Experimental and theoretical study of exciton surface polaritons on organic crystals. II. Highly reflecting faces of CTIP and PTS

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Abstract
Exciton surface polaritons have been observed at room temperature on the highly reflecting crystal faces of CTIP [γ‐cyclopropyl‐bis (1,3,3‐trimethylindolenine‐2‐yl)pentamethinium fluoroborate] and PTS [poly‐2,4‐hexadiyne‐1,6‐diol bis (p‐toluene sulfonate)]. These surface modes, which cannot be excited by light incident from air, were detected by measuring the attenuated total reflection (ATR) of a focused laser beam inside a prism separated from the crystal face by a gap of the order of the wavelength of the exciting light. The experimental dispersion curves of the surface modes were obtained by plotting the photon energy against the angle of the reflectance minimum. The surface active component of the complex dielectric tensor was calculated for both cyrstals and found to agree quite well with values obtained from normal incidence reflectivity studies. Several supporting theoretical studies are also reported, including: (i) the calculation of the free exciton surface polariton dispersion curves for the natural faces of CTIP, (ii) model calculations representing the ATR for all energies and angles as a perspective figure, and (iii) an investigation of the stop‐point region where the virtual exciton surface polariton intersects the volume polariton modes of the CTIP crystal.