Showing papers on "Light scattering published in 2001"

Light Scattering by Fractal Aggregates: A Review

Abstract: This paper presents a review of scattering and absorption of light by fractal aggregates. The aggregates are typically diffusion limited cluster aggregates (DLCA) with fractal dimensions of D

Revisiting the method of cumulants for the analysis of dynamic light-scattering data

Abstract: The method of cumulants is a standard technique used to analyze dynamic light-scattering data measured for polydisperse samples. These data, from an intensity–intensity autocorrelation function of the scattered light, can be described in terms of a distribution of decay rates. The method of cumulants provides information about the cumulants and the moments of this distribution. However, the method does not permit independent determination of the long-time baseline of the intensity correlation function and can lead to inconsistent results when different numbers of data points are included in the fit. The method is reformulated in terms of the moments about the mean to permit more robust and satisfactory fits. The different versions of the method are compared by analysis of the data for polydisperse-vesicle samples.

Laser Rayleigh scattering

Abstract: Rayleigh scattering is a powerful diagnostic tool for the study of gases and is particularly useful for aiding in the understanding of complex flow fields and combustion phenomena. Although the mechanism associated with the scattering, induced electric dipole radiation, is conceptually straightforward, the features of the scattering are complex because of the anisotropy of molecules, collective scattering from many molecules and inelastic scattering associated with rotational and vibrational transitions. These effects cause the scattered signal to be depolarized and to have spectral features that reflect the pressure, temperature and internal energy states of the gas. The very small scattering cross section makes molecular Rayleigh scattering particularly susceptible to background interference. Scattering from very small particles also falls into the Rayleigh range and may dominate the scattering from molecules if the particle density is high. This particle scattering can be used to enhance flow visualization and velocity measurements, or it may be removed by spectral filtering. New approaches to spectral filtering are now being applied to both Rayleigh molecular scattering and Rayleigh particle scattering to extract quantitative information about complex gas flow fields. This paper outlines the classical properties of Rayleigh scattering and reviews some of the new advances in flow field imaging that have been achieved using the new filter approaches.

Scattering matrices of mineral aerosol particles at 441.6 nm and 632.8 nm

Abstract: We present measured scattering matrices as functions of the scattering angle in the range 5°–173° and at wavelengths of 441.6 nm and 632.8 nm for seven distinct irregularly shaped mineral aerosol samples with properties representative of mineral aerosols present in the Earth's atmosphere. The aerosol samples, i.e., feldspar, red clay, quartz, loess, Pinatubo and Lokon volcanic ash, and Sahara sand, represent a wide variety of particle size (typical diameters between 0.1 and 100 μm) and composition (mainly silicates). We investigate the effects of differences in size and complex refractive index on the light-scattering properties of these irregular particles. In particular, we find that the measured scattering matrix elements when plotted as functions of the scattering angle are confined to rather limited domains. This similarity in scattering behavior justifies the construction of an average aerosol scattering matrix as a function of scattering angle to facilitate, for example, the use of our results for the interpretation of remote sensing data. We show that results of ray-optics calculations, using Gaussian random shapes, are able to describe the experimental data well when taking into account the high irregularity in shape of the aerosols, even when these aerosols are rather small. Using the results of ray-optics calculations, we interpret the differences found between the measured aerosol scattering matrices in terms of differences in complex refractive index and particle size relative to the wavelength. The importance of our results for studies of astronomical objects, such as planets, comets, asteroids, and circumstellar dust shells is discussed.

Surface plasmon propagation in microscale metal stripes

Abstract: Addressing the fundamental question of miniaturization of light guiding and routing towards nanoscale optics, we study experimentally surface plasmon propagation in silver and gold thin films of finite widths in the micrometer range. Spatially confined excitation of surface plasmons is realized by a prism coupling arrangement involving an opaque aluminum screen for a distinct separation of excitation and propagation (measurement) region. The surface plasmon propagation length as a function of film widths is measured by detecting stray light due to surface plasmon scattering with a conventional optical microscope.

Spherical bilayer vesicles of fullerene-based surfactants in water: a laser light scattering study

Abstract: The low solubility of fullerenes in aqueous solution limits their applications in biology. By appropriate substitution, the fullerenes can be transformed into stabilized anions that are water soluble and can form large aggregated structures. A laser light scattering study of the association behavior of the potassium salt of pentaphenyl fullerene (Ph5C60K) in water revealed that the hydrocarbon anions Ph5C60- associate into bilayers, forming stable spherical vesicles with an average hydrodynamic radius and a radius of gyration of about 17 nanometers at a very low critical aggregation concentration of less than 10(-7) moles per liter. The average aggregation number of associated particles in these large spherical vesicles is about 1.2 x 10(4).

The effects of particle size on reversible shear thickening of concentrated colloidal dispersions

Abstract: The particle size dependence of the reversible shear thickening transition in dense colloidal suspensions is explored. Five suspensions of monodisperse silica are synthesized via the Stober synthesis. The physicochemical properties of the dispersions are quantified using transmission electron microscopy, dynamic light scattering, small angle light scattering, electrophoresis, and viscometry. Rheology measurements indicate a critical stress marking the onset of reversible shear thickening that depends on the dispersion’s particle size, concentration, polydispersity, and interparticle interactions. A simplified two particle force balance between the interparticle repulsive forces and the hydrodynamic compressive forces is used to derive a scaling relationship between this critical shear stress and the suspension properties. The scaling is tested against the fully characterized silica dispersions, which span nearly a decade in particle size. Furthermore, bimodal mixtures of the dispersions are employed to ev...

Small-angle X-ray scattering of polymers.

Abstract: A. Temperature Chambers 1735 B. Pressure Cells 1735 C. Stretching Equipment 1736 D. Fiber Spinning Equipment 1737 E. Fiber Drawing Apparatus 1738 F. Shear Apparatus 1739 IV. Combined Techniques 1739 A. Simultaneous SAXS/WAXD 1739 B. Raman Spectroscopy 1740 C. Laser Light Scattering 1741 D. Fourier Transform IR Spectroscopy 1743 V. Applications 1743 A. Materials-Based Studies 1743 1. Block Copolymers 1743 2. Ionomers 1745 3. Liquid Crystalline Polymers 1745 4. Biopolymers 1746 B. Phenomenon-Based Studies 1747 1. Polymer Solutions and Gels 1747 2. Colloidal Suspensions, Micellar Solutions, and Microemulsions 1749

Scattering of Electromagnetic Waves: Advanced Topics

Abstract: Preface. Two-Dimensional Random Rough Surface Scattering Based on Small Perturbation Method. Kirchhoff Approach and Related Methods for Rough Surface Scattering. Volume Scattering: Cascade of Layers. Analytic Wave Theory for a Medium with Permittivity Fluctuations. Multiple Scattering Theory for Discrete Scatterers. Quasi-Crystalline Approximation in Dense Media Scattering. Dense Media Scattering. Backscattering Enhancement. Index.

Synthesis and Characterization of Monodisperse Core−Shell Colloidal Spheres of Zinc Sulfide and Silica

Abstract: We report on a new type of composite particles consisting of a zinc sulfide (ZnS) core and a silica (SiO2) shell or vice versa. We developed and optimized these particles for photonic applications, because ZnS has a large refractive index and does not absorb light in the visible and both ZnS and SiO2 can be easily doped with fluorophores. Both kinds of morphologies were created using a seeded growth procedure using monodisperse seeds on which homogeneous layers with a well-defined thickness were grown. Moreover, the ZnS and SiO2 cores could be completely dissolved leaving SiO2 and ZnS shells, respectively, filled with solvent or air after drying. The particle morphology was investigated by electron microscopy. The optical properties were studied by extinction measurements and angle resolved light scattering and compared to scattering theory.

Mie theory for light scattering by a spherical particle in an absorbing medium.

Abstract: Analytic equations are developed for the single-scattering properties of a spherical particle embedded in an absorbing medium, which include absorption, scattering, extinction efficiencies, the scattering phase function, and the asymmetry factor. We derive absorption and scattering efficiencies by using the near field at the surface of the particle, which avoids difficulty in obtaining the extinction based on the optical theorem when the far field is used. Computational results demonstrate that an absorbing medium significantly affects the scattering of light by a sphere.

Relationship of light scattering at an angle in the backward direction to the backscattering coefficient

Abstract: We revisit the problem of computing the backscattering coefficient based on the measurement of scattering at one angle in the back direction. Our approach uses theory and new observations of the volume scattering function (VSF) to evaluate the choice of angle used to estimate bb. We add to previous studies by explicitly treating the molecular backscattering of water (bbw) and its contribution to the VSF shape and to bb. We find that there are two reasons for the tight correlation between observed scattering near 120° and the backscattering coefficient reported by Oishi [Appl. Opt.29, 4658, (1990)], namely, that (1) the shape of the VSF of particles (normalized to the backscattering) does not vary much near that angle for particle assemblages of differing optical properties and size, and (2) the ratio of the VSF to the backscattering is not sensitive to the contribution by water near this angle. We provide a method to correct for the water contribution to backscattering when single-angle measurements are used in the back direction (for angles spanning from near 90° to 160°) that should provide improved estimates of the backscattering coefficient.

Three-dimensional confocal microscopy of colloids.

Abstract: Confocal microscopy is used in the study of colloidal gels, glasses, and binary fluids. We measure the three-dimensional positions of colloidal particles with a precision of approximately 50 nm ~a small fraction of each particle’s radius! and with a time resolution sufficient for tracking the thermal motions of several thousand particles at once. This information allows us to characterize the structure and the dynamics of these materials in qualitatively new ways, for example, by quantifying the topology of chains and clusters of particles as well as by measuring the spatial correlations between particles with high mobilities. We describe our experimental technique and describe measurements that complement the results of light scattering. © 2001 Optical Society of America OCIS codes: 110.2960, 110.0180, 180.1790.

Shape and Structure

Abstract: The convenient availability and simplicity of the Lorenz-Mie theory has resulted in a widespread practice of treating nonspherical particles (especially those in random orientation) as if they were spheres to which Lorenz-Mie results are applicable. However, the assumption of sphericity is rarely made after first having studied the effects of nonsphericity and having concluded that they are negligible. On the contrary, overwhelming evidence suggests that scattering properties of nonspherical particles, including those in random orientation, can significantly differ from those of volume- or surface-equivalent spheres. Hence, the last few decades have demonstrated major research efforts aimed at a significantly better understanding of the effects of particle shape and morphology on electromagnetic scattering. The goal of this presentation is to provide a concise summary of these efforts. The recent availability of theoretical techniques for computing single and multiple scattering of light by realistic polydispersions of spherical and nonspherical particles and the strong dependence of the Stokes scattering matrix on particle size, shape, and refractive index make polarization and depolarization measurements a powerful particle characterization tool. This presentation will focus on recent applications of photopolarimetric and lidar depolarization measurements to remote sensing characterization of tropospheric aerosols, polar stratospheric clouds (PSCs) and contrails. The talk will include (1) a short theoretical overview of the effects of particle microphysics on particle single-scattering characteristics; (2) the use of multi-angle multi-spectral photopolarimetry to retrieve the optical thickness, size distribution, refractive index, and number concentration of tropospheric aerosols over the ocean surface; and (3) the application of the T-matrix method to constraining the PSC and contrail particle microphysics using multi-spectral measurements of lidar backscatter and depolarization

Retardation-induced plasmon resonances in coupled nanoparticles.

Abstract: We study the coupling induced by retardation effects when two plasmon-resonant nanoparticles are interacting. This coupling leads to an additional resonance, the strength of which depends on a subtle balance between particle separation and size. The scattering cross section and the near field associated with this coupled resonance are studied for cylindrical particles in air and in water. Implications for surface-enhanced Raman scattering and nano-optics are discussed.

Resonance light scattering particles as ultrasensitive labels for detection of analytes in a wide range of applications.

Abstract: We have developed a new detection technology that uses resonance light scattering (RLS) particles as labels for analyte detection in a wide range of formats including immuno and DNA probe type of assays in solution, solid phase, cells, and tissues. When a suspension of nano sized gold or silver particles is illuminated with a fine beam of white light, the scattered light has a clear (not cloudy) color that depends on composition and particle size. This scattered light can be used as the signal for ultrasensitive analyte detection. The advantages of gold particles as detection labels are that (a) their light producing power is equivalent to more than 500,000 fluorescein molecules, (b) they can be detected at concentrations as low as 10(-15) M in suspension by eye and a simple illuminator, (c) they do not photobleach, (d) individual particles can be seen in a simple student microscope with dark field illumination, (e) color of scattered light can be changed by changing particle size or composition for multicolor multiplexing, and (f) they can be conjugated with antibodies, DNA probes, ligands, and protein receptors for specific analyte detection. These advantages allow for ultra-sensitive analyte detection with easiness of use and simple and relatively inexpensive instrumentation. We have shown that our RLS technology can indeed be used for ultra-sensitive detection in a wide range of applications including immuno and DNA probe assays in solution and solid phases, detection of cell surface components and in situ hybridization in cells and tissues. Most of the assay formats described in this article can be adapted for drug fast throughput screening.

Influence of counterion valency on the scattering properties of highly charged polyelectrolyte solutions

Abstract: Light and neutron scattering measurements on highly charged polyelectrolyte solutions have recently provided firm evidence for the existence of “domain structures” containing many chains, even at rather low-polymer concentrations. In the present paper, we systematically investigate the influence of counterion charge valency Zc on the scattering properties of sulfonated polystyrene (PSS) solutions in water with monovalent and divalent counterions. This study is part of a larger effort to identify essential factors governing polyelectrolyte domain formation and the geometric properties of these transient structures. Neutron scattering measurements indicate that the interchain correlation length ξd within the domains becomes larger by a factor of 1.5–2 for divalent relative to monovalent counterions. This observation is consistent with the Manning model estimate of the change in effective polymer charge density Γ* with Zc and with previous observations linking ξd [from the peak position in the scattering int...

Plasmons in metal nanostructures

Abstract: Plasmon excitations, i.e. collective oscillations of the conduction electrons, strongly influence the optical properties of metal nanostructures and are of great interest for future photonic devices. Here, plasmons in metal nanostructures are investigated by nearand far-field optical microscopic techniques. Emphasis is placed on the study of the linear interaction of light with individual nanostructures. Specifically, the light transmission through individual nanometer-sized holes in opaque metal films is investigated using a scanning near-field optical microscope (SNOM). It is shown unambiguously that excitation and lateral propagation of surface plasmons support the light transmission through these nanoholes. This process has been under discussion recently. The propagation direction is given by the light polarization, thus allowing controlled addressing of individual holes — a process that may figuratively be described as “nanogolf”. This polarization controlled addressing of specific holes may be applicable for de-multiplexing purposes in future optical systems. Furthermore, the dephasing times and local field enhancement factors of plasmons in noble metal nanoparticles are determined by spectrally investigating the light-scattering from individual nanoparticles in a dark-field microscope setup. It is found that radiation damping limits the plasmon dephasing time, T2, in the gold nanospheres studied here to 25 fs, whereas suppression of interband damping in gold nanorods leads to surprisingly long particle plasmon dephasing times and large local field enhancement factors. The record value of T2=18 fs approaches the theoretical limit given by the free-electron relaxation time. The results of this first systematic and conclusive experimental study on individual particles answer the long debated question of the amount of plasmon damping in gold nanoparticles. Good agreement with calculations using the bulk dielectric function of gold shows that purely collective dephasing and surface effects contribute negligibly to the overall damping in the particles under investigation. The experimental results allow to deduce the relative contributions of radiation, interand intraband damping in gold nanoparticles. Also, the “true” particle plasmon dephasing time is determined in the sense of a pure plasmon oscillation without coupling to photons or interband excitations. The spectroscopic investigation of plasmons in single nanoparticles can also be used to determine the refractive index of the medium surrounding the particles. It is demonstrated for the first time how this can be used to build optical nanosensors based on particle plasmons. Applications for local concentrations, binding events and redox reactions are shown. The sensing volume of such a nanosensor is on the order of attoliters. First experiments towards increasing the sensitivity show the prospect of this technique. Actively changing the environment around metal particles by electrically aligning liquid crystal molecules allows the shift of the resonance frequency over a wide spectral range. The experiments presented here show that this novel effect can be used for electrically controlled light scattering with high contrast and spectral selectivity.

Measuring cellular structure at submicrometer scale with light scattering spectroscopy

Abstract: We present a novel instrument for imaging the angular distributions of light backscattered by biological cells and tissues. The intensities in different regions of the image are due to scatterers of different sizes. We exploit this to study scattering from particles smaller than the wavelength of light used, even when they are mixed with larger particles. We show that the scattering from subcellular structure in both normal and cancerous human cells is best fitted to inverse power-law distributions for the sizes of the scattering objects, and propose that the distribution of scattering objects may be different in normal versus cancerous cells.

Scattering optics of foam

Abstract: The multiple scattering of light by aqueous foams is systematically studied as a function of wavelength, bubble size, and liquid fraction. Results are analyzed in terms of the transport mean free path of the photons and an extrapolation length ratio for the diffuse photon concentration field. The wavelength dependence is minimal and may be attributed entirely to the wavelength dependence of the refractive index of water rather than thin-film interference effects. The transport mean free path is found to be proportional to the bubble diameter and the reciprocal of the square root of liquid fraction. The extrapolation length ratio varies almost linearly with liquid fraction between the values for water-glass-air and air-glass-air interfaces.

Extinction and Scattering Properties of Soot Emitted From Buoyant Turbulent Diffusion Flames

Abstract: Extinction and scattering properties at wavelengths of 250-5200 nm were studied for soot emitted from buoyant turbulent diffusion flames in the long residence time regime where soot properties are independent of position in the overfire region and characteristic flame residence times. Flames burning in still air and fueled with gas (acetylene, ethylene, propane, and propylene) and liquid (benzene, toluene, cyclohexane, and n-heptane) hydrocarbon fuels were considered. Measured scattering patterns and ratios of total scattering/absorption cross sections were in good agreement with predictions based on the Rayleigh-Debye-Gans (RDG) scattering approximation in the visible. Measured depolarization ratios were roughly correlated by primary particle size parameter, suggesting potential for completing RDG methodology needed to make soot scattering predictions as well as providing a nonintrusive way to measure primary soot particle diameters. Measurements of dimensionless extinction coefficients were in good agreement with earlier measurements for similar soot populations and were independent of fuel type and wavelength except for reduced values as the near ultraviolet was approached. The ratios of the scattering/absorption refractive index functions were independent of fuel type within experimental uncertainties and were in good agreement with earlier measurements. The refractive index junction for absorption was similarly independent of fuel type but was larger than earlier reflectometry measurements in the infrared. Ratios of total scattering/absorption cross sections were relatively large in the visible and near infrared, with maximum values as large as 0.9 and with values as large as 0.2 at 2000 nm, suggesting greater potential for scattering from soot particles to affect flame radiation properties than previously thought.

Propagation Characteristics of Ultrahigh-Δ Optical Waveguide on Silicon-on-Insulator Substrate

Abstract: A submicron rectangular waveguide with a relative refractive index difference Δ of 45% was fabricated on a silicon-on-insulator substrate, and its propagation characteristics were evaluated using the Fabry-Perot resonance method. The propagation loss was of the order of 10 cm-1, and was dominated by light scattering at rough interfaces. However, a large modal effective index of more than 4.5 and a low bend loss of less than 1 dB at a 0.5-µm-radius bend were also observed. These results suggest the potential of an ultrasmall and high-density lightwave circuit, which accepts the relatively large propagation loss.

Light emitting device, display apparatus with an array of light emitting devices, and display apparatus method of manufacture

Abstract: A light emitting device and display apparatus using a plurality of light emitting devices can drastically reduce contrast loss due to light from an external source. The light emitting device has (a) light emitting chip(s) and a first layer covering the light emitting chip(s). A second layer including a light scattering material is provided at least over the first layer, and the surface of second layer has a plurality of protrusions which follow the topology of the light scattering material. The display apparatus is formed by disposing these light emitting devices in an array on a substrate.

Structure of Alkanoic Acid Stabilized Magnetic Fluids. A Small-Angle Neutron and Light Scattering Analysis

Abstract: Small-angle neutron scattering and dynamic and static light scattering measurements were used to probe the structures of aqueous and organic-solvent-based magnetic fluids comprising dispersed magnetite nanoparticles (∼10 nm in diameter) stabilized against flocculation by adsorbed alkanoic acid layers. A core−shell model fitted to a set of neutron scattering spectra obtained from contrast variation experiments allowed the determination of the iron oxide core size and size distribution, the thicknesses of the surfactant shells, and the spatial arrangement of the individual particles. The magnetic colloidal particles appear to form compact fractal clusters with a fractal dimension of 2.52 and a correlation length of ∼350 A in aqueous magnetic fluids, consistent with the structures of clusters observed directly using cryo-TEM (transmission electron microscopy), whereas chainlike clusters with a fractal dimension of 1.22 and a correlation length of ∼400 A were found for organic-solvent-based magnetic fluids. T...

QCD and QED corrections to light-by-light scattering

Abstract: We present the QCD and QED corrections to the fermion-loop contributions to light-by-light scattering, γγ→γγ, in the ultrarelativistic limit where the kinematic invariants are much larger than the masses of the charged fermions.

Fluorescence correlation spectroscopy: inception, biophysical experimentations, and prospectus.

Abstract: Fluorescence correlation spectroscopy examines the chemical and the photophysical dynamics of dilute molecular solutions by measurement of the dynamic optical fluctuations of the fluorescence of a few molecules, even averaging less than one molecule at a time, in open focal volumes that are usually less than a femtoliter (<10-18 m3). It applies the same principles of statistical thermodynamics as does quasi-elastic light scattering. Molecular interactions, conformational changes, chemical reactions, and photophysical dynamics that are not ordinarily detectable by quasi-elastic light scattering can be analyzed by fluorescence correlation spectroscopy in cases in which molecular fluorescence changes in the dynamic range 10-7–102 s.