The Electromagnetic Properties of Inhomogeneous Composite Materials.

A self-consistent method was modified to treat the propagation of electromagnetic waves in composite media made of coated particles. The theory can be shown to produce the Maxwell-Garnett approximation in the static limit, but unlike the latter, is not restricted to composites of particles suspended in a single host. The self-consistency condition on the effective complex propagation constant reduces to the requirement that the effective dielectric constant be chosen so that the forward scattering amplitude of particles embedded in the effective medium vanish on the average. As in an older static effective medium approximation, this dynamic approach was originally applied to the randomly inhomogeneous composites, whose microstructure is composed of adjacent grains. The underlying role of microstructure on electromagnetic properties is examined in the modified formalism, by embedding various types of coated particles, depending on how the actual microgeometry of the composite is modelled. Good agreement with experimental data from cermets of gold in silicon dioxide for volume fractions of gold from ten percent to eighty percent is obtained, if two structural units are accounted for: gold completely surrounded by insulator and insulator completely surrounded by gold. The theory reproduces both the optical resonance and metal -insulator transition, thereby illustrating the basis for a unified understanding of the optical behavior of granular metals. A detailed model of the microstructure of silver -cesium oxide photocathodes that explains the variation of the quantum yield from 300 nanometers to 1100 nanometers is developed. In this model the broad maximum near 800 nanometers is due to resonance absorption created by clusters of silver particles on the surface layer. These clusters were incorporated self-consistently using the structural unit of insulator completely surrounded by silver. Similarly, the short wave length maximum near 360 nanometers is due to an optical resonance, which is created by isolated silver islands on the rough surface structure.