Circular polarization in scattered light as a possible biomarker

Biological molecules exhibit homochirality and are optically active. Therefore, it is possible that the scattering of light by biological molecules might result in a macroscopic signature in the form of circular polarization. If this is the case, then circular polarization spectroscopy, which may be utilized in remote sensing, can offer a powerful indicator of the presence of a universal biosignature, namely homochirality. Here, we describe laboratory experiments designed to investigate this idea. We focus on photosynthetic microorganisms, and also show results from macroscopic vegetation and control minerals. In the microorganisms, we find unambiguous circular polarization associated with electronic absorption bands of the photosynthetic apparatus. Macroscopic vegetation yields a stronger and more complex signature while the control minerals produce low-levels of circular polarization unrelated to their spectra. We propose a heuristic explanation of our results, which is that the polarization is produced by circular dichroism in the material after the light has undergone its last scattering event. The results are encouraging for the use of circular polarization spectroscopy in remote sensing of a generic biomarker from space or the ground.     

Anomalous polarization effects during light scattering in random media

The dependence of the intensity of light backscattered from a layer of a randomly inhomogeneous medium on the polarization of incident light and the size of scatterers has been investigated. The results of numerical simulation have demonstrated that the direction of rotation of the plane of polarization is different in systems with small- and large-scale inhomogeneities. It is shown for the first time that the dependence of the sign of the residual circular polarization on the size of scatterers can be observed in systems described by the Henyey-Greenstein phase function used in simulating biological tissues. A similar anomalous polarization effect, which consists in changing the direction of rotation of the plane of polarization of backscattered light with an increase in the scattering angle, is revealed in studying the coherent backscattering component. These polarization effects are observed in light backscattering from optically active media.     

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。     

Polarisation properties of comet NEAT C/2001 Q4

Comets exhibit high (up to 25 %) amount of optical polarization when they are observed through ground based or space telescopes. These polarizations are caused due to the scattering of cometary dust. The observed linear polarisation of comets is generally a function of the wavelength of incident light (λ), the scattering angle (θ), the geometrical shape and size of the particle and the composition of dust particles in terms of the complex values of the refractive index. The scattering properties of cometary dust will help to know the nature of cometary dust. In the present work, the observed linear polarization data of Comet NEAT are studied through simulations using Ballistic Particle-Cluster Aggregate (BPCA) and Ballistic Cluster-Cluster Aggregate (BCCA). Using Superposition T-matrix code, the best-fitting values of complex refractive indices are calculated which can well fit the observed polarization data of Comet NEAT C/2001 Q4. The best fitting values of complex refractive indices coming out from the present analysis correspond to mixture of both silicates and organics.     

生物组织多光谱偏振特性研究

生物组织内部结构复杂且具有较强的散射特性,而光作为生物组织检测的重要信息载体,其自身特性包括颜色、幅值、偏振等都对信息获取有较大的影响.结合偏振成像,对生物组织多光谱偏振特性展开了研究,依据不同微粒尺寸的分布建立了均匀单层生物组织模型,结合瑞利和米氏散射理论模拟了基于单个微粒的两种散射事件.瑞利散射具有较好的前向后向散...     

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.     

水下物体激光圆偏振成象实验及与线偏振成象的比较

采用波长532nm激光作光源,面阵CCD作探测器,利用圆偏振技术进行水下物体成象实验研究,对实验结果进行了分析并与线偏振成象技术进行了比较。结果表明无论是采用圆偏振技术还是线偏振技术都可提高水下物体成象的衬比（度）和成象距离;水体较清时圆偏振成象清晰度远远大于线偏振成象清晰度;当水体较混时,圆偏振成象清晰度大大下降和线偏振成象效果相接近。     

Light scattering by agglomerates: Interconnecting size and absorption effects (PROGRA2 experiment)

Linear polarization of the scattered light by clouds of dust particles and by very large agglomerates deposited on a surface are studied with the PROGRA² experiment. A first series of measurements use bare silica spheres and black-coated spheres to compare the phase curves obtained by different sizes of agglomerates with varying albedos. The refractive indices are evaluated by comparison with numerical simulations. Then, the maximum polarization, P_max, on the phase curves for irregular particles is studied as a function of the size of the grains (equivalent diameters from submicron-sized to hundreds of micrometres) and of the agglomerates (from micrometres to centimetres). A minimum value of P_max is obtained for silica (about 5% for lifted agglomerates and 3% for layers of particles with a grain size of about 50 μm) and amorphous carbon (about 40% for lifted agglomerates and layers with a grain size of about 0.2 μm). For smaller grain sizes, P_max increases when the grain size decreases. For larger grain sizes, P_max increases when the grain size increases. Differences between transparent and absorbing materials are underlined. Such studies may be used to interpret remote observations of light scattering by dust particles in cometary comae and Titan's atmosphere.     

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。     

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.     

Production of high circular polarization by ion surface scattering of ferromagnets

From a theoretical analysis we predict large fractions of circular polarization of light emitted by ions scattered from surfaces of ferromagnetic single crystals. Maximum enhancement of circular polarization as compared to surface ion scattering from non-magnetic materials is again expected at grazing incidence. A circular polarization of 89% was estimated for the case of Ar⁺ ions scattered from the (110) plane of a Ni single crystal.     

Model of light scattering by dust particles in the solar system: Applications to cometary comae and planetary regoliths

We analyze both the intensity and linear polarization of cosmic dust particles by using the physically exact superposition T-matrix method in a fixed orientation for various aggregates of spheres and DDA for the aggregates of Gaussian random spheres. We study both the spherical geometry (in cometary comae) and cylindrical slabs (for regoliths) up to 2000 monomers with size parameters less than ∼3. It is straightforward to produce the observed linear polarization in both geometries while the typically convex and strong opposition spike seems to require wide regolith geometries. The dependence of various parameters on light scattering has also been studied in a rather detailed form. In applications to the cometary polarization we can fit the data in six colors from UV to the J band at a very good accuracy. We, however, emphasize that we do not claim our model to be unique. The most important parameters here are the refractive index and the size distribution of submicron particles. Rest of the parameters has only a minor role. We also found that it is critically important to use several realizations from any assumed particle geometry model because corresponding scattering characteristics can vary quite a lot.     

碳酸钙微粒光致旋转的实验和理论研究

理论分析了由于光束轨道角动量和自旋角动量传递以及微粒的特殊形状导致微粒旋转的机理.实验建立了单光束激光光镊装置,不仅可以捕获并移动直径为微米量级的微小粒子,而且利用圆偏振光与微粒之间角动量的传递,实现了对具有双折射特性的碳酸钙微粒的光致旋转.实验中发现微粒的旋转不仅取决于光束的偏振态,还与微粒本身的形状有关,解释了实验中观察到的几种旋转现象.碳酸钙微粒旋转的最高转速达到12转/秒,转速与激光功率成正比.     

Circular differential double diffraction in chiral media

In an optically active liquid the diffraction angle depends on the circular polarization state of the incident light beam. We report the observation of circular differential diffraction in an isotropic chiral medium, and we demonstrate that double diffraction is an alternate means to determine the handedness (enantiomeric excess) of a solution.     

Polarisation de la lumière diffusée par des particules agrégées fractales

Some of the dust particles present in the solar system are likely to be aggregates. The study of polarization of the light scattered by such aggregated particles is performed for the six main formation processes. The analysis of the results shows that an irregular and fluffy structure of the particles allows a better understanding of the light scattering observations.     

Giant Raman optical activity of the 128-cm-1 mode in α-quartz

The threshold intensity of stimulated Raman scattering along the c-axis in α-quartz was measured for the 128-cm^-1 optical lattice vibration as a function of pump laser polarization at T = 10 K. In a right-handed, optically active quartz crystal the Raman threshold intensity for left-handed, circularly polarized pump light was lower by a factor of about 1.5 than for right-handed, circularly polarized light. The difference in threshold intensity is discussed in terms of Raman optical activity.     

Using dynamic low-coherence interferometry to image Brownian motion within highly scattering media

Dynamic low-coherence interferometry was used to measure Brownian motion of submicrometer particles within highly scattering media. Strong rejection of multiply scattered light was obtained by combination of a coherence gate with a confocal microscope, thus allowing particle characterization methods generally reserved for optically dilute materials to be applied to optically concentrated suspensions. The Brownian diffusion coefficient of highly scattering media was determined with an accuracy better than 5%. Furthermore, we show that spatial variations in the Brownian diffusion coefficient can be imaged with an axial resolution determined by the coherence length of the light source (~30 mum) . The experiments also show broadening of the power spectrum as a function of depth into the sample, most likely as a result of detecting multiply scattered light.     

Study of the sensitivity of size-averaged scattering matrix elements of nonspherical particles to changes in shape, porosity and refractive index

Studies of the physical parameters that influence the single scattering properties of a size distribution of small particles in random orientation are fundamental in understanding the origin of the observed dependence of the scattering matrix elements on the scattering angle. We present results of extensive calculations of the single scattering matrices of small nonspherical particles performed by a computational model based on the Discrete-Dipole Approximation. We have particularly studied the sensitivity of the size-averaged scattering properties at visible wavelengths of nonspherical, randomly oriented absorbing particles considering changes in shape, porosity and refractive index. These studies have importance regarding the inversion of physical properties of small particles as measured in the laboratory and the dust properties in various astrophysical and atmospherical environments. We have found that size distributions of randomly oriented irregular particles of different shape, including large aspect ratio particles, show similar scattering matrix elements as a function of the scattering angle, in contrast with the pattern found for regularly shaped particles of varying axis ratios, for which the scattering matrix elements as a function of the scattering angle show much larger differences among them. Regarding porosity, we have found a very different pattern in the scattering matrix elements for an ensemble of compact and porous particles. In particular, the linear polarization for incident unpolarized light produced by compact and absorbing particles of large size parameter tend to mimic the pattern found for large absorbing spheres. For porous particles, however, the linear polarization for incident unpolarized light tends to decrease as the size of the particle grows, with the maximum being displaced towards smaller and smaller scattering angles.     

非球形生物气溶胶对偏振光的多次散射特性研究

基于回转椭球模型和有限长圆柱模型,采用T矩阵方法研究了非球形生物气溶胶的单次散射特性,计算了鼠疫耶尔森氏杆菌、土拉热杆菌二种生物气溶胶对氦氖激光的单次相矩阵、单次散射反照率以及不对称因子。根据矢量辐射传输理论,研究了激光在生物气溶胶中传输的偏振散射特性,基于累加-倍加法(adding-doubling method)求解矢量辐射传输方程,并计算了非球形生物气溶胶对激光多次散射的斯托克斯参量。计算结果表明,生物气溶胶的尺寸和形状对光的极化更为敏感,因此在利用激光进行生物气溶胶微观特性探测和反演时,利用激光的偏振散射特性为非常有效的方法。