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.