用基于方位分辨的漫后向散射光确定粒子尺寸分布及相对折射率

基于偏振门技术,提出了利用漫后向散射光随线偏振光入射方位的变化来测量双层模型表层的粒子尺寸分布及相对折射率。双层物理模型用来模拟上皮组织,上层分别由聚苯乙烯小球或HeLa细胞悬浮液组成,下层由脂肪乳溶液组成,每层的光学特性与上皮组织光学特性匹配。根据米氏(Mie)散射理论,建立了与散射体形态参量相关的偏振后向漫散射强度随入射偏振方位变化的理论模型,并采用浮点遗传算法反演程序获得表层粒子的尺寸分布(PSD)及相对折射率。将理论值和实验测量值进行比较。研究结果表明通过测量随方位变化的后向漫散射光的偏振差分信号,能够获得类上皮组织模型表层的形态结构信息。     

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.     

散射相函数的高阶光学参量研究

漫反射光谱技术被广泛应用于无创测量生物组织光学性质。当光源与探测器很近时,仅仅依靠吸收系数μ_a和约化散射系数μ′_s不能准确描述光源附近光的传播状态。而二阶光学参量γ的引入改善了近光源光的传播状态的描述。本文将生物组织的散射等效成特定球形颗粒的散射,基于Mie散射理论,计算了与散射相函数p(θ)有关的单粒子和多分散系粒子的二阶光学参量γ,研究了γ随尺度参数α和相对折射率m的变化规律,描述了γ与组织结构参量之间的联系,并阐述了γ对粒子特征的表征能力。研究表明,参量γ对尺度参数α小于2的微粒尺寸的改变是敏感的,并呈二次函数关系,其系数与相对折射率呈线性关系;对于相对折射率和尺度参数都不相同的两个粒子,他们的各向异性因子g相同时,二阶光学参量γ却不同,粒子越大,γ表征粒子特征的能力越强。这对于无创探究组织微观形态具有深远的意义。     

石英系单模保偏光纤的受激拉曼散射传输模式的实验分析

对石英系单模保偏光纤中的受激拉曼散射传输模式进行了实验研究，测得三种不同截止频率的单模保偏光纤中受激布曼散射光谱及其模式的分布，其结果与圆芯多模渐折射率光纤中的受激拉曼散射模式分布完全不同，斯托克斯光谱的光斑尺寸基本不变，而且具以有高阶模传输的特点。这些结果在国内外尚未见报道。     

组织模型偏振后向散射差分光谱的测量

建立了一套用于测量生物组织模型偏振后向散射差分光谱的实验系统。在系统中,线偏振光入射到样品表面,检偏器和光谱仪配合记录后向散射光中的平行和垂直分量,随后计算出差分光谱。采用了平均直径分别为5.0μm、9.0μm的两种聚乙烯小球悬浮液进行实验,结果与蒙特卡罗模拟进行对照,验证了实验系统的正确性。初步研究了粒子尺寸分布变化对差分光谱的影响,结果表明,偏振散射差分光谱能敏感地反映粒子尺寸分布的变化,在大小粒子的混合液中,大粒子浓度一定程度的增加,光谱仍保留小粒子的振荡趋势,但差分光强减小。该方法对于早期癌症检测具有潜在应用意义。     

由吸收系数和粒度分布计算浮游植物的散射光谱特征

散射特性在水色卫星遥感模式的开发、水下光辐射传输、优势藻光谱特征的提取以及赤潮监测等方面有着重要的应用价值.在假定藻细胞为均匀球形结构的条件下,建立了浮游植物的散射正演模型.模型输入参量为实际测量的藻细胞吸收系数和粒度分布,输出参量为浮游植物的散射效率和吸收效率光谱.对短伪海链藻和威氏海链藻进行实验,模拟结果与实验结果的比较发现:该模型估箅得到的散射光谱能与实验的散射光谱得到很好的吻合,两种藻的模拟散射效率相对实验散射效率的误差分别为7%和10.6%,吸收效率的误差分别为7.4%和13.4%.模拟与实验结果的吻合表明,由吸收系数和粒度分布可以模拟出藻类的散射光谱特性.单细胞近似球形藻类散射正演模型可行.     

混浊介质中后向单次漫散射米勒矩阵特征

采用斯托克斯(Stokes)矢量形式,推导出当无限窄的连续光束垂直入射到混浊介质表面时,后向单次漫散射米勒(Mueller)矩阵的解析表达式。基于米氏(Mie)散射模式,详细分析了单次散射米勒矩阵元素的分布模式,以及与介质粒子数密度,粒子尺寸参量之间的关系。研究表明:单次散射米勒矩阵的方位变化随粒子数密度的增加,逐渐消失,而矩阵元素m22,m33,m23,m32随粒子数密度的变化,具有更显著的方位变化特征。矩阵元素m22,m33在方位角=45°时的值随尺寸参量的变化有一定的规律性,当尺寸参量小于某一特征参量时,其值呈下降趋势,反之则呈波动上升趋势。当介质粒子数密度以及粒子尺寸参量改变时,米勒矩阵元素强度的径向分布模式不变,即在任何方位,强度随径向距离都近似成指数规律衰减,方位变化呈周期性。     

一种动-静态光散射颗粒折射率测量法

提出一种动静态光散射颗粒折射率测量方法.该方法分为三部分,一部分是用动态光散射技术测得颗粒样品的线宽分布函数,然后将此线宽分布连同一组折射率值代入静态光散射公式推算出不同折射率值下的散射光强空间分布;第二部分是用光电探测器测出实际的散射光空间分布;第三部分是将不同折射率下的推算值与实测值做均方偏差运算以做比较,取均方偏差最小时所对应的代入折射率值作为颗粒折射率的最终测量值.此方法对样品的分散度无要求,适用于微米到亚微米级的颗粒物.     

光在水中吸附膜层气泡上的散射特性

光在水中大尺度气泡上散射特性的研究多是基于Davis模型.该模型没有考虑到吸附膜层对光在气泡上散射的影响,而海水中的大多数气泡都有膜层附着,这些膜层会影响到气泡的光散射特性.本文从几何光学的角度出发,建立了吸附膜层气泡的体积散射函数简化公式.在此理论基础上,模拟计算了尺度远大于入射光波长的大气泡散射光强分布曲线,得出光照射下气泡上散射光强的远场特性,讨论了影响气泡散射光强分布的主要因素.并与无膜气泡光散射分布曲线比较,讨论了油膜膜厚、折射率等参量对气泡的光散射特性影响.得出结论:吸附膜层气泡的光强分布曲线与无膜气泡相似,但吸附膜层会削弱前向散射光,增强后向散射光.     

离轴球形粒子对高斯波束的散射光通量的计算

利用矢量球谐函数展开的方法,研究了离轴球形粒子对椭圆高斯波束的散射。根据其远区散射场的形式,得出了归一化散射场的斯托克斯参量(散射强度)与颗粒直径、折射率以及散射角的关系。建立了计算离轴球形粒子对高斯波束散射通量的解析模型,计算了散射光在任意散射方向上的光通量,得到了前向任意立体角内散射通量的计算公式,为激光散射探测提供了理论依据。     

高散射媒质中的光程可分割动态光散射研究

为研究散射光强度随光子在散射媒质中散射光程的变化,基于单散射理论与扩散波光谱理论,采用了低相干动态光散射装置对不同粒径大小的聚苯乙烯悬浮液进行研究。将测量得到的背散射光光场强度谱的线宽与相应的理论计算结果相比较发现,在短光程区域,考虑容器壁附近拖曳效应的影响后,对于不同粒径的颗粒,光程为约5倍粒子平均自由程的区域可看成为单散射区域;对于光程大于225 m的区域可看成为扩散光波区域。实验结果表明低相干动态光散射法可实现高散射媒质从单散射区域到低次散射再到扩散区域的全光程的可分割的光场强度谱测量。     

雷达波段内磁性吸波颗粒光散射特性分析

针对目前在微波雷达隐身技术中广泛运用的吸波材料颗粒,根据米氏(Mie)散射理论对磁性吸波颗粒在雷达波段内的光散射特性进行了数值模拟和计算分析.在Mie系数中引入磁导率变量,分别计算了磁导率、折射率及颗粒粒径等参数对吸波颗粒光散射特性的影响;同时对比分析了磁性与非磁性吸波颗粒的散射光强、散射系数、吸收系数等散射特性规律.数值计算结果表明,颗粒磁特性的变化对其散射性能造成影响,磁导率的增大将使颗粒的吸收性能增强而后向散射强度减弱,有利于吸波颗粒雷达散射截面的减小,同时磁导率变化对颗粒散射特性的影响受到复折射率的制约.     

Interference effects in backscattering of light by a layer of a discrete random medium

Multiple backscattering of light by a layer of a discrete random medium is considered. A brief derivation of equations for describing the coherent and incoherent components of scattered light is presented. These equations are solved numerically in the approximation of doubled scattering of light by a semi-infinite medium of spherical scatterers having a size comparable with the wavelength in order to study the effect of the properties of particles on the angular dependence of interference effects. Calculations show that the half-width of the interference peak decreases upon an increase in lateral scattering by particles and that the degree of polarization has a complex angular dependence on the properties of the particles. For an optically thin layer of the medium, the relations defining the interference peak half-width and the scattering angle upon extreme linear polarization as functions of the effective refractive index are given.     

米散射理论在新型导光板中的应用

根据米散射理论,提出了新型导光板的设计思路,计算并分析了对于一定波长的入射光,不同粒径的微粒的散射特性。总结了随着微粒粒径的变化,散射效率、消光效率与背向散射效率的变化规律,分析了散射过程中的偏振度随粒子粒径几散射角变化的情况,同时模拟计算了多个微粒对同一波长的入射光经过多次散射后的概率统计结果。     

Random Backscattering in the Parabolic Scaling

In this paper we revisit the parabolic approximation for wave propagation in random media by taking into account backscattering. We obtain a system of transport equations for the moments of the components of reflection and transmission operators. In the regime in which forward scattering is strong and backward scattering is weak, we obtain closed form expressions for physically relevant quantities related to the reflected wave, such as the beam width, the spectral width and the mean spatial power profile. In particular, we analyze the enhanced backscattering phenomenon, that is, we show that the mean power reflected from an incident quasi-plane wave has a maximum in the backscattered direction. This enhancement can be observed in a small cone around the backscattered direction and we compute the enhancement factor as well as the shape of the enhanced backscattering cone.     

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.     

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.     

Light scattering by aggregates of varying porosity and the opposition phenomena observed in the low-albedo particulate media

The rule that the opposition phenomena in brightness and linear polarization observed in many regolith surfaces usually accompany each other is violated in the cases of very dark asteroids and particulate samples: practically no nonlinear surge of brightness to opposition is observed while the branch of negative polarization at small phase angles exists. To explain this fact, we model the light scattering by particulate media with ensembles of spherical particles (with size comparable to the wavelength) of varying packing density and refractive index. The superposition T-matrix method is used. The increase in the absorption and/or packing density diminishes the amplitude of the brightness opposition peak, and its profile becomes wider. The influence on the branch of negative polarization is more complex and depends on the relation between the size parameters of the constituents, the refractive index, and the porosity. However, the feature common to all considered cases is that the negative branch changes its shape and the polarization minimum moves to the inversion point. This behavior radically differs from that observed in nonabsorbing ensembles of particles and reflects the fact that the efficiency of the coherent backscattering, which mainly determines these characteristics in nonabsorbing ensembles (to the packing density of about 30%), decreases. Moreover, since the angular profiles are not simply damped, but the polarization minimum changes its angular position, we may conclude that the near-field interaction of the constituents becomes important: the shielding of particles by each other eliminates many constituents from the scattering and the near-field effects promote the negative polarization and smooth the backscattering brightness surge. Due to this, when the packing density exceeds 10-20%, the opposition phenomena in absorbing ensembles are caused not only by the coherent backscattering, and situations, when the opposition brightness surge is practically suppressed, but the negative branch of polarization still survives, are possible. This may explain the fact that the dark regolith surfaces show no brightness opposition effect, but produce the branch of negative polarization with the minimum shifted from opposition.     

利用Turbiscan多重光散射技术评价pH值与钙离子对脱脂乳的热稳定性的影响

热稳定性直接影响牛乳的加工与感官特性，准确判断牛乳的热稳定性对优化液态乳制品的加工条件具有重要意义。然而，当前液态奶的稳定性评价主要是通过加速实验后肉眼观察分层、沉淀等情况，以及动态光散射等手段，尚无快速可靠以及量化的评价标准，严重制约了热处理工艺的选择效率。Turbiscan多重光技术在测试流体的稳定性时，无需对样品进行前处理，可实时检测样品的背向散射光和透射光的强度，计算出体系内部颗粒的迁移速率、沉淀层的厚度、体系的不稳定指数等参数，因此是评价流体物理特性的有效手段。研究以动态光散射测试结果为对照，利用Turbiscan多重光技术测定了脱脂乳经过80 ℃条件下加热30 min后，分别在pH值6.3，6.5，6.7和6.9，CaCl₂的浓度在0，20，40，60和80 mmol·L-1条件下的热稳定性。结果表明，当CaCl₂添加量为20，40，60和80 mmol·L-1时，脱脂乳的z-平均直径从152.7增加到1 284.4 nm，20 h后的不稳定指数值从0.98增加到17.04，样品顶端的背散射光强变化值从-4.3降低到-37.4，底端的背散射光强变化值从2.2增加到14.7；当样品的pH值分别为6.3，6.5，6.7和6.9时，脱脂乳的z-平均直径分别为148.1，152.7，132.4和122.4 nm，20 h后的不稳定指数值分别为1.32，1.02，0.98和1.41，样品顶端的背散射光强变化值分别为-3.1，-4.7，-4.2和-5.6，底端的背散射光强变化值分别为5.7，3.4，4.1和6.8。结果表明，随着CaCl₂添加量的增加，脱脂乳的热稳定性显著降低，pH值对脱脂乳热稳定性的影响较小。同时发现，与动态光散射技术相比，Turbiscan多重光散射技术可更精确、方便、快捷地获得热处理后的牛乳背散射光强值和体系不稳定指数等牛乳稳定性指标，从而明确热处理导致的乳蛋白分散体系的不稳定性发生的机理。Turbiscan多重光散射技术比动态光散射技术测定更方便快捷，该研究对优化乳制品加工工艺具有重要指导意义。