Polaritons in highly ordered cyanine dye films

The interaction of dye crystals with light lead to an excitation (excitons) and polarization of the material. The coupling of the excitons with the polarization is described by polaritons. In anisotropic materials, they cause a strong dependence of absorption energy on the crystal direction. A possible shift of absorption wavelength from 635 to 336 nm was calculated from the reflection spectra of single crystals of 1.7-bis(dimethylamino)heptamethine hexafluorophosphate. The use of this effect in transmission requires highly ordered dye layers of 50–100 nm thickness. We have prepared such layers by a spherulitic crystallization process (Thin Layer Aggregation, TLA). Three different crystal faces in millimeter size but only 80 nm thick were obtained, which showed quite different dichroic colors although the material, the crystal structure, and the layer thickness did not change. The spectra were described by the polariton theory.