Spherical particle absorption over a broad range of imaginary refractive index

Abstract

This paper presents studies of the optical absorption cross section for homogeneous spheres of radius R interacting with light of wavelength λ as calculated with the Mie equations. Four regimes of behavior are disclosed: the Rayleigh Regime, the Geometric Regime, the Reflection Regime, and a Crossover Regime. Two parameters govern these regimes, the imaginary part of the refractive index, κ, and the product of κ with the sphere size parameter kR where k = 2π/λ, i.e. κkR. Simple, approximate functionalities on κ, k, sphere volume and projected geometric cross section are derived for these regimes. Interesting aspects of our observations include: Rayleigh absorption can apply to all particle sizes, Fresnel reflection can occur for sub-wavelength spheres, and while κ is the agent of absorption, large κ can increase scattering to the detriment of absorption.