A microphysical model of scattering, absorption, and extinction in electromagnetic theory

Abstract

This work presents a microphysical model of the classical interaction of electromagnetic waves with arbitrary single and multiple particles. The model is based on the volume integral equation solution to the macroscopic time-harmonic Maxwell equations. The integral is discretized over a particle's volume. The near and far-field scattered wave is then described by the secondary radiation from the discretized elements. The physical origin of the angular structure of the scattered wave is characterized by the superposition of these secondary waves. A graphical technique is developed to visualize how this superposition relates to the physical features of a particle, e.g., its size, shape, and refractive index. Numerical and analytical implementations of the model are presented for spherical and spheroidal particles and fractal-like spherical-particle aggregates. The connection between the reflection symmetry of a particle and the polarization state of its far-field scattered wave is illustrated. The model is used to explain the cause of the angular power-law patterns in a particle's scattered intensity. An analysis of the internal field distribution in fractal-like aggregates is performed and the results are compared to the Rayleigh-Debye-Gans theory. Extinction and the optical theorem are examined within the context of the model, resulting in a new understanding for the physical mechanism causing extinction and implications regarding its measurement. The culmination of this work is the unification of multiple scattering-concepts, often regarded as distinct, and the resulting insight afforded by the unified microphysical picture. This unified view is shown to reveal a new and simple explanation for the famous extinction paradox.