Backscatter effects of surfaces composed of dry biological particles

We present the backscattering of particulate surfaces consisting of dry biological particles using two laboratory photopolarimeters that measure intensity and degree of linear polarization in a phase-angle range 0.2-60°. We measure scattering properties from three samples composed of dry biological particles, Bacillus subtilis var. niger (BG) spores and samples of fungi Aspergillus terreus and Sporisorium cruentum spores. We find that the surfaces display a prominent brightness opposition effect and significant negative polarization near backscattering angles. The brightness and polarimetric phase curves are different for B. subtilis and the fungi.     

Particle size effect on the opposition spike and negative polarization

We report results of experiments designed to increase our understanding of the influence of particle size on the photometric opposition spike and negative polarization observed in the reflectance and polarization phase curves of particulate surfaces. We concentrate our studies on particle-size separates of alumina (bright powders) and boron carbide (absorbing powders). We use two photopolarimeters that span small (0.2-17°) and large (2-160°) phase-angle ranges. The results suggest that the negative polarization has two dominant mechanisms: (1) the coherent-backscatter enhancement and (2) single-particle scatter, and that the contributions of the mechanisms are a function of particle size. The measured photometric and polarimetric phase functions can be applied to evaluate models used to calculate scattering properties of particulate surfaces.     

Polarization properties of odd comet 17P/Holmes

We present results of polarimetric observations of comet 17P/Holmes that in 2007 surprised the observers by its outburst that resulted in dramatically increased brightness and unusual shape of the coma. Polarimetric properties of the comet also appeared to be very peculiar. Even though the comet showed negative polarization typical for the small phase angles, its absolute values were much lower than usually observed at these phase angles and the spectral gradient of the polarization was much more pronounced than usually observed for comets (although negative that is typical for other comets too). Both these results are different from the regular trends and indicate some specifics of the comet Holmes dust during the outburst. We discuss how such unusual polarimetric properties of the comet can be explained and show how a peculiar combination of aggregated and compact particles or a composition with a unique ratio of silicates to organics can explain the observational results.     

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.     

Discrete-dipole analysis of backscatter features of agglomerated debris particles comparable in size with wavelength

We study the backscattered light of agglomerated debris particles using the discrete dipole approximation (DDA) in order to isolate specific physical components that contribute to negative polarization and the brightness opposition surge. We examine a few specific particle systems that display a prominent brightness surge and significant negative polarization over a range of near backscatter angles. In all cases, removal of the far-field interaction components results in the disappearance of the brightness surge and negative polarization branch; whereas, these phenomena remained if the near-field or radial components of the interaction fields are removed. This suggests that the mechanisms for these phenomena are embedded within the far-field interaction component.     

Coherent opposition effects for semi-infinite discrete random medium in the double-scattering approximation

The rigorous equations of the theory of multiple scattering of light by a layer of disordered medium have been used in the double-scattering approximation for semi-infinite medium to determine the influence of the particle properties on the coherent opposition effects. The effects were found to be strongly dependent on the concentration of scatterers in the medium. The polarization opposition effect is more sensitive to the properties of the scatterers than the photometric opposition effect. The interference of waves could result in the negative polarization at the backscattering direction as well as in the positive polarization.     

Shadowing effect in clusters of opaque spherical particles

The shadowing effect is studied for clusters of opaque spherical particles. The present modeling allows geometric optics computations of cluster scattering phase functions and shadowing effects with internal accuracy better than 1%. Three types of cluster structures are treated—uniform, ballistic, and hierarchical (physical fractal)—and three types of elementary surface scattering laws are examined—Lambertian, flux-isotropic, and specular. All structures investigated give rise to an opposition effect, that is, a nonlinear brightening toward zero phase angle. The amplitude and width of the opposition effect depend on the cluster parameters. For uniform clusters, the volume fraction and number of particles are the parameters that characterize the shadowing effect. The opposition effect becomes sharper with increasing number of constituent particles and with decreasing particle volume fraction. For ballistic clusters, the only parameter is the number of particles: when it increases, the opposition effect becomes sharper. For hierarchical clusters, the number of cluster structural levels plays a crucial role. With increasing number of cluster levels, the opposition behavior of brightness becomes markedly more nonlinear, mostly due to the decreasing particle volume fraction. It is notable, however, that the opposition effects of the hierarchical clusters and the uniform clusters with the same particle volume fraction differ from each other underscoring the importance of the detailed cluster structure on shadowing. It is shown that, with reasonable accuracy, the cluster scattering phase functions can be factorized as the products of the corresponding single-particle phase function and the so-called shadowing factor almost independently of the elementary surface scattering law. While the opposition effect due to shadowing is presently confirmed, it is typically wider than the opposition effect due to coherent backscattering, an interference mechanism in multiple scattering. The present work helps us to understand, e.g., the opposition effects of the Moon, asteroids, and other atmosphereless celestial bodies.     

Coherent backscattering effects for discrete random media: Numerical and theoretical results

Manifestation of the backscattering enhancement phenomenon in the reflection matrix elements of the coherent component of scattered radiation is considered. The dependence of the coherent backscattering effects on the microphysical properties of the medium scatterers are investigated. It is shown that random media of fractal-like clusters exhibit brightness and polarization opposition effects, which are like those observed for some atmosphereless Solar system bodies. Conditions for a bimodal angle dependence in the degree of linear polarization are discussed and the manifestation of the enhanced backscattering phenomenon in the intensity of scattered radiation is studied.     

Polarimetric observations and laboratory simulations of asteroidal surfaces: The case of 21 Lutetia

The Rosetta spacecraft flew by 21 Lutetia on July 2010. This event provides a unique opportunity to enhance our knowledge of solar system small bodies, by comparing the surface properties measured in situ and the properties deduced from the linear polarization of scattered light, and prepare future observations. The linear polarization is studied as a function of the phase angle at different wavelengths and compared to phase curves of M-type and C-type asteroids. In a second part of the work, 21 Lutetia's polarization phase curves are compared to phase curves measured in the laboratory for powdered carbonaceous chondrites suggested as eventual analogs by spectroscopic studies. The importance of the variation of the linear polarization as a function of the wavelength is emphasized. CV3-class meteorite is found to be the best polarimetric laboratory analog with an average size of regolith grain lower than 50 μm.     

Microwave scattering properties of sand particles: Application to the simulation of microwave radiances over sandstorms

The single-scattering properties of sand/dust particles assumed to be ellipsoids are computed from the discrete dipole approximation (DDA) method at microwave frequencies 6.9-89.0 GHz in comparison with the corresponding Lorenz-Mie solutions. It is found that the single-scattering properties of sand particles are strongly sensitive to the shapes of the particles. The bulk scattering properties of sandstorms composed of spherical or nonspherical particles are investigated by averaging the single-scattering properties of these particles over log-normal particle size distributions. Furthermore, a vector radiative transfer model is used to simulate microwave radiances. The microwave brightness temperatures in the vertical polarization model are essentially not sensitive to sand particle habit, whereas microwave brightness temperature polarization differences are influenced by particle habit. It is shown that microwave brightness temperatures and brightness temperature polarization differences may be useful for estimating the effective particle sizes and mass loading of sandstorms.     

Light-scattering properties of fractal aggregates: numerical calculations by a superposition technique and the discrete-dipole approximation

Dust particles in space often grow by mutual collisions and appear to be an agglomeration of individual grains, the morphology of which can be described by the concept of fractals. In this paper, we study light scattering by fractal aggregates of identical spheres (monomers) using the superposition technique incorporated into the T-matrix method where the orientationally averaged scattering matrix is analytically obtained. We also apply the discrete-dipole approximation, in which the dipole polarizability of spherical monomers is determined by the first term of the scattering coefficients in the Mie theory. Two cases of the ballistic aggregation process (particle–cluster and cluster–cluster aggregations) are considered to model fractal aggregates consisting of silicate or carbon material. The dependences of light-scattering properties on the monomer sizes, aggregate structures and material compositions are intensively investigated. The light-scattering properties of the fractal aggregates strongly depend on the size parameters of the monomers. The difference in the scattering function between the particle–cluster and cluster–cluster aggregates can be seen in the case of monomers much smaller than the wavelength of incident radiation. When the size parameter of monomers exceeds unity, the material composition of the monomers influences the light-scattering properties of the aggregates, but different morphologies result in similar scattering and polarization patterns. We show that silicate aggregates consisting of submicron-sized monomers, irrespective of the aggregate size and morphology, produce a backscattering enhancement and a negative polarization observed for dust in the solar system.     

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。     

Polarimetric weak-localization effect in scattering of natural light in the region of small phase angles

Results of laboratory measurements and interpretations of the polarimetric effect of weak localization (negative polarization) appearing under scattering of natural light by a dark ultradisperse surface (soot with albedo of 2.5%) are presented. The measurements were carried out in red light in a range of phase angles of 0.2°–4.2°. It was revealed that the soot, despite the low albedo, shows the polarimetric weak-localization effect inherent in bright surfaces such as the surface formed by smoked MgO. The results of measurements are interpreted using numerical simulation of multiple light scattering in a medium consisting of particles whose characteristics are close to those observed for soot. As the result of simulation, it was found that the scattering with the multiplicity exceeding two can give rise to a negative polarization branch, which becomes narrower with increasing scattering order. For the case of soot, four orders of scattering is sufficient to describe the observed polarimetric effect of weak localization with a necessary accuracy.     

Optical modeling of mineral dust particles: A review

Dust particles are uniquely and irregularly shaped, they can be inhomogeneous, form agglomerates, be composed of anisotropic materials, and have a preferred orientation. As such, modeling their light scattering is very challenging. This review takes a look at the advances in dust optical modeling over the last decade. It is obvious that our ability to model the single-scattering properties of dust particles accurately depends on the size parameter. Unfortunately, our ability to account realistically for all the relevant physical properties in light-scattering modeling is the best for small particles; whereas, the realistic treatment of the particles would be most important for large size parameters. When particles are not much larger than the wavelength, even simple model shapes such as homogeneous spheroids appear to perform well; practically any reasonable shape distribution of non-spherical model particles seems superior compared to the Mie theory. Our ability to model scattering by dust particles much larger than the wavelength is very limited: no method presently exists to predict reliably and accurately the single-scattering properties of such particles, although there are models that can be tuned to agree well with the laboratory-measured reference scattering matrices. The intermediate size parameters between the resonance domain and the geometric-optics domain appear to be almost uncharted territory and, consequently, very little can be said about the impact of different physical properties on scattering in this region. Despite the challenges, the use of Mie theory should be avoided: contrary to the popular belief, the use of Mie spheres is a major source of error even in radiation-budget considerations.     

切比雪夫雾霾粒子的紫外光散射偏振特性研究

雾霾天气已严重影响人们的日常生活,通过检测雾霾粒子的紫外光散射偏振特性可以有效分析雾霾成因。矿物粒子、灰尘粒子等雾霾颗粒均有小规模表面粗糙度的形态学特征,因此可用切比雪夫粒子作为模型分析。“日盲”紫外光与切比雪夫雾霾粒子相互作用发生散射,散射光偏振特性可反演切比雪夫雾霾颗粒物理性质(如粒子尺寸参数、复杂折射率、粒子形变、波纹参数)。采用紫外光单次散射模型和T矩阵方法,仿真分析切比雪夫雾霾粒子物理参数与散射光偏振特性(Stokes矢量和偏振度)之间的关系,结果表明：粒径对散射光Stokes矢量Is和Qs随散射角的变化趋势影响很大,粒子的粒径和复杂折射率虚部的变化会造成散射光偏振度随散射角的变化趋势的改变;具体分析散射角度为10°时,得到粒径对Is和Qs的数值影响最大,当粒径r<1 μm时,Is随粒径呈现抛物线趋势;切比雪夫粒子形变的增大,Is呈现先增大后减小的趋势。     

T-matrix approach to calculating circular polarization of aggregates made of optically active materials

Optical activity is a typical property of the biological materials where left-handed amino-acids and right-handed carbohydrates dominate (so called homochirality). Observationally, optically active materials reveal themselves through the circular polarization in the light they scatter. Thus, circular polarization produced by the optically active particles can serve as a biomarker. It is known that biological (e.g. colonies of bacteria) and pre-biological (e.g. dust in comets) particles often have a complex structure that can be modeled presenting them as aggregates of small monomers. This motivated the development of the T-matrix code presented in this paper, which enables calculation of the scattering matrix - including circular polarization - of the light scattered by aggregated optically active particles. The code can be used for modeling the light scattering by biological objects (e.g. colonies of bacteria, blood cells) and for interpretation of the circular polarization produced by the cosmic dust that contains (pre)biological organic, e.g. comet dust or planetary aerosols.     

Comparative study on opposition effect of icy solar system objects

Opposition effect is a nonlinear increase in the brightness of an object close to astronomical opposition. We present a comparison and summary of the phase curves of some icy solar system objects, and terrestrial ice. We have constructed a method of approximating the height and width of the opposition peak and evaluating the practicality of the method in each case, with the aid of a four-parameter fit to a linear-exponential function and computing the a posteriori probability distribution of the height and width of the opposition peak. The procedure described is most applicable in finding a realistic fit and evaluating the suitability of data, even with moderate inaccuracy. We have studied the effect of the number of data points to the feasibility of the fit, in order to estimate the minimum amount of data required for a desired accuracy. Furthermore, we present an interpretation of the phase curves by means of two scattering mechanisms with important applications to the recent study of the opposition effect: shadowing and coherent backscattering.     

Static Wiener spectrum of a thin film of polymer-encapsulated ferroelectric liquid crystals

Results of computer simulation of the shading effect in systems of opaque spherical particles scattering light in accordance with the Lambert law are presented. Two types of systems are studied: a semi-infinite medium and statistically uniform clusters of a finite number of particles. The simulation makes it possible to obtain photometric characteristics of systems with an accuracy better than 1%. The phase dependence of the shading effect is shown to become steeper as the packing density of particles in clusters decreases and their number increases. For statistically uniform media, the following relation takes place: The lower the packing density, the more pronounced the shadow decrease in brightness with an increasing phase angle.     

SOS: 对称操作相似原理

凝聚态物理学往往为对称性所支配。自发对称性破缺将导致相变，而且许多可观测量或物 理现象都与破缺的对称性有关，如铁电极化、铁磁磁化、旋光性（包括法拉第旋转和磁光克尔旋 转）、二次谐波发生、光伏效应、非互易性、霍尔效应型输运以及多铁性。在这里，我们提出以 下观点：从对称性破缺的角度来看，当构成量（如自旋排列、晶格畸变，或处于外场以及其他环 境中等）或测量方式（如光学探测，或处于不同极性态中电子和其它粒子探测，或对体极化[磁 化] 等的测量）遵循对称操作相似(symmetry operational similarity, SOS) 原理时，我们便可以 观测到某些特定的物理现象。这种SOS原理提供了一种在复杂材料中用简洁而清晰的物理图像描 述非直观物理现象的途径。反之，也可以用于甄别具有潜在需求特性的新材料，或发现已知材料 中新的物理现象。     

Backscattering and negative polarization of agglomerate particles

We used the discrete dipole approximation to study the backscattering of agglomerate particles consisting of oblong monomers. We varied the aspect ratio of the monomers from approximately 1 (sphere) to 4, while we kept the total particle volume equivalent to that of an x = 10 sphere for m = 1.59 + i0 and 1.50 + i0 and considered two values of agglomerate packing density: rho = 0.25 and rho = 0.1. We found that these particles do not display a prominent brightness opposition effect but do produce significant negative polarization over a range of near-backscattering angles. Increasing the monomers' aspect ratio can make the negative polarization much more prominent. We have noted also that decreasing m and p can reduce the amplitude of the negative polarization for these particles.