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 po larization is described by polaritons. In anisotropic materials, they cause a strong dependence of absorption energy on the crystal direction. A possi ble shift of absorption wavelength from 635 to 336 nm was calculated from t he reflection spectra of single crystals of 1.7-bis(diInethylamino)heptamet hine hexafluorophosphate. The use of this effect in transmission requires h ighly ordered dye layers of 50-100 nm thickness. We have prepared such laye rs 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 materia l, the crystal structure, and the layer thickness did not change. The spect ra were described by the polariton theory. (C) 1999 Elsevier Science S.A. A ll rights reserved.