Several Steps toward Fundamentals of Physics of Light-Matter Coupling

Abstract

The microscopic theory of optical phenomena in crystals is closely related to the theory of excitons, which provides an appropriate tool for elucidation of specifics of interaction between electromagnetic fields and crystals and for construction of the physical picture of energy transfer in crystalline media. It is well established that in a crystal, where the translation symmetry allows for excitation of any of its elementary cells (or any of the comprising structural units), the energy transfer of electronic excitations occurs due to the motion of quasi-particles called excitons. The concept of “exciton” was originally introduced into physics by the pioneering works of Frenkel [1]. Among the first to consider excitonic states was also Peierls [2]. These works gave a theoretical explanation for the experimentally observed photoelectrically negative light absorption. Utilizing HeitlerLondon-Heisenberg method in his study of electronic states Frenkel [1] demonstrated that the absence of photoconductivity under the light absorption by an electronic subsystem in an ideal crystal stems from excitations of crystalline structural units moving in waves (due to translation invariance) along a crystal. Such excited states were given the name of “Frenkel excitons”.