角度的变化，并与T矩阵法的计算结果进行了对比，发现椭球形粒子的长短轴比和有限长圆柱形粒子的径长比大于0.5时，新的相函数在大角度后向散射部分与T矩阵法的吻合程度较高。考虑波长变化，对比了尺寸谱满足对数正态分布的四种气溶胶粒子的Henyey-Greenstein*相函数和新的相函数与T矩阵法的计算结果。研究表明，对于椭球形粒子和有限长圆柱形粒子，在大角度(大于90°)后向散射部分，除了0.694时的椭球形海洋性气溶胶，新的相函数均方根差较小的占100%，证明了新的相函数可以较好的模拟非球形粒子的后向散射特征。新的相函数对准确模拟辐射传输过程具有重要意义。     

非球形椭球粒子参数变化对光偏振特性的影响

针对自然界中多数沙尘、烟煤粒子的非球形问题, 在球形粒子偏振特性的基础上, 进一步研究非球形椭球粒子的折射率、有效半径、粒子形状等参数变化对光偏振特性的影响, 采用基于T矩阵的非球形粒子仿真方法, 模拟非偏振光经椭球粒子传输后光的偏振特性及其与球形粒子间的差异, 并以实际沙尘、海洋、烟煤三种气溶胶粒子为例说明结果的正确性. 结果表明: 当折射率实部越小, 虚部越大时, 球形粒子与非球形粒子的偏振差异越不明显; 当粒子有效半径增加时, 球形粒子偏振度的变化比非球形粒子更为明显, 且最大值分别出现在散射角为150°和120°的位置; 当粒子形状不同时, 不同形状椭球及球形粒子的差异在散射角小于60° 时并不明显, 且当椭球粒子纵横比互为倒数时, 两种粒子的偏振特性近似相同. 通过以上分析可知, 在光传输过程中, 椭球粒子多数情况下无法被近似为球形粒子进行计算.     

雾霾粒子的紫外光散射特性研究

紫外光与雾霾粒子发生散射后,其散射信道特性能够反映雾霾粒子的相关物理信息,利用无线紫外光单次和多次散射信道模型,采用Mie散射和T矩阵理论分析了霾粒子在不同形态和浓度下的紫外光散射信道特性,以及散射角对散射光强的影响,并完成了紫外光在雾霾环境下的实测。通过理论及仿真分析,得到了不同霾粒子形态下的紫外光通信路径损耗以及光强分布。结果表明：紫外光直视通信方式下,路径损耗随着霾粒子浓度的增大而增大,且通信质量差于晴朗天。非直视通信方式中,在短距离通信时,高霾浓度下的路径损耗小于中低霾浓度,然而随着通信距离的继续增大,高雾霾浓度下的通信质量急剧下降,低霾浓度下通信质量最终达到最优,且距离为200 m时通信质量能优于晴朗环境。当通信距离相同时,三种雾霾浓度下的紫外光散射光强分布均随着散射角的增大而减小,当散射角继续增大并超过90°时,低霾浓度下的散射光强最大。主要原因是虽然散射角继续增大,但是有效散射体体积逐渐减小,因此低霾浓度下的散射光强较大。且当粒子粒径相同时,球形粒子的衰减较非球形粒子大。雾霾环境下实测结果与仿真结果相类似,证明了仿真结果的正确性,并在一定程度上证明了实际大气中雾霾非球形粒子多于球形粒子。     

Morphological characterization of cells in concentrated suspensions using multispectral diffuse optical tomography

Based on a non-spherical model of particle scattering, we investigate the capabilities and limitations of a T-matrix based inverse algorithm to morphologically characterize cells in concentrated suspensions. Here the cells are modeled as randomly orientated spheroidal particles with homogenous dielectric properties and suspended in turbid media. The inverse algorithm retrieves the geometrical parameters and the concentration of cells simultaneously by inverting the reduced scattering coefficient spectra obtained from multispectral diffuse optical tomography (MS-DOT). Both round and spheroidal cells are tested and the role of multiple and higher order scattering of particles on the performance of the algorithm is evaluated using different concentrations of cells.     

Experimental and theoretical analyses on the microwave backscattering ability and differential radar reflectivity of non-spherical hydrometeors

This paper experimentally and theoretically examines the scattering properties of simulated non-spherical hydrometeors including water oblates, ice oblates and ice sphere-cone-oblates, in terms of the backscattering cross-section and the differential reflectivity. The experimental measurements of the backscattering cross-sections of non-spherical hydrometeor samples were performed in the Electromagnetic Scattering Laboratory of China National Space Industrial Corporation. Meanwhile, the backscattering cross-sections have been computed with the transition (T) matrix method. The theoretical results are compared with the experimental data, showing that the calculations are consistent with the observations in general. Experimental and theoretical analyses indicate that the backscattering cross-section of non-spherical particles increases as the particle size parameter increases, and fluctuates when the sizes are larger under the effect of resonance scattering. Differential reflectivity Z_DR of water oblates in natural rainfall is always greater than 0 dB whereas Z_DR of hailstones may be negative. There is a good linear relationship between differential reflectivity and aspect ratio of a particle. These derivations agree with the literature and can be used to identify the presence of hail particles and distinguish between plate-type and columnar-type hydrometeors. In this study, the measuring experiment and the T-matrix method calculations for the scattering of simulated raindrop and ice particles are also briefly described.     

Sensitive and accurate dual-wavelength UV-VIS polarization detector for optical remote sensing of tropospheric aerosols

An UV-VIS polarization lidar has been designed and specified for monitoring aerosols in the troposphere, showing the ability to precisely address low particle depolarization ratios, in the range of a few percent. Non-spherical particle backscattering coefficients as low as 5×10^?8 m^?1?sr^?1 have been measured and the particle depolarization ratio detection limit is 0.6 %. This achievement is based on a well-designed detector with laser-specified optical components (polarizers, dichroic beamsplitters) summarized in a synthetic detector transfer matrix. Hence, systematic biases are drastically minimized. The detector matrix being diagonal, robust polarization calibration has been achieved under real atmospheric conditions. This UV-VIS polarization detector measures particle depolarization ratios over two orders of magnitude, from 0.6 up to 40 %, which is new, especially in the UV where molecular scattering is strong. Hence, a calibrated UV-VIS polarization-resolved time-altitude map is proposed for urban and free tropospheric aerosols up to altitude of 4 kilometers, which is also new. These sensitive and accurate UV-VIS polarization-resolved measurements enhance the spatial and time evolution of non-spherical tropospheric particles, even in urban polluted areas. This study shows the capability of polarization-resolved laser UV-VIS spectroscopy to specifically address the light backscattering by spherical and non-spherical tropospheric aerosols.     

Characteristics of the crystalline and luminescence properties of a-plane GaN films grown on γ-LiAlO2 （302） substrates

A method of clarifying bioaerosol particles is proposed based on T-matrix. Size and shape characterizations are simultaneously acquired for individual bioaerosol particles by analyzing the spatial distribution of scattered light. The particle size can be determined according to the scattering intensity,while shape information can be obtained through asymmetry factor(AF) . The azimuthal distribution of the scattered light for spherical particles is symmetrical,whereas it is asymmetrical for non-spherical ones,and the asymmetry becomes intense with increasing asphericity. The calculated results denote that the 5 –10 scattering angle is an effective range to classify the bioaerosol particles that we are concerned of. The method is very useful in real-time environmental monitoring of particle sizes and shapes.     

利用T-matrix计算非球形粒子散射特性的研究

本文用T-matrix方法计算了非球形气溶胶粒子的光学特性,得到了气溶胶粒子的消光截面、散射截面、吸收截面与气溶胶粒子形状的关系,不同形状气溶胶粒子的有着相同的散射相函数和不同的偏振度,非球形气溶胶粒子的散射相函数对其复折射指数的实部和虚部都不太敏感,而偏振相函数对其实部和虚部都比较敏感.此结论为研究大气辐射传输提供了较好的方法,尤其是偏振度与偏振相函数的提出为用偏振的方法进一步的反演气溶胶的光学参数提供了理论基础.     

Preliminary results of a non-spherical aerosol method for the retrieval of the atmospheric aerosol optical properties

There is experimental evidence that the non-sphericity of certain atmospheric particles can cause scattering properties different from those predicted by standard Mie theory. Numerous studies indicate the need to consider the presence of non-spherical particles in modeling the optical properties of atmospheric aerosols. On the other hand, natural aerosols show a great variety of shapes, making difficult a realistic choice of a particle shape (or shape mixture) model. In this paper, we test a parameterization of the particle shape in the retrieval of size distribution, phase function, single scattering albedo and asymmetry parameter from direct and sky-radiance measurements. For this purpose we have substituted the Kernel based on the Mie theory included in the model SKYRAD.PACK by one derived for non-spherical particles. The method is applied under different atmospheric conditions, including Saharan dust outbreak, polluted and local mineral episodes. We compare the results with those obtained by the well known spheroids algorithm used in the AERONET network.     

Splitting of a surface plasmon polariton beam by chains of nanoparticles

The operation of a micro-optical beam splitter for surface plasmon polaritons (SPP’s) formed by lined up scatterers is modeled and studied in the framework of a vectorial dipolar approach for multiple SPP scattering by equivalent non-spherical nanoparticles. It is shown that the inclusion of anisotropic polarizability of individual scatterer in the vectorial dipolar model of multiple SPP scattering allows one to obtain, in some cases, quantitative agreement between modeling and experimental results. As an example, we apply this approach to model an SPP beam-splitter formed by a chain of spheroidal particles. The dependencies of the splitting efficiency on the shape of particles, the incidence angle and the waist of an incident SPP Gaussian beam are considered. It is found that the efficiency is very sensitive to the shape of scatterers and the angle of SPP beam incidence. Comparison of numerical results with experimental data shows good agreement with respect to the particle shape and incident angular dependences. PACS 02.70.-c; 71.36.+c; 78.68.+m     

Discrete sources method for light scattering analysis from 3D asymmetrical features on a substrate

The discrete sources method is extended to analyze polarized light scattering by three-dimensional asymmetrical features on a plane penetrable substrate. The strict mathematical model and corresponding numerical scheme are described. Computer simulation results of non-spherical micro-particle scattering show that particle shape has a stronger effect on measured response for normal incident scanners.     

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：     

基于光热效应的显微光谱技术在单粒子检测中应用和发展

高灵敏度的单粒子检测技术是纳米粒子在生物医学、化学、光电子等领域应用的前提条件。常见的单粒子检测技术主要包括基于粒子的荧光、拉曼、散射和吸收等信号而发展起来的光学显微成像及光谱技术。其中,拉曼光谱和荧光光谱技术主要适用于一些具有拉曼活性的分子/粒子或可发光的荧光分子或粒子,然而即使对于荧光效率高的有机染料分子和半导体纳米粒子,固有的光漂白和blinking现象也对单粒子探测形成了挑战。散射光谱测量是应用于单粒子检测的另外一种方法,从理论上讲,由于瑞利散射随着尺寸的减小而呈六次方减弱的趋势,在细胞或生物组织内,小尺寸粒子的散射信号很难从背景散射噪声中分离出来。众所周知,介质吸收激发光后会引起介质内的折射率变化,进而在光加热区附近出现折射率的梯度分布,称为光热效应(photothermal effect)。基于粒子光热效应的光学显微成像和光谱测量技术具有信号灵敏度高、无背景散射、原位和免标记等优点,在单粒子检测领域展现了良好的应用潜力。综述了近年来基于光热效应的显微光谱技术在单粒子检测中应用和研究发展,首先介绍了光热效应的测量原理;接着分别讨论了光热透镜测量技术、微分干涉相差测量技术和光热外差测量技术的实验装置,比较了各种测量技术的信噪比、灵敏度、分辨率等特点,并且介绍这些测量技术在单粒子检测中的应用研究进展;接着,论述了近年来研究人员在提高光热显微测量的信噪比、改善动态测量性能以及在红外波段拓展等方面的最新研究成果;最后,简单总结了光热测量技术在单粒子检测领域所面临的挑战。     

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.     

Linear time domain model of the acoustic potential field

A new time domain formulation of the acoustic wave is developed to avoid approximating assumptions of the linearized scalar wave equation that limit its validity to low Mach particle velocity modeling or to a smooth potential field in a stationary medium. The proposed model offers precision of the moving frame while retaining the form of the widely used linearized scalar wave equation although with respect to modified coordinates. It is applicable to field calculations involving transient waves with unlimited particle velocity, propagating in inhomogenous fluids or in those with time varying density. The model is based on the exact flux continuity equation and the equation of motion, both using the moving reference frame. The resulting closed-form free space scalar wave equation employing total derivatives is converted back to the partial differential form by using modified independent variables. The modified variables are related to the common coordinates of space and time following integral expressions involving transient particle velocity representing wave radiated by each point of a stationary source. Consequently, transient field produced by complex surface velocity sources can be calculated following existing surface integrals of the radiation theory although using modified coordinates. The use of the proposed model is presented in a numerical simulation of a transient velocity source vibrating at selected magnitudes, leading to the determination of the propagating pressure and velocity wave at any point.     

均匀椭球粒子对拉盖尔-高斯光束的散射特性研究

基于广义Mie理论, 研究了椭球粒子对在轴入射的拉盖尔-高斯光束的散射特性. 通过局域近似法求解椭球坐标系中的波束因子, 计算得到了波束因子之间满足的普遍关系. 对散射强度随椭球粒子不同尺寸参数和扁圆程度的变化特性进行了数值计算, 并针对不同拓扑荷时的散射强度进行了对比分析. 结果表明: 当椭球粒子尺寸在与入射光波长可比拟的范围内变化时, 散射强度随尺寸参数的增大而增大, 随椭球长短轴之比和拓扑荷的增大而减小. 本文的理论研究能够为拉盖尔-高斯光束在粒径测量、大气激光通信、 大气遥感等领域的应用提供更准确的粒子模型和参考价值. 关键词： 椭球粒子 拉盖尔-高斯光束 波束因子 散射强度     

Impact of ice particle shape on short-wave radiative forcing: A case study for an arctic ice cloud

We used four different non-spherical particle models to compute optical properties of an arctic ice cloud and to simulate corresponding cloud radiative forcings and fluxes. One important finding is that differences in cloud forcing, downward flux at the surface, and absorbed flux in the atmosphere resulting from the use of the four different ice cloud particle models are comparable to differences in these quantities resulting from changing the surface albedo from 0.4 to 0.8, or by varying the ice water content (IWC) by a factor of 2. These findings show that the use of a suitable non-spherical ice cloud particle model is very important for a realistic assessment of the radiative impact of arctic ice clouds. The differences in radiative broadband fluxes predicted by the four different particle models were found to be caused mainly by differences in the optical depth and the asymmetry parameter. These two parameters were found to have nearly the same impact on the predicted cloud forcing. Computations were performed first by assuming a given vertical profile of the particle number density, then by assuming a given profile of the IWC. In both cases, the differences between the cloud radiative forcings computed with the four different non-spherical particle models were found to be of comparable magnitude. This finding shows that precise knowledge of ice particle number density or particle mass is not sufficient for accurate prediction of ice cloud radiative forcing. It is equally important to employ a non-spherical shape model that accurately reproduces the ice particle's dimension-to-volume ratio and its asymmetry parameter. The hexagonal column/plate model with air-bubble inclusions seems to offer the highest degree of flexibility.