Extraordinary exciton polaritons in the optical spectra of molecular crystals

The highly anisotropic reflectivity spectra of molecular crystals (cyanine dye, TCNQ⁰) are discussed in terms of macroscopic dielectric theory. The resonance energies defined as the maxima in the ?₂-spectra depend on the relative orientation of the wavevector of the light and the transition moment. These directional effects are quantitatively explained by directional dispersion of the extraordinary polariton modes. Vibrational satellites are responsible for the detailed structure of the reflectance bands. Strong mixing of the various polariton branches shows up as additional broadening of the peaks in the ?₂-spectra.