Biomedical subjects

Brian Stout

Publications and source records attributed to Brian Stout.

6 recordsLinked to original sources

PubMed2006

Longitudinal optical binding of high optical contrast microdroplets in air.

Binding along the beam axis (which we shall call "longitudinal optical binding") has been observed between micron-sized oil droplets in a three dimensional optical trap in air. We argue that it is the high optical contrast which is responsible for the exceptionally stable doublet structures observed experimentally. It was also observed that optically bound doublets tend to cling to interference fringes created by the two counterpropagating beams. Our observations are qualitatively discussed in the context of both the ray model (optics) approximation, and in the Rayleigh (dipolar) range. Our observations were consistent with calculations of binding and trapping forces which we carried out by employing an exact multiple-scattering theory.

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PubMed2006

Mie scattering by an anisotropic object. Part I. Homogeneous sphere.

Establishing a vector spherical harmonic expansion of the electromagnetic field propagating inside an arbitrary anisotropic medium, we extend Mie theory to the diffraction by an anisotropic sphere, with or without losses. The particular case of a uniaxial material leads to a simpler analysis. This work opens the way to the construction of a differential theory of diffraction by a three-dimensional object with arbitrary shape, filled by an arbitrary anisotropic material.

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PubMed2006

Mie scattering by an anisotropic object. Part II. Arbitrary-shaped object: differential theory.

The differential theory of diffraction by an arbitrary-shaped body made of arbitrary anisotropic material is developed. The electromagnetic field is expanded on the basis of vector spherical harmonics, and the Maxwell equations in spherical coordinates are reduced to a first-order differential set. When discontinuities of permittivity exist, we apply the fast numerical factorization to find the link between the electric field vector and the vector of electric induction, developed in a truncated basis. The diffraction problem is reduced to a boundary-value problem by using a shooting method combined with the S-matrix propagation algorithm, formulated for the field components instead of the amplitudes.

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PubMed2005

Light diffraction by a three-dimensional object: differential theory.

The differential theory of diffraction of light by an arbitrary object described in spherical coordinates is developed. Expanding the fields on the basis of vector spherical harmonics, we reduce the Maxwell equations to an infinite first-order differential set. In view of the truncation required for numerical integration, correct factorization rules are derived to express the components of D in terms of the components of E, a process that extends the fast Fourier factorization to the basis of vector spherical harmonics. Numerical overflows and instabilities are avoided through the use of the S-matrix propagation algorithm for carrying out the numerical integration. The method can analyze any shape and/or material, dielectric or conducting. It is particularly simple when applied to rotationally symmetric objects.

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PubMed2005

Scattering efficiency of aggregated clusters of spheres: dependence on configuration and composition.

We study the orientation average scattering cross section of various isolated aggregates of identical spherical particles as functions of their size, optical properties, and spatial configurations. Two kinds of aggregates are studied: latex particles in water and rutile titanium dioxide pigments in a polymeric resin, with size parameters varying from 0.6 to 2.3. Calculations are performed by using a recursive centered T-matrix algorithm solution of the multiple scattering equation that we previously developed [J. Quant. Spectrosc. Radiat. Transfer 79-80, 533 (2003)]. We show that for a specific size of the constituent spheres, their respective couplings apparently vanish, regardless of the aggregate configuration, and that the scattering cross section of the entire cluster behaves as if its constituents were isolated. We found that the particular radius for which this phenomenon occurs is a function of the relative refractive index of the system. We also study the correlations between the strength of the coupling among the constituent spheres, and the pseudofractal dimension of the aggregate as it varies from 1 to 30.

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PubMed2003

Absorption in multiple-scattering systems of coated spheres.

We derive formulas for rigorous transfer matrix calculations of absorption in multiple-coherent-scattering systems in which the scatterers are multiply coated spheres (not necessarily concentric). Any of the spherical coatings, cores, or host media may be composed of absorbing materials. For a nonabsorbing host media, the total absorption may be deduced from overall energy conservation. A more detailed description of the absorption is obtained through formulas yielding the absorption within individual scatterers and/or coatings. We present some illustrative applications of these formulas to the design of heterogeneous coated-sphere media exhibiting enhanced absorption.

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