DISPERSION OF EXCITON POLARITONS IN A MONOCLINIC CYANINE DYE CRYSTAL - EVALUATION OF THE REFLECTIVITY ON ARBITRARY FACES

Normal incidence reflectance spectra of the optically biaxial dye bis(N-ethyl-thiazoline-2yl)-trimethin cyanine iodide are studied on various crystal faces. The spectra vary considerably because the mixing of different electronic transitions by the electromagnetic wave depends on the orientation of the wavevector k. The polarization of the excited polariton modes rotates in the absorbing region if k is not parallel or perpendicular to a symmetry axis. Strength and position of the optical resonances may deviate strongly from the properties of the underlying excitonic transitions. The spectra agree well with the solutions of the Fresnel equation using a model dielectric tensor which approximates the excitonic transitions as Lorentz oscillators. Copyright © 1984 WILEY-VCH Verlag GmbH & Co. KGaA