Estimates of the albedo of powderlike surfaces in the geometrical optics approximation

Results of computer simulation of light scattering by powderlike media composed of large semitransparent particles of different shapes are presented. The geometrical optics approach is used. The cases of particles of spherical, cubic, and random shapes are considered. The one-dimensional geometrical optics model of light scattering by powderlike media, which finds application in investigations of planetary surfaces, meteorites, and lunar samples, is briefly described. The numerical simulation allowed us to estimate the error of this model, which turned out to be within the limits of several percent. The albedo calculated by using the one-dimensional model is shown to be closest to the “three-dimensional” reflectance of the surfaces at a phase angle of about 60°. This albedo also approximates fairly well the integrated reflectance, which in laboratory measurements is determined with the help of an integrating sphere.     

Similarity and diversity in photometric and polarimetric opposition effects of small Solar System bodies

The phase-angle dependences of brightness and polarization of light scattered by atmosphereless Solar System bodies (satellites, asteroids, planetary rings) as well as comets and zodiacal light are analysed on the basis of available ground-based and spacecraft observations. We study similarity and diversity in the photometric and polarimetric opposition effects. The similarity of the brightness and polarization phase functions for polydisperse dust media (cometary and interplanetary dust) and for atmosphereless bodies at small phase angles gives grounds to state that at least some physical properties of dust particles and light-scattering mechanisms should be similar for the two classes of objects. The aggregate structure of particles in different objects can be precisely the property that determines the photometric and polarimetric effects observed in the opposition region. Differences observed in opposition effects for different objects are likely caused by different physical properties (composition, sizes, structure, density) of the scattering particles and, therefore, by different relative contributions of the light-scattering mechanisms. However, the relationship between the photometric opposition effect, spike effect, negative polarization branch, and polarization opposition effect for different bodies is ambiguous.     

偏振遥感在伪装目标识别上的应用及对抗措施

 为了研究偏振遥感对伪装目标的识别特点,分别对不同颜色的伪装网进行了偏振参数测量和成像实验。通过数据分析发现,伪装网的散射偏振度受观测条件和材料自身特性（如反射率、折射率和表面粗糙度）影响很大;伪装涂层对入射光的散射作用可以分为面散射和体散射,其中面散射具有起偏振作用而体散射具有消偏振作用;与自然背景相比,伪装目标的偏振特征非常显著,利用偏振遥感可以有效地识别出常规侦察手段所不能发现的伪装目标。通过偏振遥感的侦察原理和特点分析,提出了可以利用表面结构设计和烟雾等方法对抗偏振遥感的侦察。     

Polarimetry of the Galilean satellites and Jupiter near opposition

We present new polarimetric observations of the Galilean satellites Io, Europa, Ganymede, and Callisto as well as the North and South polar regions of Jupiter in the V filter within the range of phase angles ?0.13?0.62°. The differences between our results and observations of other authors are discussed. It is shown that there is no second inversion point near the phase angle 0.5°. New observations fully confirmed the presence of the polarization opposition effect for the Galilean satellites Io, Europa, and Ganymede in the region of phase angles smaller than 2°. Observations of Jupiter verified a sharp turn of the angle between the observed polarization plane and the plane perpendicular to the scattering plane by 90°.     

光源的偏振态对动态光散射颗粒测量结果的影响

研究了在动态光散射纳米颗粒测量中,光源的偏振态对测量结果的影响。采用了粒径为100nm、体积浓度为0.5%的标准颗粒作为样品,使He-Ne激光通过起偏器得到0°～180°方向的偏振光,测量了散射光强、偏振度和粒径测量值的变化,计算了相应的粒径均值偏差和标准差,并将这一结果与无偏振He-Ne激光入射进行了比较。结果表明,当入射光为线偏振光时,偏振方向垂直于散射面时测量效果最好;另一方面,由于颗粒系散射迭加造成的散射光偏振度降低,使线偏振光源与无偏振光源产生的散射光偏振度无明显差别,证明在测量中可以使用无偏振He-Ne激光代替。     

基于双旋延迟器结构的偏振BRDF测量系统的设计

偏振双向反射分布函数（BRDF）不仅可以表示物体散射光辐强度的空间分布情况,还包含了丰富的偏振信息。与标量BRDF相比,偏振BRDF可以更加精确地、全面地表示物体表面的光散射情况。设计了基于双旋延迟器结构的偏振BRDF测量系统,通过同步旋转波片调制入射光和散射光的偏振态,得到一系列变化的光强值,再由光强的Fourier分解系数计算获得样品的偏振BRDF值。系统内设计了一对正交反射镜,用以减小系统中器件后向散射光的影响。通过铝板偏振BRDF的测量,说明了该系统具有较高的准确性。     

Plasmonic laser nanoablation of silicon by the scattering of femtosecond pulses near gold nanospheres

We present the fabrication of nanostructures ablated on silicon(100) by the plasmonic scattering of 780 nm, 220 fs laser pulses in the near-field of gold nanospheres. We take advantage of the enhanced plasmonic scattering of ultrashort laser light in the particle near-field to ablate well-defined nanocraters. Gold nanospheres of 150 nm diameter are deposited onto a silicon surface and irradiated with a single laser pulse. We studied the effect of laser polarization on the morphology of ablated nanostructures and estimated the minimum fluence for plasmonic nanoablation. When the polarization of the incident radiation is directed at a 45° angle into the substrate surface, a near-field enhancement of 23.1±7.6 is measured, reducing the required silicon ablation fluence from 191±14 mJ/cm² to 8.2±2.9 mJ/cm². Enhancements are also measured for laser polarizations parallel to the substrate surface when the substrate is angled 0° and 45° to the incident irradiation, giving enhancements of 6.9±0.6 and 4.1±1.3, respectively. Generated nanocrater morphologies show a direct imprint of the particle dipolar scattering region, as predicted in our theoretical calculations. The measured near-field enhancement values agree well with the maximum field enhancements obtained in our calculations. The agreement between theory and measurements supports that the nanocraters are indeed formed by the enhanced plasmonic scattering in the near-field of the nanoparticles. PACS 42.62.-b; 52.38.Mf; 81.65.Cf; 81.16.-c; 78.67.Bf     

伪装涂层材料的二向偏振散射研究

目标的偏振散射特征研究是偏振遥感应用的基础。在不同的条件下测量了伪装涂层的偏振参数,研究了入射角、观测方位角对涂层偏振散射特征的影响。研究发现,伪装涂层的面散射会产生较大的偏振度,而体散射具有消偏振效应;深色涂层因为面散射起主要作用而具有较大的偏振度;涂层散射光的偏振度与入射角成正比,在观测方位角接近镜面反射方向时最大,随着方位角的增加而迅速减小;草地的偏振度很小,与伪装涂层形成鲜明对比。利用偏振遥感可以有效地识别草地背景中的伪装目标,偏振测量在遥感和目标识别方面具有重要意义。     

Feasibility of an optical experimental method for the sizing of primary spherules in sub-micron agglomerates by polarized light scattering

The possibility of inferring by a non-invasive experimental method the size of primary particles (spherules), which constitute the agglomerated soot generated in an ethylene–air diffusion flame, is investigated. In contrast to the predictions from the Mie theory for isolated spheres, experimental evidence is provided here about the fact that the size of spherules (some tens of nanometers), which stick together to form agglomerates (some hundreds of nanometers), can be recognized from polarization ratio measurements. Validation of the proposed scattering technique is obtained by first performing standard measurements of the primary particle size by SEM analysis of soot samples taken on quartz inserted directly in the flame along the burner axis. Then, the polarization ratio P(θ)≡σ_HH/σ_VV of scattered light is measured at the same locations and for the same flame conditions for different polar scattering angles θ. As major result, evidence is provided of a linear relationship existing between the primary sizes, obtained independently by SEM analysis, and the measurements of the polarization ratio P(90°). Finally, a procedure is reported and applied to retrieve the absolute spherule size from the direct observation of the transition between the power-law and Porod’s scattering regimes, which correspond to the domains of long-range (fractal) and short-range (not fractal) interactions between primary particles, respectively. Received: 24 February 1999 / Final version: 6 December 1999 / Published online: 1 March 2000     

沙尘气溶胶粒子群的散射和偏振特性

根据Mie散射理论,以对数正态分布函数描述沙尘气溶胶粒子群的粒径尺度分布,计算了沙尘气溶胶粒子群在0.2～40μm波段间对太阳短波辐射和地球大气长波辐射的单次散射反照率、散射相矩阵函数,揭示了不同相对湿度时,沙尘粒子群对入射辐射的散射和偏振的特征。结果表明,沙尘粒子群的单次散射反照率随着入射波长的增加有较大起伏,不同相对湿度条件下,变化趋势基本一致;在可见光、近红外波段单次散射反照率随湿度增加而变大,湿度95%时非常接近于1;大于10μm的热红外波段单次散射反照率随相对湿度增加而减小,具有较强的吸收辐射能力。散射辐射强度受湿度影响较小,随散射角的增加呈现先减小后增大的趋势,且增大的趋势随着波长的增加而减弱;不同波段上,线偏振和圆偏振随散射角和相对湿度变化存在差异;在前向和后向仅对入射辐射为圆偏振辐射产生圆偏振散射;散射光的偏振特性及其湿度差异主要表现在后向散射区,多以拱形形式体现。拱顶峰值散射角位置存在差异,且峰值散射角随相对湿度的降低向后向漂移。     

Weak localization in monolayer and bilayer graphene

We describe the weak localization correction to conductivity in ultra-thin graphene films, taking into account disorder scattering and the influence of trigonal warping of the Fermi surface. A possible manifestation of the chiral nature of electrons in the localization properties is hampered by trigonal warping, resulting in a suppression of the weak anti-localization effect in monolayer graphene and of weak localization in bilayer graphene. Intervalley scattering due to atomically sharp scatterers in a realistic graphene sheet or by edges in a narrow wire tends to restore weak localization resulting in negative magnetoresistance in both materials.     

The scattering matrix of transparent particles of random shape in the geometrical optics approximation

A numerical model that allows one to calculate elements of the scattering matrix for transparent particles of random shape in the geometrical optics approximation is presented. It is shown that a deviation from sphericity, which, in particular, is modeled by a reduction of the number of triangular facets approximating a sphere, essentially affects the magnitude, position, and width of peaks of the photometric and polarimetric indicatrices. Thus, when 1500 facets were used for the approximation, the amplitude of the polarimetric peak associated with the first rainbow, which is located close to the scattering angle 160°, decreases by a factor of two. Calculations showed that, in the region of backscattering, for particles of an arbitrary shape, the linear polarization ?F ₁₂/F ₁₁ has no negative branch, which is well observed for spherical particles. In going from spherical to nonspherical particles, the backscattering peak also disappears. The indicatrices for particles of irregular shape that were calculated for small distances from the center of a particle noticeably differ from the indicatrices at infinity. Thus, when simulating multiple scattering in dense powderlike media, the use of particle scattering indicatrices that were calculated for infinite distances is incorrect even in the geometrical optics approximation.     

Observation of Ramsey fringes in an atomic cell with buffer gas

Temporal Ramsey fringes that are due to light scattering by coherently prepared rubidium atoms diffusing through a cell containing neon as a buffer gas have been observed. The effect leads to increasing magneto-optical rotation of cw light polarization at weak magnetic fields.     

Weak-localization effect for light backscattered by surfaces with a complex structure

Results of laboratory measurements of the weak-localization effect for light scattered by surfaces with different characteristics are presented. The measurements were made in a range of phase angles of 0.2°–3.5°. The aim of the measurements was to study the influence of the choice of surface material (metal, dielectric), the size of particles of the scattering surface, their packing density, the reflectivity of a sample, and the wavelength of incident radiation on the effect. For the measurements, both nonpolarized and linearly polarized light was used. Powders of dielectric materials were found to enhance the weak-localization effect with increasing particle size, but the effect weakened starting with a certain size of the order of wavelength. The pressing of powders of dielectric transparent materials enhanced the opposition effect. This was most pronounced for samples with small-sized particles. The phase dependence of brightness became wider and more linear with decreasing reflectivity of the surface. A qualitative difference in the behavior of the phase dependence for the ratio of cross-and copolarized components was observed. For metals and metal-like materials, the ratio increased with decreasing phase angle. Dielectrics were found to have an opposite dependence.     

Assessment of LES of intermittent soot production in an aero-engine model combustor using high-speed measurements

Soot production in turbulent flames is an extremely intermittent phenomenon since it is the result of specific thermochemical conditions occasionally occurring in space and time. In realistic configurations such as the swirling flames used in gas-turbines, the presence of large-scale flow motions can additionally affect soot formation processes, leading to even more pronounced intermittency. Classically, the validation of numerical simulations is performed by comparing time-averaged results with experimental data of the phenomenon under investigation. This comparison can be considered as rigorous only if a statistically converged numerical representation is obtained. In case of sporadic events such as intermittent soot formation in turbulent flames, this means to perform the simulation over thousands of milliseconds of physical time, which can have extremely high CPU demands when performing Large Eddy Simulation (LES). In this work, a possible strategy to overcome this issue is proposed based on the use of high-speed measurements and numerically synthesized signals from LES. To illustrate the approach, numerical and experimental soot light scattering signals are considered here by looking at the model aero-engine combustor developed at DLR for the study of pressurized swirled sooting flames. The light scattering signal is numerically synthesized from an LES. Experimental high-speed measurements are used to statistically account for the high temporal and spatial variability of soot when considering time intervals similar to what is today achievable with LES. The feasibility of this approach is finally demonstrated by comparing numerical results to the ensemble of possible soot production states observed experimentally in the DLR burner allowing to eventually validate the present LES results.     

A Dissymmetry Ratio Optical Technique as Applied to Scattering Pattern Recognition of Differently Shaped Soot Aggregates

We investigated the shape of soot aggregates as retrieved from light scattering measurements. In particular, the ratios of scattered light intensities at two angles, called the dissymmetry ratios (DRs), will be considered. These are attractive diagnostic tools as they are insensitive to variations in the complex refractive index (m = n + i k) and to the electromagnetic effect of self-interaction that each individual scatterer in the aggregate produces with itself depending on its finite size. The aim of the paper is both to correlate the DRs with the information about the aggregate shape and to establish what is the minimum number of scattering measurements to be performed to characterize the aggregate scattering patterns. The main results can be summarized as follows: (a) an independent evaluation of the primary spherule size is required in order to interpret the scattering data correctly; (b) some of the DRs can share common values for different morphologies when the number of primary spherules per aggregate is sufficiently high; in this respect, we demonstrate the significance of the dissymmetry ratio R_vv(10°/90°) for the identification of the prevailing morphology of aggregates at different levels of agglomeration; (c) the information about light intensities scattered at three angles seems to suffice for the recognition of prevailing aggregate morphology.     

旋转波片偏振仪波片方位误差研究

旋转波片Stokes偏振仪是最常用的测量光束偏振态的仪器。波片快轴方位误差是影响旋转波片Stokes偏振仪的主要误差源之一。为了研究波片方位偏差对测量精度的影响,提出了一种描述波片快轴方位误差向最终的偏振测量误差传递的数学模型,并引入协方差矩阵法表征偏振测量误差。根据这一模型,获得最优的偏振仪配置参数。在推导过程中,假设波片方位误差服从同一高斯分布。基于此误差模型,得到如下结论：(1)由波片方位误差引入的测量误差与光强测量次数N成反比;(2)测量误差独立于入射光强度,但是依赖于入射光偏振态(s₁, s₂, s₃)和波片的位相延迟量δ;(3)波片位相延迟量在(103.22°, 116.13°)范围内时波片方位误差引入的测量误差最小。最后,经过仿真实验证明,所得解析结果与仿真模拟结果相一致。     

Influence of complex component and particle polydispersity on radiative properties of soot aggregate in atmosphere

The effects of morphological structure, water coating, dust mixing and primary particle size distribution on the radiative properties of soot fractal aggregates in atmosphere are investigated using T-matrix method. These fractal aggregates are numerically generated using a combination of the particle-cluster and cluster-cluster aggregation algorithms with fractal parameters representing soot aggregate in atmosphere. The radiative properties of compact aggregate notably deviate from that of the branched one, and the effect of morphology changes on the radiative properties in wet air cannot be neglected. However it is reasonable to use realization-averaged radiative properties to represent that of the aggregates with certain morphology. In wet air, the scattering, absorption and extinction cross-section and symmetry parameter of soot aggregates coated with water notably increase with water shell thickness. The mixing structures of dust have little effect on radiative properties of aggregates, but the volume fraction of dust has an obvious effect on extinction, scattering and absorption cross-section of aggregates when the size parameters are above the Rayleigh limit. Although the primary particle size distribution of soot aggregate has mild effect on the scattering albedo and asymmetry parameter, the deviations of the extinction, scattering, absorption cross-section among the three size distributions are significant in this study. The size distribution has a significant effect on forward scattering of phase function, while the effect can be neglected as the size parameter approaches to the Rayleigh limit.     

基于野外测量研究植被冠层偏振光谱反射特性

植被冠层反射太阳光会产生部分偏振光,这为偏振遥感探测提供了信息源。利用地基平台开展野外偏振测量是非常重要的研究,它不仅能获取植被冠层本质的偏振反射特性,而且还可以与机载或者星载平台数据进行匹配;虽然研究者们已经通过机载或者星载偏振探测器获取了大量植被冠层的偏振反射信息,但是关于地面植被冠层偏振反射分布特性的研究却很少。因此,为了详细研究植被冠层的偏振反射特性,基于野外多角度高光谱偏振测量与偏振反射物理机制,分析了植被冠层偏振光谱特性及其偏振反射分布特性;随后,反演了两种偏振模型的参数,将测量值与模拟值进行对比分析。研究结果发现,植被冠层的偏振反射具有明显的各项异性特征,而且与波长之间存在微弱的关系;同时,偏振模型可以很好地模拟植被冠层的偏振反射信息,可以作为研究植被冠层的理论依据。该研究测量的偏振反射信息与模型计算值之间呈现良好的吻合性,既描述了植被冠层偏振反射分布特性,又证明模型的有效性和测量结果的准确性。对植被冠层偏振特性的详细研究,不仅有助于完整地解释植被冠层与电磁波之间的辐射响应特征,而且对于偏振遥感对地观测以及地表偏振信息对大气偏振信息的研究也具有重要科学价值。     

Optical properties of pulsed laser heated soot

To investigate the transient change of soot optical properties resulting from pulsed laser heating of soot in a cooled exhaust plume we have simultaneously performed cw light extinction at 405 and 830 nm and elastic light scattering at 1064 nm. A reversible increase to the 830-nm light extinction of up to 7%, observed during the time period where the soot was hot, suggests a temperature-dependent light absorption refractive index function, E(m _λ ). At low fluence, small permanent increases of E(m _λ ) of <2% were also observed. 405-nm extinction measurements revealed that the soot likely contained material which continued to absorb 405-nm radiation when desorbed, thus complicating measurement interpretation. 1064-nm light scattering measurements showed a gradual decrease of scattering propensity with increasing laser fluence up to the point of material loss, which is consistent with the expected decrease of the structure factor of the soot aggregates as they expand. It is concluded that variations of the optical properties are occurring at the time of laser-induced incandescence (LII) emission, which should be accounted for in time-resolved LII measurement interpretation.