Effective Dielectric Tensor for Electromagnetic Wave Propagation in Random Media

TL;DR

This paper derives exact and approximate formulas for the effective dielectric tensor of electromagnetic waves in random media, emphasizing the role of microstructure correlations and disorder-induced wave attenuation.

Contribution

It introduces strong-contrast expansions for the dielectric tensor in random media and demonstrates the significance of higher-order correlation functions for high contrast ratios.

Findings

Higher-order correlation functions are crucial for accurate modeling.

Disorder induces an imaginary component in the dielectric tensor.

Wave attenuation is linked to microstructural fluctuations.

Abstract

We derive exact strong-contrast expansions for the effective dielectric tensor $\epeff$ of electromagnetic waves propagating in a two-phase composite random medium with isotropic components explicitly in terms of certain integrals over the $n$-point correlation functions of the medium. Our focus is the long-wavelength regime, i.e., when the wavelength is much larger than the scale of inhomogeneities in the medium. Lower-order truncations of these expansions lead to approximations for the effective dielectric constant that depend upon whether the medium is below or above the percolation threshold. In particular, we apply two- and three-point approximations for $\epeff$ to a variety of different three-dimensional model microstructures, including dispersions of hard spheres, hard oriented spheroids and fully penetrable spheres as well as Debye random media, the random checkerboard, and power-law-correlated materials. We demonstrate the importance of employing $n$-point correlation functions of order higher than two for high dielectric-phase-contrast ratio. We show that disorder in the microstructure results in an imaginary component of the effective dielectric tensor that is directly related to the {\it coarseness} of the composite, i.e., local volume-fraction fluctuations for infinitely large windows. The source of this imaginary component is the attenuation of the coherent homogenized wave due to scattering. We also remark on whether there is such attenuation in the case of a two-phase medium with a quasiperiodic structure.