Directional Dispersion of an Exciton Polariton in a Pentamethinium Cyanine Dye

Reflectance spectra of γ‐cyclopropyl‐bis(1, 3, 3‐ trimethyl‐indolenine‐2‐yl)pentamethiniumtetrafluoroborate are measured on different crystal faces. The complex dielectric constant, derived from the reflectivity data, reveals that the broad and highly polarized reflectance bands (R ≈︁ 90%) in the visible spectral range are caused by a narrow but strong molecular excitation (ϵ2 > 300). The resonance energy is different for the various crystal faces and depends on the angle between the wave vector of light, k, and the transition moment of the molecular excitation. The large shift of the resonance energy from 1.73 eV on a {110} face to 2.61 eV on a {100} face is described quantitatively as directional dispersion of an extraordinary polariton.