Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals

TLDR

The crystal–radiation system’s eigenstates are mixed photon–exciton states. The authors develop a more complete theory in which absorption requires three‑body processes and incorporates local‑field effects yielding a Lorentz correction. The work shows that the standard semiclassical exciton absorption theory is inadequate, that excitons act as approximate bosons and classical polarization fields, that single‑quantum excitations have infinite lifetime, and that a Smakula equation links oscillator strength to the integrated absorption constant.

Abstract

It is shown that the ordinary semiclassical theory of the absorption of light by exciton states is not completely satisfactory (in contrast to the case of absorption due to interband transitions). A more complete theory is developed. It is shown that excitons are approximate bosons, and, in interaction with the electromagnetic field, the exciton field plays the role of the classical polarization field. The eigenstates of the system of crystal and radiation field are mixtures of photons and excitons. The ordinary one-quantum optical lifetime of an excitation is infinite. Absorption occurs only when "three-body" processes are introduced. The theory includes "local field" effects, leading to the Lorentz local field correction when it is applicable. A Smakula equation for the oscillator strength in terms of the integrated absorption constant is derived.