Exact Backscattering Direction Research Articles

Optical backscattering and linear polarization properties of the colony forming cyanobacterium Microcystis.

Light scattering characteristics of the cyanobacterium Microcystis are investigated with numerical models for sphere aggregates. During summer bloom seasons, Microcystis is prevalent in many inland waters across the globe. Monitoring concentrations with remote sensing techniques requires knowledge of the inherent optical properties (IOPs), especially the backscattering properties of Microcystis cells and colonies in natural settings. In situ measurements in waters dominated by Microcystis blooms have previously detected extremely high backscattering ratios, i.e., bb/b>0.043 at 443 nm [1], the highest to our knowledge in the natural environment. These highbb/bvalues could hold promise as a diagnostic tool in identifying and monitoring Microcystis using optical approaches. However, it has been unclear how this type of optically 'soft' organic particle can generate such highbb/bvalues. In this study, the Multiple Sphere T-matrix (MSTM) model is used to calculate the IOPs of model colonies, including bb/b. Colony sizes in the model ranged from several cells to several hundred and both colony packing density and cell gas vacuole content were varied. Results are compared with model results for equivalent-volume spheres (EVS) and direct in situ measurements. Colony formation was required in the modeling to reproduce the high bb/bconsistent with in situ measurements. The combination of moderate to very dense colony (packing density >30%) and high gas vacuole content in individual cells (volume percentage >20%) was the most favorable condition leading to rapid increases in bb/bwith increasing number of cells Ncell of the colony. Significant linear correlations were observed betweenbb/b and Ncell1/3 for these colonies, wherebb/b increased beyond 0.04 once cell number reached about 1000 cells in the case with the most densely packed cells and highest gas vacuole content. Within commonly observed colony sizes (Ncell <106), colonies with high gas vacuole content exhibited bb/bvalues up to 0.055, consistent with direct measurements from Lake Erie. Polarized scattering was also of interest as a diagnostic tool, particularly with future Earth-orbiting polarimeters being deployed for the NASA Plankton, Aerosols, Cloud, ocean Ecosystem (PACE) mission. The Degree of Linear Polarization (DoLP), expressed by the ratio of two Mueller matrix elements-P12/P11, decreased with increasing colony cell number for Microcystis. Another ratio of two Mueller matrix elementsP22/P11, an index for nonsphericity, also decreased with increasing colony size. In addition to higher relative backscattering, greater colony packing density and larger gas vacuole sizes both led to lower DoLP peak magnitude and lowerP22/P11. An optical opposition feature due to constructive phase interference that was observed previously for cosmic dusts is also present for these modeled colonies, manifested by a narrow intensity peak and negative polarization dip near exact backscattering direction, gradually forming as colony size increases.

Coherence of Raman light arises from disorder

Light propagation in random materials is a topic of great interest for the scientific community, not only for the possible relevant applications in the fields of photonics and renewable energies but even more since it allows to unveil new fascinating phenomena related to wave physics. Among these physical events, the most robust and always surviving any ensemble average is the coherent backscattering of light (CBS). It is a very subtle interference effect in disordered scattering media (such as semiconductor powders or micro-particle suspensions like milk or fog), in which wave coherence is preserved even after a very large number of random scattering events, eventually manifesting as a maximum of interference in the exact backscattering direction. CBS is related to the well-defined wave character and to the preservation of the optical information, for this reason it has been so far experimentally observed and theoretically studied only for elastic scattering, while the occurrence of inelastic scattering is known to reduce the degree of coherence in the diffusion process, affecting the visibility of the effect. Fazio et al. (2017) have demonstrated that this experimental evidence surprisingly survives also for the inelastic light scattering, such as the spontaneous Raman process, as long as the optical information of the propagating wave is retained. In this kind of inelastic scattering events, light loses a small part of its energy by slightly changing wavelength. Its phase coherence, however, is preserved for a very short time, thus making interference between Raman scattered waves still possible. The observed maximum of interference in the exact backscattering direction is therefore a signature of the coherent nature of individual Raman scattering processes. To date, indications on the coherence properties of Raman scattering have been reported only by looking at the nanoscopic scale, through complex near-field experiments making use of very sharp tips or through ultra-fast time resolved techniques. This time, however, we did not rely on complex experiments or advanced techniques. Conversely, the combination of an accurate experimental procedure and the unique structural properties of a silicon-based material were the only simple ingredients for the observation of a new unexpected physical phenomenon. In particular, a dense forest of ultrathin silicon wires arranged in a disordered fashion, where light waves bounce back and forth countless times before coming out, was the medium that allowed us to reveal this new effect, which opens the way for new and important discoveries.

Study of light scattering properties of dust aggregates with a wide variation of porosity

We study the light scattering properties of moderately large dust aggregates ($0.8\mu m \lesssim R \lesssim 2.0\mu m$) with a wide variation of porosity ($\mathcal{P}$) from 0.57 to 0.98. The computations are performed using the Superposition T-matrix code with BAM2 cluster ($\mathcal{P} \sim 0.57-0.64$), BAM1 cluster ($\mathcal{P} \sim 0.74$), BA or BPCA cluster ($\mathcal{P} \sim 0.85-0.87$) and BCCA cluster ($\mathcal{P} \sim 0.98$). The effect of aggregate size parameter ($X$) on the light scattering properties of aggregates (BA and BAM2) having different porosities is explored in this study. We find that the positive polarization maximum ($P_{max}$), the amplitude of the negative polarization ($P_{min}$) and phase function at the exact backscattering direction ($S_{11}$(180$^\circ$)) are correlated with the porosity of aggregates. Compact aggregates show deeper negative polarization as compared to porous aggregates when the characteristic radius ($R$) of the aggregates are considered to be the same. Further lower porosity aggregates show higher $S_{11}$(180$^\circ$) and vice versa. When $\mathcal{P}$ is increased in a range from 0.64 to 0.98, both ${S}_{11}(180^\circ$) and $P_{min}$ decrease linearly, whereas P$_{max}$ increases linearly. We also find that the porosity of the aggregates plays a crucial role in determining the polarimetric color for high absorbing organic refractories. The compact clusters (BAM1 and BAM2) show the negative polarimetric color whereas BA clusters show almost positive polarimetric color at all values of scattering angle. We have also made some comparisons of our simulated results with PROGRA$^2$ experimental results.

Polarization-resolved exact light backscattering by an ensemble of particles in air

We present the first experimental observation of exact backscattering of light by an ensemble of particles in ambient air. Our experimental set-up operates in the far-field single scattering approximation, covers the exact backscattering direction with accuracy (θ = π ± ε with ε = 3.5 × 10(-3) rad) and efficiently collects the particles backscattering radiation, while minimizing any stray light. Moreover, by using scattering matrix formalism, the observation of the particles UV-backscattering signal allowed to measure the particles depolarization of water droplets and salt particles in air, for the first time, in the exact backscattering direction. We believe this result may be useful for comparison with the existing numerical models and for remote sensing field applications in radiative transfer and climatology.

Revisiting the vegetation hot spot modeling: Case of Poisson/Binomial leaf distributions

The accuracy of spaceborne/airborne sensor measurements in the solar domain keeps increasing over time. High resolution, multi-directional and hyperspectral image acquisitions start to be abundant. With regard to the multi-angular remote sensing data, the hot spot, i.e. the exact backscattering direction of direct sunlight together with its neighboring directions, is of special interest. Accurate hot spot models have to be used to adequately simulate the hot spot signature and to allow reliable inversion of multi-angular data. In this paper, we propose a physical hot spot model (Leaf Spatial Distribution based Model, LSDM) assuming that for a given point inside the vegetation to be sunlit (respectively, observed) it should be located within a cylinder free from leaf centers. The cylinder is oriented to the sun (respectively, sensor) direction. Assuming a leaf random, regular or clumped spatial distributions, the gap probabilities in the sun and sensor directions are expressed as a function of these cylinder volumes. Based on the same hypothesis, the bidirectional gap probability is estimated as a function of the total volume of the two cylinders. The evaluation of the needed common volume of two cylinders having different radii is reduced to calculation of some elliptic integrals. Finally, the hot spot signature is estimated based on the bidirectional gap probability distribution. Different model versions with different leaf spatial distribution functions are compared. Particularly, it is shown that compared to the random distribution, the regular (the clumped, respectively) distribution increases (decreases, respectively) the reflectance due to single scattering contribution from foliage. The proposed model is validated using the ROMC web-based tool and its better performance compared to the Semi-Discrete Model and Kuusk's model is confirmed.

Influence of particle size and shape on the backscattering linear depolarisation ratio of small ice crystals – cloud chamber measurements in the context of contrail and cirrus microphysics

Abstract. The article presents the laser scattering and depolarisation instrument SIMONE that is installed at the large aerosol and cloud chamber facility AIDA of the Karlsruhe Institute of Technology. SIMONE uses a 488 nm cw laser to probe simulated atmospheric clouds by measuring the scattered light from the 1.8° and 178.2° directions. At 178.2°, the scattered light is analysed for the linear polarisation state to deduce the particle linear depolarisation ratio δp which is a common measurement parameter of atmospheric lidar applications. The optical setup and the mathematical formalism of the depolarisation detection concept are given. SIMONE depolarisation measurements in spheroidal hematite aerosol and supercooled liquid clouds are used to validate the instrument. SIMONE data from a series of AIDA ice nucleation experiments at temperatures between 195 and 225 K were analysed in terms of the impact of the ice particle microphysics on δp. We found strong depolarisation values of up to 0.4 in case of small growing and sublimating ice particles with volume equivalent diameters of only a few micrometers. Modelling runs with the T-matrix method showed that the measured depolarisation ratios can be accurately reproduced assuming spheroidal and cylindrical particles with a size distribution that has been constrained by IR extinction spectroscopy. Based on the T-matrix modelling runs, we demonstrate that in case of small ice crystals the SIMONE depolarisation results are representative for the lidar depolarisation ratio which is measured at exact backscattering direction of 180°. The relevance of our results for the interpretation of recent lidar observations in cirrus and contrails is discussed. In view of our results, the high depolarisation ratios observed by the spaceborne lidar CALIOP in the tropical upper troposphere might be a hint for the presence of small (sublimating) ice particles in the outflows of deep convective systems.

Noninvasive identification of subcellular organization and nuclear morphology features associated with leukemic cells using light-scattering spectroscopy

Leukemia is the most common and deadly cancer among children and one of the most prevalent cancers among adults. Improvements in its diagnosis and monitoring of leukemic patients could have a significant impact in their long-term treatment. We demonstrate that light-scattering spectroscopy (LSS)-based approaches could serve as a tool to achieve this goal. Specifically, we characterize the light scattering properties of leukemic (NALM-6) cells and compare them to those of normal lymphocytes and granulocytes in the 440-710 nm range, over ±4 deg about the exact backscattering direction. We find that the LSS spectra are well described by an inverse power-law wavelength dependence, with a power exponent insensitive to the scattering angle but significantly higher for leukemic cells than for normal leukocytes. This is consistent with differences in the subcellular morphology of these cells, detected in differential interference contrast images. Furthermore, the residual light-scattering signal, extracted after subtracting the inverse power-law fit from the data, can be analyzed assuming a Gaussian distribution of spherical scatterers using Mie theory. This analysis yields scatterer sizes that are consistent with the diameters of cell nuclei and allows the detection of the larger nuclei of NALM-6 cells compared to those of lymphocytes and granulocytes.

Polarimetry in turbid, birefringent, optically active media: A Monte Carlo study of Mueller matrix decomposition in the backscattering geometry

Determination of the intrinsic polarization properties of a complex turbid medium such as biological tissue in the backscattering geometry (a geometry that is convenient for in situ applications) is complicated due to the confounding influence of scattering, and due to simultaneous occurrence of several polarization effects. We have investigated the polar decomposition approach of Mueller matrices to delineate individual intrinsic polarization parameters (specifically linear retardance δ and optical rotation ψ) of a birefringent, chiral, turbid medium in the backscattering geometry, using Mueller matrices generated with polarization-sensitive Monte Carlo simulations. The results show that near the exact backscattering direction, the interplay of the scattering-induced linear retardance and diattenuation on the intrinsic values for δ and ψ is coupled in a complex interrelated way, due to contribution of the backscattered photons. In contrast, these effects were significantly reduced for detection positions at distances larger than a transport length away from the point of illumination. Simultaneous determination of the intrinsic values for δ and ψ of a birefringent, chiral, turbid medium in the backward detection geometry can thus be accomplished by decomposing the Mueller matrices recorded at distances larger than a transport length away from the point of illumination. Determination of the intrinsic values for these polarization parameters in backscattering geometry could be significant in, for example, for quantification of tissue structural anisotropy and for noninvasive blood glucose measurements of diabetic patients.

Low-coherence enhanced backscattering beyond diffusion

An analytical theory for coherent backscattering (CBS) of low-coherence light is presented. An expression linking the CBS profile to the radial distribution of the incoherent backscattered light is derived when the incident light is partially spatially coherent. The backscattered snake light, which has experienced exactly two large-angle scatterings, is taken into account together with the diffuse light in the analysis. Monte Carlo simulations demonstrate that the model describes well the CBS profile as long as the spatial coherence length, L(c), of the incident beam is larger than the scattering mean free path of light in the medium. The intensity of the enhanced backscattered light in the exact backscattering direction and the width of the CBS cone are found to be proportional to L(c) and L(c)(-1), respectively, in the limit of small L(c).

Incoherent and coherent backscattering of light by a layer of densely packed random medium

The problem of light scattering by a layer of densely packed discrete random medium is considered. The theory of light scattering by systems of nonspherical particles is applied to derive equations corresponding to incoherent (diffuse) and interference parts of radiation reflected from the medium. A solution of the system of linear equations describing light scattering by a system of particles is represented by iteration. It is shown that the symmetry properties of the T -matrices and of the translation coefficients for the vector Helmholtz harmonics lead to the reciprocity relation for an arbitrary iteration. This relation is applied to consider the backscattering enhancement phenomenon. Equations expressing the incoherent and interference parts of reflected light from statistically homogeneous and isotropic plane-parallel layer of medium are given. In the exact backscattering direction the relation between incoherent and interference parts is identical to that of sparse media.

Comparison of the `glory' with coherent backscattering of light in turbid media

The 'glory' results from light scattering of single, sub-millimetre to millimetre-sized spheres in the exact backscattering direction. The so-called coherent backscattering by disordered media is the intensity enhancement in the very same direction due to the interference between each light path and its reversed path. This pair of paths always exists in multiple-scattering media. The two phenomena have very similar properties, which will be studied and compared in more detail, experimentally as well as by numerical calculations.

Polarization preservation in diffusive scattering from in vivo turbid biological media: effects of tissue optical absorption in the exact backscattering direction

There is considerable recent interest in using polarized light to investigate turbid biological media. Although tissue multiple scattering randomizes incident polarization states, there are circumstances when appreciable degree of polarization can be observed in diffusive scattering. In this study, we use polarization modulation and synchronous detection to examine in the exact backscattering direction the polarization properties of diffusely reflected visible light from hands of human volunteers of varying pigmentation levels. The surviving polarization fraction increases with increasing pigmentation, likely due to preferential loss of highly scattered, long-pathlength photons; this mechanism lowers the average pathlength traversed by the detected light and hence increases the measured polarization preservation. This behavior is contrasted with the overall diffuse reflectance intensity, whose magnitude decreases with increasing absorption. These experiments demonstrate the important influences of medium optical properties on the polarization characteristics of multiply scattered light, which must be further investigated to enable quantitative polarization evaluation of turbid media such as biological tissues.

Magnetic field effects on coherent backscattering of light

We have studied the influence of magneto-optical Faraday rotation on coherent backscattering of light experimentally, theoretically and by computer simulations of Monte-Carlo type. The consistency of these three approaches reveals new aspects of the propagation of vector waves in turbid media with and without Faraday rotation. Experimentally, we have demonstrated that the Faraday rotation may almost completely destroy the reciprocity of light paths. However, as shown by the simulations, the cone of coherent backscattering may not only be destroyed but also shifted off the exact backscattering direction, parallel to the magnetic field, as long as the amount of circular polarization is not completely destroyed by the multiple scattering. The relationship between coherent backscattering, depolarization and Faraday rotation are explained by a simple path model of vector waves. This leads to a new characteristic correlation length required to properly describe the influence of Faraday rotation on multiple light scattering.

An accurate technique to record the angular distribution of backscattered light

We report on a new technique to record the angular distribution of light which is scattered from a (disordered) material around the backscattering direction. The technique is accurate over a large scanning range (500 mrad) which includes the exact backscattering direction, and the angular resolution is high (100 μrad). The technique is particularly suitable for the study of coherent backscattering from very strongly scattering random media, but it can be applied in any situation where the angular distribution of backscattered light is studied. It allowed us to measure for the first time the theoretical value of two of the enhancement factor in coherent backscattering.