Two-particle characterization by pulse induced and time resolved Mie scattering

With this numerical investigation we demonstrate the determining of particle size and particle distance for a two-particle system by time resolved Mie scattering. The optical interaction of the fs-laser pulse with the particle configuration produces a sequence of scattered light signals on the femtosecond time scale. The temporal differences between these signals represent typical dimensions of the particle system. The different ray tracks of the specific scattering orders, which are the reason of the temporal behaviour, have been verified by models of geometric optics. We have simulated the Mie scattered light by an algorithm for multi-particle scattering. For all examples the detector was positioned in the back scattering region at a scattering angle of θ=150°.     

Particle sizing by time-resolved Mie calculations—A numerical study

If a small transparent particle is illuminated with a short laser pulse, the signals of the individual scattering light orders appear temporally successively. Since to each scattered light order belongs a specific optical path through the particle, the particle size can be determined from the time difference between the detected scattered light signals. For the case of a detector position within the backscatter region, which especially is important in measuring practice, the time difference between the specular reflection signal and the signal after a single internal reflection (refraction of second order) must to be evaluated. In the numerical simulation we generate the concerned scattered light signals by using time-resolved Mie calculations and in this paper we present the geometrical models, which permit a correct interpretation of the temporal behavior of these pulse-induced scattered light signals.     

Polarization of near-forward-scattered light from particulate substrates illuminated at near-grazing angles

Laboratory photopolarimetric measurements of light scattered by substrates consisting of semitransparent particles with sizes significantly larger than the wavelength show a polarization shoulder at small scattering angles near θ=10-30° in addition to the Brewster maximum positioned near θ=50°. With ray-tracing simulations, we find that the shoulder appears to be related to light passing through particles in the upper layers of the substrates. We study the dependence on particle absorption and packing density of particulate substrates. The studies show that the shoulder weakens with increased absorption and packing density.     

Molecular dynamics simulation of low-energy bombardment of Pt (1 1 1) surface by Cu atoms with various incident angles

The low-energy bombardment of Pt (1 1 1) surface by Cu atoms with various incident angles (θ) is studied with MD simulations. In the case of near-normal incidence (θ≤20°), the result of energy deposition is similar to that of θ=0°. In contrast, in the case wherein the incident angles are higher than 60°, the incident atom cannot penetrate through the first layer and is scattered directly on the surface. The low-energy deposition has no obvious effect on the substrate. For 20°≤θ≤60°, the oblique incidence contributes to uniformity of nucleation and layer-by-layer growth of film as well as the layer-by-layer removal of atoms in the surface layers. Based on our MD simulations, the mechanism behind the deposition and thin film formation is related to the horizontal component and the vertical component of the impact momentum.     

关于折射率对散射光场分布影响的研究

根据Mie散射理论,采用理论计算和实验相结合的手法,研究了光散射现象以及散射介质的折射率对散射光场分布的影响.通过对空气中不同折射率的散射介质形成的散射光场光强的实验比较,论证了散射介质折射率的实部变化对散射光强的影响不大,其主要影响是通过对相位的变化来实现的,也即散射介质折射率的虚部变化对光强的影响很大,在实际应用中不可忽略.这一结论对以散射光场的分布为基础的各种研究具有一定的指导意义.     

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.     

Light scattering properties of sea-salt aerosol particles inferred from modeling studies and ground-based measurements

Direct climate radiative forcing depends on the aerosol optical depth τ, the single scattering albedo ?, and the up-scatter fraction β; these quantities are functions of the refractive index of the particles, their size relative to the incident wavelength, and their shape. Sea-salt aerosols crystallize into cubic shapes or in agglomerates of cubic particles under low relative humidity conditions. The present study investigates the effects of the shape of dried sea-salt particles on the detection of light scattering from the particles. Ground-based measurements of scattering and backscattering coefficients have been performed with an integrating nephelometer instrument for a wavelength . The measurements are compared to two models: the Mie theory assuming a spherical shape for the particles and the Discrete Dipole Approximation (DDA) model for the hypothesis of cubic shape of the sea-salt aerosols. The comparison is made accurately by taking into account the actual range of the scattering angles measured by the nephelometer in both models that is from 7° to 170° for the scattering coefficient and from 90° to 170° for the backscattering coefficient. Modeled scattering and backscattering coefficients increase for nonspherical particles compared to spherical shape of particles with diameter larger than about 1 μm. However, the comparison of the modeling results with the measurements gives best agreement for particles diameter less than about 1 μm. The size distribution of the particles is measured with two instruments with different size bins: an electrical low-pressure impactor (ELPI) and an aerodynamic particle sizer (APS). It is found that the size of the bins of the instruments to determine the number concentration of the particles in accordance with their diameter is critical in the comparison of measurements with modeling.     

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.     

测量单泡声致发光中气泡R(t)曲线的前向Mie散射技术

光干涉原理和Mie理论计算结果表明,单泡声致发光中气泡前向Mie散射的振荡信号,主要是由于气泡的透射光束和表面反射光束之间光干涉产生的. 这些干涉波峰形成了测量气泡半径的空间标尺,标尺的单位长度δR由散射角θ,检测光波波长和流体光折射率确定,而每个波峰就是标尺的刻线,它们与该时刻的气泡半径大小一一对应. 在30°—50°散射角范围内,利用前向Mie散射实验测定了单泡声致发光中气泡的最大半径,R(t)曲线及平衡半径,表明前向Mie散射是一种便捷的测定气泡运动特性的有效方法. 关键词： 单泡声致发光 前向Mie散射 光干涉     

Un nouveau theoreme d'echantillonnage pour le degre de coherence complexe en vue de la reconstitution d'une source lumineuse isotrope

We present some results obtained by using the standard technique of light scattering on so-called “microemulsions”. From the data of single-photon counting of light scattered from 0° to 90° the particle scattering factor P(θ) is obtained. Fitting with the theoretical curves gained for various kinds of particles enables us to deduce in a rather direct and simple way the shape and the size of particles.     

Particle sizing by ultrashort laser pulses – numerical simulation

This paper contains the results of our numerical investigations into particle sizing by analysis the time-dependent formation of the scattered light. We use an extended Mie theory for calculation the differences in time between the signals of reflection and higher order of refraction. The corresponding optical path lengths of light rays are computed by the principles of geometrical optics. By using a Debye series expansion it is possible to take into account single orders of scattered light. In detail we demonstrate the pulse-induced generation of scattered light for the refraction of first and second order as function of the detection angle.     

微小颗粒的光散射数值模拟

简单介绍了以经典Mie理论为基础的光散射测量技术在颗粒直径和颗粒浓度测量中广泛的应用。分别以Mie理论和离散偶极子近似理论(DDA)为基础, 用数值计算方法分析了球型颗粒的光散射特性，给出了微小颗粒对平行入射光散射的强度函数和散射偏振度的数值计算方法。得到了强度函数和偏振度随相关物理参量变化的三维图，为微小颗粒散射研究提供了一种三维视图。计算结果表明：当尺度参量x＜4时，2种方法所得结果差异不大；随尺度参量增大，2种方法所得结果出现较大差异。与经典Mie理论相比，由于离散偶极子近似理论可以解决各种形状的颗粒散射问题，其应用前景更广泛。     

Au粒子在不同基质中Mie散射特性的研究

根据Mie散射理论,给出了金属粒子的散射、消光和吸收截面以及散射场强度的计算公式,并数值计算了在λ=r=1μm时,金属Au粒子在五种不同的基质中的散射截面和散射光强,结果表明基质折射率越大散射特性越强。     

Performance of discrete dipole approximation for prediction of amplitude and phase of electromagnetic scattering by particles

Electromagnetic scattering provides useful signatures for nonintrusive particle characterization. Scattered wave which carries characteristic information about particles is identified completely by its intensity, polarization state and phase. Recent developments in measurement techniques have enabled measurement of phase of the scattered wave which is a source of additional information about particles. In the present study, accuracy of discrete dipole approximation (DDA) in predicting amplitude and phase of scattered wave is investigated via publicly available DDSCAT code by Draine and Flatau, which is a well-established tool for DDA and has found wide range of applications in the literature due to its flexibility. DDSCAT routine is modified to enable accurate computation of phase of complex amplitude scattering matrix (ASM) elements as well as their magnitude. DDA method was implemented by using lattice dispersion relation for dipole polarizabilities, generalized prime factor algorithm for fast-Fourier transformation and pre-conditioned bi-conjugate gradient method with stabilization for the solution of the complex linear system of equations. Accuracy of ASM elements predicted by DDA is assessed on single sphere problems with various size parameters and refractive indices by validation against Mie theory solutions. Excellent agreement between predictions and exact solutions proves the reliability of the modified DDSCAT code for prediction of amplitude and phase of scattered electromagnetic wave. Applicability conditions and requirements of the present DDA application to ensure accurate prediction of complete set of scattering parameters are mapped for single spheres, on an extensive domain of size parameters and refractive indices. A correlation is presented to estimate the magnitude and phase errors associated with given size parameter, refractive index and cubic lattice subdivision. Assessment of computational time requirements for different optical constants shows that implementation of DDA with the present specifications is unfeasible for size parameters larger than 4 when Re(m)>2 and Im(m)<0.1 at the same time, due to slow convergence rate.     

油砂混合水体后向散射系数光谱贡献分离算法Ⅰ: 实验理论

在石油类污染水体中,油会吸附在悬浮颗粒物表面而形成一个双层结构,影响水体后向散射系数光谱特征, 分离水体石油类物质与悬浮颗粒物对后向散射系数光谱的贡献, 能提高水体石油类污染后向散射理论模型的准确性。将美国Wyatt公司生产DAWN HELEOS Ⅱ18角度散射测量仪、美国SEQUOIA公司生产的LISST-100x B粒径仪和美国Hobilabs公司的后向散射仪HydroScat-6 Sprctral Backscattering Sensor(HS6)联动观测,构成后向散射系数光谱测量系统,分别测量不同水样的散射强度电压值、粒径分布及粒径浓度、后向散射系数等参数,提出了利用Mie散射理论计算未知折射系数物质的体散射函数β(λ,θ)的新思路及分离后向散射系数光谱的算法。选择已知折射系数m的石英砂作为颗粒物与采自不同油田区域的油污水进行配比,获取不同特性水样,测定相关数据。首先, 根据Mie散射理论计算出各样本对应的水体体积散射函数β(λ,θ);其次,建立的DAWN HELEOS Ⅱ 18角度激光散射仪测定散射强度对应的电压值V(θ)转化为体积散射函数β(λ,θ)的关系式;再次,根据最优化方法估算出油砂混合的等效折射系数m_os以及油的折射系数m_o;最后,利用β(λ,θ)和估算的m_os值及m_o计算出各类样本的后向散射系数b_b(λ),分别建立油污水b_b,o(λ)和石英砂b_b,s(λ)与油砂混合总b_b,os(λ)的分离算法。分离算法的建立一方面提高了水体石油类污染后向散射理论模型的准确性,另外一方面拓展了米散射理论在海洋水色遥感中的应用。     

Giant Enhancement and Sign Inversion of Optical Kerr Nonlinearity in Random High Index Nanocomposites near Mie Resonances

High-index dielectric nanoantennas excited at Mie-type resonances have exhibited enormous enhancement of optical nonlinearity. Such nanostructures have been actively studied by researchers in recent years. Here, a numerical analysis study of the optical Kerr effect of nanocomposites consisting of high refractive index (GaP) spheres at the wavelength of 532 nm is presented. This is done by means of 3D finite-difference time-domain simulations. The effective nonlinear refractive index of 0.8 µm thick nanocomposites and metasurfaces is evaluated. It is shown that the optical Kerr nonlinearity of the nanocomposites rises by orders in proximity to Mie resonances and may exceed the second-order refractive index of the bulk material. It is revealed that the sign of the effective optical Kerr coefficient is inverted near the Mie resonances. This effect may be of interest in developing nonlinear optical metadevices.     

Stokes vector for small rough particles: Regression analysis

The Stokes parameters and the linear polarization P_l of small rough particles composed of ice and silicate are studied with statistical methods. The probability density function of P_l and a regression analysis for the first two Stokes parameter I_s² and Q_s² are presented for three typical scattering angles θ_s=60°, 90° and 150°. The width of the probability density function of P_l given by its standard deviation increases with the roughness of the particles. A linear relationship between I_s² and Q_s² is found by the regression analysis. The slope of the linear model reflects the correlation between I_s² and Q_s² that, for a specific composition of the particles, depends on the roughness of the particles and the scattering angles. The value of the constant of the linear regression is characteristic of the composition of the particles and is not sensitive to the roughness of the particles and the scattering angles. The scattering angle θ_s=150° is the most appropriate angle for a study of rough particles of different composition.     

激光等离子体受激Raman散射光谱的时间分辨测量

采用光学多道谱仪和光学条纹相机耦合,组成时间分辨的Raman散射光谱测量系统,可实现0.5nm的光谱分辨和好于10ps的时间分辨。采用该测量系统,在神光Ⅱ装置上开展了脉宽1ns、波长351nm的激光与两种不同尺寸柱腔靶相互作用的物理实验,获得了时间分辨的SRS光谱实验结果。研究表明,SRS光谱在时间上相对于入射激光有一定的延迟,腔靶尺寸减小时,延迟时间随之减小。通过长、短波截止波长分析电子密度方法,计算得出了Ⅰ型和Ⅱ型腔靶SRS散射光最短波长光谱发生的密度区分别为0.069nc和0.027nc。     

Polarization of light backscattered by small particles

We study scattering of light by wavelength-scale spherical, cubic, and spheroidal particles as well as clusters of spherical particles for equal-volume-sphere size parameters 4≤x≤10 and refractive indices 1.1≤m≤2.0. Such particles exhibit three specific features in the regime of backscattering: first, the intensity shows a backscattering peak; second, the degree of linear polarization for unpolarized incident light is negative; and, third, the depolarization ratio is double-lobed. We find that the overall characteristics of the scattering-matrix elements can be explained by an internal field composed of waves propagating in opposite directions near the particle perimeter and forming standing waves, as well as a wave propagating forward with the wavelength of the internal medium. When moving from the central axis of the particle toward its perimeter, the internal field changes from a forward-propagating wave with a wavelength dictated by the particle refractive index toward a standing wave with an apparent wavelength of the surrounding medium. The mapping of the internal field to the scattered far field is like an interference dial where rotation of the dial by a quarter of a wavelength on the particle perimeter results in a change from a destructive to constructive interference feature in the angular patterns (or vice versa). The dial is a manifestation of a well-known rule of thumb: the number of maxima or minima in the scattering-matrix elements is given by the size parameter. We explain the backscattering peak as deriving from the backward-propagating internal wave near the particle perimeter. Negative polarization follows from the spatial asymmetry of the internal fields: inside the particle, the fields are amplified near the central plane perpendicular to the polarization state of incident light, resulting in more pronounced interference effects for the perpendicular polarization than for the parallel polarization. The double-lobe feature in the depolarization results from the same internal-field structure with leading cross-polarized fields located slightly different from the copolarized fields. We discuss practical implications of these findings for the retrieval of particle sizes, shapes, and refractive indices from observations and laboratory experiments.     

双层散射介质的单次后向散射光谱分析

从上皮组织的结构特点出发,基于米氏(Mie)散射理论,建立了双层散射介质的单次后向散射光谱的理论模型,该模型通过偏振门屏蔽来自下层的噪声背景,只保留来自上层的单次散射光。计算分析了粒子的形态学参量如平均尺寸及其分布、相对折射率变化时,单次散射光谱的特征。并用傅里叶波形分析法研究了这些参量对单次后向散射光谱曲线形状及其谐波幅值的影响。结果表明,这些只经历了表层粒子单次散射的光谱信号对表层粒子的平均尺寸及其分布、相对折射率具有灵敏性。对光谱曲线波纹结构的幅值、频率,散射强度,光谱谐波的幅值有直接的影响。研究结果对早期癌症的散射光谱特征识别及其特征提取有重要的实用价值。