Kirk A. Fuller

Optical resonances and two-sphere systems

After a historical review of previous research into cooperative scattering, a theoretical investigation of the effects of interparticle coupling on morphology-dependent resonances of spheres is conducted. Bispheres composed of identical, slightly dissimilar, and very different monomers are considered. Calculations are presented of resonance spectra for selected orientations of the bispheres relative to the incident wave vector along with spectra that should prove useful in describing the scattering properties of a monodisperse ensemble of randomly oriented bispheres. (The bispheres in this dispersion may be constituted from dissimilar monomers.) Normalized source functions for regions inside and near the scatterers are also provided. Finally a numerical simulation of an interesting experiment is carried out in which a resonating aerosol passes through the focal volume of a relatively large, spherical microlens.

Consummate solution to the problem of classical electromagnetic scattering by an ensemble of spheres. I: Linear chains

An outline is provided of the derivation of an order-of-scattering technique that we apply here for the first reported time to the study of light scattering by two or more interacting spheres. This new method provides what is to our knowledge the first complete physical description of the classical processes involved in cooperative EM scattering by an aggregate of spheres. In addition, the algorithms that it employs are often more efficient than those used in an already established method. Comparisons between selected calculations and experimental results are made for linear chains of three and five spheres, and effects of particle orientation are investigated theoretically.

Consummate solution to the problem of classical electromagnetic scattering by an ensemble of spheres. II: Clusters of arbitrary configuration

The order-of-scattering approach developed earlier [Opt. Lett. 13,90 (1988)] and applied there to the case of linear chains of spheres is extended to the more difficult problem of scattering by clusters of spheres, the centers of which no longer need lie on a common axis. To help establish the validity of this most general calculation, comparisons are made between theoretical and experimental results for triangular and tetrahedral arrays of spheres. We also perform calculations based on an older method that requires the inversion of matrices, and we find that for the cases considered here the order-of-scattering method is substantially faster.

Scattering and absorption cross sections of compounded spheres. III. Spheres containing arbitrarily located spherical inhomogeneities

The theory of light scattering by spheres possessing one or more spherical inhomogeneities is developed. The inhomogeneities, or subspheres, are of uniform but otherwise arbitrary composition and are restricted in size and number by only the volume of the host. Numerical results for single inclusions are considered in regard to questions that have arisen in the course of recent experimental research on morphology-dependent resonances in droplets. The modification of the light-absorbing properties of carbon by its entrainment in droplets is also studied. The predicted absorption by mass of soot particles in cloud droplets tends to be higher when the particles are centered in the droplets than when the more realistic eccentric inclusion model is used. Absorption by carbon that is internally mixed in sulfate hazes is more sensitive to the relative sizes of the sulfate and carbon particles, but absorption by eccentrically included grains tends not to differ greatly from that predicted by concentric models.

Scattering and absorption cross sections of compounded spheres. I.Theory for external aggregation

The cross section for total scattering by clusters of spheres is derived from an integration, over a closed spherical surface, of the scattered Poynting flux associated with the different pairs of spheres in the ensemble. With the use of the addition theorem for vector spherical harmonics the integral can be evaluated analytically. The pairwise cross sections can be rearranged into an expression for the scattering cross section of sphere aggregates that is analogous to that obtained from Lorenz–Mie theory for a single sphere. The latter formulation, however, is more difficult to treat numerically than is the summation over pairwise cross sections.

Scattering and absorption cross sections of compounded spheres. II. Calculations for external aggregation

The cross section for total scattering by a cluster of spheres derived previously [ J. Opt. Soc. Am. A11, 3251 ( 1994)] is applied to a study of the effects of scavenging and aggregation on the specific absorption of carbon. Results are presented for absorption cross sections of sulfate haze elements and cloud droplets with small carbon grains (spheres) attached to their surfaces. Soot typically occurs as aggregates of carbonaceous spherules. In order to address the validity of certain assumptions that are made in the analysis of such structures by fractal optics, comparisons are provided among the absorption cross sections of free carbon, linear chains, and tightly clumped carbon spheres. Polarization- and orientation-dependent cross sections of composite and aggregate particles are profiled, and orientation-averaged cross sections are tabulated.

Electromagnetic scattering from two dielectric spheres: further comparisons between theory and experiment

Results of experimental measurements and theoretical calculations concerning the scattering of plane wave radiation by a system of two interacting spheres are presented. In particular, the complex scattering amplitude of such a system and the phenomena of specular resonances are addressed. Preliminary calculations of the two-sphere Muller matrix are also considered.

Scattering by two spheres in contact: comparisons between discrete-dipole approximation and modal analysis

This paper applies two different techniques to the problem of scattering by two spheres in contact: modal analysis, which is an exact method, and the discrete-dipole approximation (DDA). Good agreement is obtained, which further demonstrates the utility of the DDA to scattering problems for irregular particles. The choice of the DDA polarizability scheme is discussed in detail. We show that the lattice dispersion relation provides excellent improvement over the Clausius-Mossoti polarizability parameterization.

Scattering of light by coated spheres

Treatment of the scattering of radiation by a spherical concentric core–shell system as a problem of multiple scattering by two spheres leads to a fuller understanding of the physical processes that produce the scattered fields of such a particle. The problem of scattering from a sphere containing multiple spherical inhomogeneities must, as a special case, be reducible to the problem considered here. Such a reduction leads to an expression of the scattering coefficients of coated spheres solely in terms of the scattering coefficients associated with a homogeneous host and with an isolated core, in agreement with the multiple scattering treatment.

Complex refractive index of ammonium nitrate in the 2–20-μm spectral range

Using high-resolution Fourier-transform infrared absorbance and transmittance spectral data for ammonium sulfate (AMS), calcium carbonate (CAC), and ammonium nitrate (AMN), we made comparisons with previously published complex refractive-index data for AMS and CAC to infer experimental parameters to determine the imaginary refractive index for AMN in the infrared wavelength range from 2 to 20 μm. Subtractive Kramers-Kronig mathematical relations were applied to calculate the real refractive index for the three compositions. Excellent agreement for AMS and CAC with the published values was found, validating the complex refractive index obtained for AMN. We performed backscatter calculations using a log-normal size distribution for AMS, AMN, and CAC aerosols to show differences in their backscattered spectra.