Showing papers on "Light scattering published in 1998"

Mechanisms of light scattering from biological cells relevant to noninvasive optical-tissue diagnostics

Abstract: We have studied the optical properties of mammalian cell suspensions to provide a mechanistic basis for interpreting the optical properties of tissues in vivo. Measurements of the wavelength dependence of the reduced scattering coefficient and measurements of the phase function demonstrated that there is a distribution of scatterer sizes. The volumes of the scatterers are equivalent to those of spheres with diameters in the range between ~0.4 and 2.0 mum. Measurements of isolated organelles indicate that mitochondria and other similarly sized organelles are responsible for scattering at large angles, whereas nuclei are responsible for small-angle scattering. Therefore optical diagnostics are expected to be sensitive to organelle morphology but not directly to the size and shape of the cells.

Non-volatile holographic storage in doubly doped lithium niobate crystals

Abstract: Photorefractive materials are being widely investigated for applications in holographic data storage1. Inhomogeneous illumination of these materials with an optical interference pattern redistributes charge, builds up internal electric fields and so changes the refractive index. Subsequent homogeneous illumination results in light diffraction and reconstructs the information encoded in the original interference pattern. A range of inorganic and organic photorefractive materials are known2, in which thousands of holograms of high fidelity can be efficiently stored, reconstructed and erased. But there remains a problem with volatility: the read-out process usually erases the stored information and amplifies the scattered light. Several techniques for ‘fixing’ holograms have been developed3,4,5,6, but they have practical disadvantages and only laboratory demonstrators have been built7,8,9,10. Here we describe a resolution to the problem of volatility that should lead to the realization of a more practical system. We use crystals of lithium niobate — available both in large size and with excellent homogeneity — that have been doped with two different deep electron traps (iron and manganese). Illumination of the crystals with incoherent ultraviolet light during the recording process permits the storage of data (a red-light interference pattern) that can be subsequently read, in the absence of ultraviolet light, without erasure. Our crystals show up to 32 per cent diffraction efficiency, rapid optical erasure of the stored data is possible using ultraviolet light, and light scattering is effectively prevented.

Extensive studies on pi-stacking of poly(3-alkylthiophene-2,5-diyl)s and poly(4-alkylthiazole-2,5-diyl)s by optical spectroscopy, NMR analysis, light scattering analysis, and X-ray crystallography

Abstract: Stacking of poly(3-alkylthiophene)s P3RThs and poly(4-alkylthiazole)s P4RTzs has been studied. Light scattering analysis indicates that head-to-tail (HT) type HT-P3HexTh (R = n-C6H13) gives a degree of depolarization (ρv) of 0.26 in CHCl3, which reveals that HT-P3HexTh takes a stiff structure even in the good solvent. Addition of CH3OH to CHCl3 solutions of HT-P3HexTh and head-to-head (HH) type HH-P4HepTz (R = n-C7H15) leads to π-stacking of the polymer molecules to form stable colloidal particles. The light scattering analysis of the colloidal solution of HT-P3HexTh in a 2:1 solution of CHCl3 and CH3OH reveals that HT-P3HexTh is aggregated in a parallel style. Results of filtration experiments using membranes with 0.20 and 0.02 μm pores agree with the degree of the aggregation. P3HexThs with irregular structures (P3HexTh (Fe) and P3HexTh (Ni) with HT/HH ratios of about 7/3 and 1/2, respectively) show a weaker trend to aggregate; however, P3HexTh (Fe) is considered to stack in a surface region of a stretc...

Ultraviolet lasing in resonators formed by scattering in semiconductor polycrystalline films

Abstract: A semiconductor laser whose cavities are “self-formed” due to strong optical scattering in highly disordered gain media is demonstrated. The lasers are made of zinc oxide polycrystalline films grown on amorphous fused silica substrates. Lasing occurs at an ultraviolet wavelength of ∼380 nm under optical pumping. Actual images of the microscopic laser cavities formed by multiple scattering have been captured. These results suggest the possibility of using disordered semiconductor microstructures as alternative sources of coherent light emission.

Optical Scattering Properties of Soft Tissue: A Discrete Particle Model

Abstract: We introduce a micro-optical model of soft biological tissue that permits numerical computation of the absolute magnitudes of its scattering coefficients. A key assumption of the model is that the refractive-index variations caused by microscopic tissue elements can be treated as particles with sizes distributed according to a skewed log-normal distribution function. In the limit of an infinitely large variance in the particle size, this function has the same power-law dependence as the volume fractions of the subunits of an ideal fractal object. To compute a complete set of optical coefficients of a prototypical soft tissue (single-scattering coefficient, transport scattering coefficient, backscattering coefficient, phase function, and asymmetry parameter), we apply Mie theory to a volume of spheres with sizes distributed according to the theoretical distribution. A packing factor is included in the calculation of the optical cross sections to account for correlated scattering among tightly packed particles. The results suggest that the skewed log-normal distribution function, with a shape specified by a limiting fractal dimension of 3.7, is a valid approximation of the size distribution of scatterers in tissue. In the wavelength range 600 ≤ λ ≤ 1400 nm, the diameters of the scatterers that contribute most to backscattering were found to be significantly smaller (λ/4–λ/2) than the diameters of the scatterers that cause the greatest extinction of forward-scattered light (3–4λ).

Light scattering and chlorophyll concentration in case 1 waters: A reexamination

Abstract: An analysis is presented based on a large dataset (N = 2,787) made up of recent measurements of the beam attenuation coefficient at 660 nm and of the chlorophyll concentration by using the SeaTech transmissometer and the high-pressure liquid chromatography technique, respectively. This analysis, restricted to case 1 waters, aims at reassessing a previous nonlinear relationship established between the particle scattering coefficient, b,, (very close to the particle attenuation coefficient, c,,), and the chlorophyll concentration, [Chl]. As a first result, nonlinearity is fully confirmed over the whole range of oceanic chlorophyll concentration (about 3 orders of magnitude). Despite more accurate measurements, the scatter in this relationship remains large and is actually comparable to that observed within the old dataset. Rather than establishing a single relationship between c,, (or b,,) and [Chl] for the entire upper water column, the deep layer and the near-surface layer (important for remote-sensing application) have been studied separately. This separation has led to two distinct expressions. A more appropriate parameterization is thus proposed when dealing specifically with, and modeling, the near-surface layer. As a consequence, a modified criterion is also suggested with a view to identifying turbid case 2 waters. Understanding or predicting the propagation of radiant energy within a water body requires that the boundary conditions (at the interface and bottom) and the inherent optical properties (IOP) within the medium are both known or prescribed. Strictly speaking, these properties (IOP) within the medium are both known or prescribed. Strictly speaking, these properties comprise the absorption coefficient, a, and the volume scattering function p(0); the scattering coefficient, b, derives from p(0) by integrating over the whole space, and the attenuation coefficient, c, represents the sum of a and b. These last three coefficients are expressed as m--l. The IOP result from the presence in a water body of colored dissolved organic substances and of scattering as well as absorbing particulate matter. In the open ocean, far from notable terrigenous influence, these optically active materials are locally and permanently Acknowledgments OMEX data were made available with the originators’ permission from the EU-MAST OMEX 1 Programme by the British Oceanographic Data center (BODC). The HPLC chlorophyll measurements for this project were undertaken by R. Barlow and R. E C. Mantoura at the Plymouth Laboratory. They are gratefully acknowledged for having made their data available to us. The transmissometer data for the same cruise were calibrated and quality controlled by the BODC. We thank R. Lowry for his help in the control of, and access to, these data. The BOFS data, published on a CD-ROM, were also produced under the BODC, and are duly appreciated here. The HPLC data for the EUMELI and OLIPAC (French JGOFS) cruises and those (unpubl.) for the MlNOS campaign were performed by H. Claustre with the collaboration of J. C. Marty, C. Cailliau, and E Vidussi. We thank them for these data. We also thank H. Claustre for helpful suggestions and discussions on a first draft of this paper: Substantial help with fieldwork and maintenance-calibration of the transmissometers used during the French cruises were provided by D. Tailliez, who is gratefully thanked. We acknowledge and thank our colleagues who worked at the two U.S. JGOFS-WOCE timeseries and during the EqPac cruises or were involved in the related databanks-these colleagues rendered this study feasible through their continuous effort. This work is a contribution to the French JGOFS Programme “Prosope.”

Single-scattering properties of complex ice crystals in terrestrial atmosphere

Abstract: Various ice crystal shapes including fernlike geometry, plates with dendritic extensions and with sector-like and broad branches, fractal geometry, and aggregates, have been numerically defined on the basis of available observations. The surface roughness of ice crystals is also accounted for by specifying the facet-tilt distribution in terms of Gram Charlier series for the small facets of which the rough surface consists. These ice crystal geometries along with those defined in our previous studies may approximately represent the ice crystal shapes frequently observed in cirrus clouds. An improved Monte Carlo/ray-tracing method has been developed to compute the single-scattering parameters for these complex ice crystals. The polarization configurations of the localized waves associated with Fresnelian rays are comprehensively accounted for by the improved method in ray-tracing procedure. Complex ice crystals, in particular, those with fernlike structures, scatter more energy in the angular region 2°-20° and in the lateral and backward directions than hexagonal ice crystals. The former ice crystal geometries normally produce smaller polarization values. In particular, a substantial reduction for the negative polarization associated with the backscattering is found as a result of the complex crystal geometries. The surface roughness of ice crystals incorporated into the single-scattering calculation tends to smooth out the scattering peaks corresponding to halos. The roughness also significantly reduces the backscattering. When a substantial roughness condition is imposed, the computed phase function and the polarization configuration of scattered light are essentially featureless. A database of the single-scattering parameters of ice crystals at solar wavelengths covering 0.2-5 μm has been established for various ice crystal shapes and sizes. This database can be useful in the parameterization of the bulk radiative properties of cirrus clouds to account for the effects of ice crystal size distribution and the percentage of various crystal habits. By applying this database to cold and warm cirrus clouds, it is demonstrated that the scattering and absorption characteristics of these clouds depend on both the size distribution and the shapes of ice crystals.

Electromagnetic scattering in polarizable backgrounds

Abstract: We develop a fully vectorial formalism for the investigation of electromagnetic scattering in polarizable backgrounds, i.e., where the scatterers are not in vacuum but situated in a medium with a dielectric permittivity different from unity. Our approach is based on the Green's tensor technique and the corresponding Green's tensors for two-dimensional (2D) and three-dimensional (3D) systems are developed. The analysis of 2D systems is not restricted to the case where transverse electric (TE) and transverse magnetic (TM) modes are decoupled, but treated in a general manner. Practical examples illustrate the application of the method: scattering by a microcavity for two dimensions and color formation in opal for three dimensions.

Multipolar molecules and multipolar fields: probing and controlling the tensorial nature of nonlinear molecular media

Abstract: A general class of multipolar molecules is introduced in the context of quadratic nonlinear optics by way of extension of the more specific cases of dipolar and octupolar molecules. An adequate irreducible tensor formalism permits us to define rotationally invariant molecular features that couple to corresponding field tensor components, thereby enabling us to account for a variety of coherent and noncoherent processes such as harmonic light (hyper-Rayleigh) scattering, coherent second-harmonic generation in electrically poled media, and the recently proposed optical poling scheme. Experiments in both harmonic light scattering in solution (for some multipolar molecules) and optical poling (in Disperse Red 1–methyl methacrylate thin films) are analyzed in light of this model. A general tensorial permutation lemma of broad validity allows nonlinear light–matter interactions to be condensed in a statistical medium in compact rotationally invariant expressions: The main tensorial symmetry features for both molecular susceptibility and read–write field polarization tensors that jointly drive these interactions are clearly revealed.

Evidence of intrinsic differences in the light scattering properties of tumorigenic and nontumorigenic cells

Abstract: BACKGROUND. The objective of this study was to determine whether there are intrinsic differences in the light scattering properties of tumorigenic and nontumorigenic cells from a multistep carcinogenesis model. METHODS. Wavelength-dependent and polarization-dependent light scattering properties of cell suspensions were measured. RESULTS. Statistically significant differences were found between the tumorigenic and nontumorigenic cells. CONCLUSIONS. Differences in the light scattering properties of tumorigenic and nontumorigenic cells are attributed to a change in the average size of the scattering centers on the order of a few ten of nanometers. This work is relevant to the development of noninvasive optical methods for cancer diagnosis.

Single scattering by red blood cells

Abstract: A highly diluted suspension of red blood cells (hematocrit 0.01) was illuminated with an Ar or a dye laser in the wavelength range of 458-660 nm. The extinction and the angle-resolved intensity of scattered light were measured and compared with the predictions of Mie theory, the Rayleigh-Gans approximation, and the anomalous diffraction approximation. Furthermore, empirical phase functions were fitted to the measurements. The measurements were in satisfactory agreement with the predictions of Mie theory. However, better agreement was found with the anomalous diffraction model. In the Rayleigh-Gans approximation, only small-angle scattering is described appropriately. The scattering phase function of erythrocytes may be represented by the Gegenbauer kernel phase function.

Increasing the sensitivity of an in-situ particle monitor

Abstract: A method and apparatus for increasing the sensitivity of an in situ particle monitor. A light scattering technique, preferably using laser light, is employed to monitor particle concentrations within the processing chamber of a plasma-based substrate processing system. Particle concentrations are increased in the light field of the sensor by creating an electric or magnetic field in the processing chamber to concentrate the particles suspended therein.

Changes in spectral shape of tissue optical properties in conjunction with laser-induced thermotherapy

Abstract: We measured the optical properties on samples of rat liver tissue before and after laser-induced thermotherapy performed in vivo with Nd:YAG laser irradiation This made it possible to monitor not only the influence of coagulation on the scattering properties but also the influence of damages to vessels and heat-induced damage to blood on the absorption properties An experimental integrating-sphere arrangement was modified to allow the determination of the g factor and the absorption and scattering coefficients versus the wavelength in the 600 ‐1050-nm spectral region, with the use of a spectrometer and a CCD camera The results show a relative decrease in the g factor of on average 21 6 7% over the entire spectral range following thermotherapy, and a corresponding relative increase in the scattering and absorption coefficients of 23 6 8% and 200 6 100%, respectively An increase of on average 200 6 80% was consequently found for the reduced scattering coefficient The cause of these changes in terms of the Mie-equivalent average radius of tissue scatterers as well as of the distribution and biochemistry of tissue absorbers was analyzed, utilizing the information yielded by the g factor and the spectral shapes of the reduced scattering and absorption coefficients These results were correlated with the alterations in the ultrastructure found in the histological evaluation The average radius of tissue scattering centers, determined by using either the g factors calculated on the basis of Mie theory or the spectral shape of reduced scattering coefficients calculated on the Mie theory, was estimated to be 21‐32% lower in treated than in untreated liver samples The Mie-equivalent average radii of scattering centers in untreated liver tissue deduced by the two methods corresponded well and were found to be 031 and 029 mm, respectively, yielding particle sizes in the same range as the size of a mitochondrion © 1998 Optical Society of America OCIS codes: 1403460, 1701420, 1703890, 1706930

Rotating-compensator multichannel ellipsometry: Applications for real time Stokes vector spectroscopy of thin film growth

Abstract: A multichannel spectroscopic ellipsometer based on the rotating-compensator principle was developed and applied to measure the time evolution of spectra (1.5–4.0 eV) in the normalized Stokes vector of the light beam reflected from the surface of a growing film. With this instrument, a time resolution of 32 ms for full spectra is possible. Several advantages of the rotating-compensator multichannel ellipsometer design over the simpler rotating-polarizer design are demonstrated here. These include the ability to: (i) determine the sign of the p-s wave phase-shift difference Δ, (ii) obtain accurate Δ values for low ellipticity polarization states, and (iii) deduce spectra in the degree of polarization of the light beam reflected from the sample. We have demonstrated the use of the latter spectra to characterize instrument errors such as stray light inside the spectrograph attached to the multichannel detector. The degree of polarization of the reflected beam has also been applied to characterize the time evo...

A Review of Elastic Light Scattering Theories

Abstract: Mie scattering is an important tool for diagnosing microparticles or aerosol particles in technical or natural environments. Mie theory is restricted to spherical, homogeneous, isotropic and non-magnetic particles in a non-absorbing medium. However, as microparticles are hardly ever spherical or homogeneous, there is much interest in more advanced scattering theories. During recent decades, scattering methods for non-spherical and non-homogeneous particles have been developed and even some computer codes are readily available. Extension of Mie theory covers coated spheres, stratified spheres and clustered spheres. For homogeneous non-spherical particles such as spheroids, ellipsoids and finite cylinders, surface discretization methods have been developed. Scattering by inhomogeneous particles may be computed by volume discretization methods.

Laboratory measurements of angular distributions of light scattered by phytoplankton and silt

Abstract: We present laboratory measurements of scattering properties of 15 different types of coastal and inland water phytoplankton species and two types of estuarine sediments. These properties are the scattering function as well as the angular distribution of a ratio of scattering matrix elements, which in practice equals the degree of linear polarization of the scattered light if the incident light is unpolarized. Laser light with a wavelength of 633 nm was used, and a scattering angle range from 20 degrees to 60 degrees was covered. The results can be used in the context of water-quality studies and to test results of theoretical models. The measured scattering functions are all strongly peaked in forward directions, but not equally so. For the covered angles, they vary significantly as a function of scattering angle. The measured angular distributions of the degree of linear polarization are mostly bell shaped, showing a maximum near 90 degrees, whose magnitude is clearly different for the phytoplankton compared to the silt particles. We find that the morphology and structural features of the particles studied play an important and complex role in their light-scattering behavior. In particular, internal cell structures such as gas vacuoles alter the scattering patterns of the phytoplankton species considerably. The external shape of the cells appears to have a much smaller influence. The experimental results are compared with results of Mie calculations and with the "standard scattering function" of San Diego Harbor water. In most cases, Mie calculations cannot provide an adequate approximation of the measured scattering behavior, which indicates that more sophisticated models are required. Only 3 of the 17 measured scattering functions resemble the San Diego Harbor standard scattering function. One of these pertains to small silt particles, showing that this function is representative for water dominated by these particles. [KEYWORDS: Refractive-index; optical-properties; mueller matrix; cells; particles; chlorella; growth; water; attenuation; backscatter]

Structure and formation of a gel of colloidal disks

Abstract: We have performed static scattering experiments on the gel state of a suspension of disk-shaped charged colloidal particles. The combination of static light scattering and small-angle x-ray scattering experiments probes more than three orders of magnitude in the scattering vector q. We observe that, after application of shear, the form factor develops nematic-Iike order. This order decays to random orientation in the gel state. This suggests that local reorientation of the disks leads to the gel state, as opposed to aggregation.

Fiber-optic bundle design for quantitative fluorescence measurement from tissue

Abstract: We demonstrate a new design for a fiber-optic bundle to measure fluorescence signals from tissue. In this design, the intensity of the signal is not significantly affected by the medium’s absorption and scattering coefficients and hence depends only on the fluorophore’s properties. Monte Carlo simulations of light scattering were used for designing and verifying the results obtained. The fiber-optic bundle was tested on tissue-simulating phantoms and compared with a standard nonimaging fiber-optic bundle. The new bundle was composed of 30 individual 100-μm fibers. Fibers on the end of the bundle that touch the tissue surface were separated from one another by approximately 1 mm. This design permits integration of the signal over several locations while maintaining localized sampling from regions smaller than the average mean free scattering path of the tissue. The bundle was tested by measurement of the fluorescence signals from tissue-simulating solutions containing fluorescent compounds. These studies demonstrate that the new bundle reduces the effect of the intrinsic absorption in the medium, permitting detection of fluorescence that is linearly proportional to the fluorophore concentration.

Practical Model for the Calculation of Multiply Scattered Lidar Returns

Abstract: An equation to predict the intensity of the multiply scattered lidar return is presented. Both the scattering cross section and the scattering phase function can be specified as a function of range. This equation applies when the cloud particles are larger than the lidar wavelength. This approximation considers photon trajectories with multiple small-angle forward-scattering events and one large-angle scattering that directs the photon back toward the receiver. Comparisons with Monte Carlo simulations, exact double-scatter calculations, and lidar data demonstrate that this model provides accurate results.

Thixotropic behavior of clay dispersions: Combinations of scattering and rheometric techniques

Abstract: The thixotropic behavior of a colloidal dispersion of clay consisting of disk-shaped particles was studied by means of a combination of rheometric measurements, static light scattering, and small-angle neutron scattering. At rest, the structure of the gel consists of dense micrometer-sized aggregates assembled into a fractal mass of dimension D. Under shear, in the case of volume fractions with (1⩽D⩽1.2) at rest, a butterfly-type light scattering pattern is observed. This is attributed to the formation of rollers within the dispersion, which align themselves on average perpendicular to the direction of shearing. This produces a fall in resistance to flow and in viscosity. The influence of shear rate on this disaggregation process was studied and linked to the rheometric measurements. Under shear flow conditions, the fall in viscosity is due to orientation and disaggregation processes occurring at length scales on the order of 1 μm. During recovery, the two time scales identified correspond, respectively, ...

Influence of bubbles on scattering of light in the ocean

Abstract: The scattering and backscattering properties of bubble populations in the upper ocean are estimated with Mie theory and a generalized bubble size spectrum based on in situ observations. Optical properties of both clean bubbles and bubbles coated with an organic film are analyzed; the results are compared with the corresponding optical properties of micro-organisms of similar size. Given a bubble number density (from ~10(5) to ~10(7) m(-3)) frequently found at sea, the bubble populations significantly influence the scattering process in the ocean, especially in oligotrophic waters. Bubbles appear to make a large contribution to the missing terms in constructing the observed total backscattering coefficient of the ocean. This contribution to backscattering is strongly enhanced if the bubbles are coated with organic film. The injection of bubbles will shift ocean color toward the green, resembling phytoplankton blooms, and hence introducing error in ocean color remote sensing if its effect is not corrected.

Dielectric and far-infrared spectroscopy of glycerol

Abstract: Broadband dielectric spectroscopy has been performed on supercooled glycerol for temperatures between 184 and 413 K. The measurements cover 18 decades of frequency and extend into the far infrared regime. The timescale of the α-process is observed over 14 decades of frequency as a function of temperature. In the THz range an additional loss peak was observed which is identified as the boson peak known from neutron and light scattering experiments. Between the α and boson peak the loss minimum has been observed as a function of temperature down to the glass temperature. It is very broad and cannot be ascribed to a simple superposition of α and boson peak. The data are compared to the susceptibilities obtained from neutron and light scattering.