Optical Reflection of Crystals of Dye Molecules and Strong Coupling between Light and Matter

Materials consisting of organic dye molecules play an important role as pigments, as active layer in organic light emitting diodes or solar cells, and as bistable medium in optical transistors and switches. Yet there is currently no working protocol to accurately predict one of the most basic optical properties of these solids: their reflection spectrum. Here we develop a method to calculate the reflection spectrum of crystals of dye molecules based on the crystal structure and optical absorption of the dye in solution. We treat the interaction between light and matter in the crystals as strong. Electromagnetic four-potentials are gauged consistently, and their boundary conditions at the reflecting interface are derived. Finally, we include both excitonic and charge transfer interactions between molecules in the crystals. We test our approach on a large data set of reflection spectra and crystal structures including several industrial pigments.