From angle-resolved ellipsometry of light scattering to imaging in random media

A procedure is described to allow selective cancellation of polarized scattering within optical substrates and multilayers. It is shown how bulk scattering (respectively surface) can be directly eliminated while the remaining roughness (respectively bulk) signal is still measurable. The same procedure can be applied to isolate a single interface or bulk within a stack or to detect slight departure from perfect correlation within multilayers. Experiments and a procedure for selective imaging in random media are described.     

Determination of ocean wave spectra from images of backscattered incoherent light

The application of imaging of sea surfaces has been investigated with respect to determination of sea wave spectra. Incoherent light is projected toward the sea surface, and the backscattered light is imaged with a camera. The primary scattering mechanism is assumed to be from particles suspended in the sea, so the backscattered intensity is determined primarily by the Fresnel coefficients. The ratio of the images detected at two orthogonal polarizations contains the desired information on the local slope of the sea surface, pixel by pixel, in one dimension. By integration, one can obtain the surface-height profile.     

Efficiency of integrated waveguide probes for the detection of light backscattered from weakly scattering media

A semianalytical model for light collection by integrated waveguide probes is developed by extending previous models used to describe fiber probes. The efficiency of waveguide probes is compared to that of different types of fiber probes for different thicknesses of a weakly scattering sample. The simulation results show that integrated probes have a collection efficiency that is higher than that of small-core fiber probes, and, in the particular case of thin samples, also exceeds the collection efficiency of large-core highly multimode fiber probes. An integrated waveguide probe with one excitation and eight collector waveguides is fabricated and applied to excite and collect luminescence from a ruby rod. The experimental results are in good agreement with the simulation and validate the semianalytical model.     

Anisotropic scattering of polarized light in a ferrofluid layer

Previously unknown effects wherein the scattering of plane-polarized light in a ferrofluid layer depends on the orientation of the electric field of the polarized radiation relative to the direction of the applied magnetic field are found experimentally, and their physical nature is explained. Pis’ma Zh. Tekh. Fiz. 23, 7–10 (September 12, 1997)     

Laser backscattered from partially convex targets of large sizes in random media for E-wave polarization

The characteristics of a radar cross section (RCS) of partially convex targets with large sizes up to five wavelengths in free space and random media are studied. The nature of the incident wave is an important factor in remote sensing and radar detection applications. I investigate the effects of beam wave incidence on the performance of RCS, drawing on the method I used in a previous study on plane-wave incidence. A beam wave can be considered a plane wave if the target size is smaller than the beam width. Therefore, to have a beam wave with a limited spot on the target, the target size should be larger than the beam width (assuming E-wave incidence wave polarization. The effects of the target configuration, random medium parameters, and the beam width on the laser RCS and the enhancement in the radar cross section are numerically analyzed, resulting in the possibility of having some sort of control over radar detection using beam wave incidence.     

Size determination by use of two-dimensional Mueller matrices backscattered by optically thick random media

Here we are concerned with the systematic study of polarized light transport in thick, isotropic, homogeneous random media and of the associated inverse problem. An original spatial and intensity rescaling of the polarization transport allows one to account implicitly for the volume fraction. This parameter elimination permits a complete exploration, by means of Monte Carlo simulations of the dependence of polarized light transport on microscopic parameters. Analysis of the Mueller matrices obtained from the simulations show that additional correlations (with respect to scalar transport) are obtained between the microscopic parameters and the spatial distribution of specific elements of the Mueller matrix. As a consequence, using carefully chosen polarization states, one can determine an average particle size independently of the volume fraction of particles, with only the knowledge of the refractive-index ratio being required. This analysis is validated with experimental Mueller matrices obtained for emulsions of various size, concentration, and polydispersity.     

Effect of multiple light scattering and self-absorption on the fluorescence and excitation spectra of dyes in random media

The absorption, fluorescence, and excitation spectra of a dye in a highly scattering random medium were studied experimentally. The intrinsic absorption spectrum of the dye does not change in the presence of scatterers, but the presence of scatterers in the media will change the observed fluorescence spectra. The observation is accounted for by the change in the photon trajectory path length for the fluorescence emission.     

Average path-length parameter of diffuse light in scattering media

We use Monte Carlo simulations to study in detail the propagation of light in a plane-parallel medium containing scattering particles. In particular, we compute the forward and backward average path-length parameters (FAPP and BAPP, respectively) of four-flux radiative transfer models as functions of the optical depth. Strong dependence on the single scattering albedo and phase function asymmetry is found for both quantities. In general the values of the FAPP decrease with increasing absorption, whereas the opposite occurs for the BAPP. A similar effect is produced when changing from isotropic phase functions to phase functions with a large asymmetry in the forward direction. We present analytical results for the asymptotic values of the FAPP and BAPP as functions of albedo for the particular case of isotropic scattering. Our results differ markedly from the predictions obtained recently with two multiple-scattering models by Vargas and Niklasson [J. Opt. Soc. Am. A 14, 2243 (1997); Appl. Opt. 36, 3735 (1997)]. The differences found point out the intrinsic limitations of these models.     

Anisotropic conical scattering and isotropic scattering oscillation in Cu:KNSBN

Abstract

A new type of anisotropic scattering ring and the isotropic scattering oscillation are observed on a screen behind a Cu:KNSBN crystal pumped by an extraordinary beam. The latter plays a key role in the forming of the former.     

Yet another look at light scattering from particles in absorbing media

We examine the scattering properties of particles contained in absorbing media. Rather than consider energy fluxes through arbitrary integrating spheres, we examine the extinction from its fundamental definition: the energy removed from the plane wave, or incident beam. The resulting energy received by a detector contains two terms: one the result of the incident beam traversing through the medium that would have occurred if the particle were not present, and a correction term due to the presence of the particle. Both terms have the same dependence on the pathlength that the beam travels between two arbitrarily located parallel planes and are independent of where the particle is located within the medium. The result is that the definition of the extinction cross section is not dependent on a reference plane or the particle location within the medium.     

Spatial distribution of single-photon and two-photon fluorescence light in scattering media: Monte Carlo simulation

Three-dimensional fluorescence spatial distributions under single-photon and two-photon excitation within a turbid medium are studied with Monte Carlo simulation. It is demonstrated that two-photon excitation has an advantage of producing much less fluorescence light outside the focal region compared with single-photon excitation. With the increase of the concentration of scattering particles in a turbid medium, the position of the maximum fluorescence intensity point shifts from the geometric focal region toward the medium surface. Further studies show that the optical sectioning property of two-photon fluorescence microscopy is degraded in thick turbid media or when the numerical aperture of an objective becomes low.     

Model of light scattering that includes polarization effects by multilayered media

We present a model for calculating the angular distribution of light, including polarization effects from multilayered inhomogeneous media, with an index of refraction mismatch between layers. The model is based on the resolution of the radiative transfer equation by the discrete ordinate method. Comparisons with previous simpler models and examples of simulations are presented.     

Fiber-optic microsensors to measure backscattered light intensity in biofilms

We have constructed tapered fiber-optic microsensors with a tip diameter of less than 10 mum to measure profiles of backscattered light in biofilms, which are thin layers of micro-organisms firmly attached to surfaces. The observed response agrees well with local effective diffusivity microelectrode measurements, with R(2) > 0.85. A strong relation between signal intensity and wavelength has been observed at 670 and 1320 nm. These sensors have the potential to replace local effective diffusivity microelectrodes for true in situ biofilm measurements.     

Coherence and polarization of light propagating through scattering media and biological tissues

The degree of polarization of light propagating through scattering media was measured as a function of the sample thickness in a Mach-Zehnder interferometer at a wavelength of lambda = 633 nm. For polystyrene microspheres of diameters 200, 430, and 940 nm, depolarization began to appear for thicknesses larger than 23, 19, and 15 scattering mean free paths (SMFP's), respectively, where the coherently detected scattered component dominates the ballistic component. For large particles (940 nm) the initial polarization survived partially in the scattering regime and progressively vanished up to the detection limit of our setup. This phenomenon was similarly observed in diluted blood from 12.5 to 280 SMFP's. Beyond this thickness the fluctuating parallel and crossed components of polarization became random. A dual-channel interferometer allowed us to detect simultaneously the low-frequency fluctuations of both polarized components through a few millimeters in liver tissue.     

Recrystallization of polysilicon films using incoherent light

A novel system has been designed and constructed for recrystallization of polysilicon films. This system utilizes tungsten light sources to heat, zone melt and subsequently crystallize polysilicon films on insulating substrates. The system has been successfully used to crystallize 0.5 μm polysilicon films deposited on 0.5 μm SiO₂₁ grown on three inch Si wafers. Initial results are presented.     

Role of higher-order scattering in solutions to the forward and inverse optical-imaging problems in random media

From analytical and numerical solutions that predict the scattering of diffuse photon density waves and from experimental measurements of changes in phase shift theta and ac amplitude demodulation M caused by the presence of single and double cylindrical heterogeneities, we show that second- and higher-order perturbations can affect the prediction of the propagation characteristics of diffuse photon density waves. Our experimental results for perfect absorbers in a lossless medium suggest that the performance of fast inverse-imaging algorithms that use first-order Born or Rytov approximations might have inherent limitations compared with inverse solutions that use iterative solutions of a linear perturbation equation or numerical solutions of the diffusion equation.     

Light scattering by a random distribution of particles embedded in absorbing media: diagrammatic expansion of the extinction coefficient

We address the problem of the modeling of the extinction coefficient into an absorbing medium, including a random distribution of identical scatterers of arbitrary size. We show that the extinction coefficient, including losses in the host medium, can be derived from a diagrammatic expansion arising from the rigorous multiple-scattering theory of electromagnetic waves in random media. While in previous approaches the contribution to the extinction coefficient due to the absorption in the host medium and due to the absorption and scattering by the particles were evaluated separately and heuristically, our approach is based on a derivation from first principles.     

Decrease in the incoherent near-edge X-ray scattering in narrow waveguides

The role of incoherent scattering in the attenuation of transmitted radiation has been studied using the statistical theory of X-ray beam scattering in a narrow rough collimator. Based on the obtained approximation expressions for the coefficients of waveguide mode damping, it is predicted that the level of scattering near the absorption edge must exhibit a significant decrease due to dispersion, which is also manifested by decreasing intensity of the backscattered radiation.