Scattering and extinction by spherical particles immersed in an absorbing host medium

TL;DR

This paper investigates electromagnetic scattering by spherical particles in absorbing media, revealing how absorption and size distribution affect extinction, phase functions, and polarization, with implications for scattering theory and applications.

Contribution

It introduces a comprehensive analysis of scattering in absorbing media using a first-principles FORTRAN code, including effects of absorption, size distribution, and ensemble averaging on scattering properties.

Findings

Negative extinction is interference-based and can be eliminated with narrow size distributions.

Absorption in the host medium suppresses interference effects and alters polarization.

Large particles exhibit deep minima in phase functions at side-scattering angles.

Abstract

Many applications of electromagnetic scattering involve particles immersed in an absorbing rather than lossless medium, thereby making the conventional scattering theory potentially inapplicable. To analyze this issue quantitatively, we employ the FORTRAN program developed recently on the basis of the first-principles electromagnetic theory to study far-field scattering by spherical particles embedded in an absorbing infinite host medium. We further examine the phenomenon of negative extinction identified recently for monodisperse spheres and uncover additional evidence in favor of its interference origin. We identify the main effects of increasing the width of the size distribution on the ensemble-averaged extinction efficiency factor and show that negative extinction can be eradicated by averaging over a very narrow size distribution. We also analyze, for the first time, the effects of absorption inside the host medium and ensemble averaging on the phase function and other elements of the Stokes scattering matrix. It is shown in particular that increasing absorption significantly suppresses the interference structure and can result in a dramatic expansion of the areas of positive polarization. Furthermore, the phase functions computed for larger effective size parameters can develop a very deep minimum at side-scattering angles bracketed by a strong diffraction peak in the forward direction and a pronounced backscattering maximum.