Far-infrared properties of DAST

Using terahertz time-domain spectroscopy, we have measured the index of refraction and the absorption coefficient of the organic ionic salt $4‐N,N‐\mathrm{dimethylamino}‐4^{\prime }‐N^{\prime }‐\mathrm{methyl}-\mathrm{stilbazolium}$ tosylate (DAST). This promising organic electro-optic material exhibits strong absorption and dispersion for frequencies above 1 THz at both room temperature and 83 K. No reduction in the absorption strength is observed when DAST is cooled, suggesting a single-phonon origin of the resonances. A simple vibration of the anion and cation of the salt is suggested as the origin of the exceptionally strong far-infrared absorption and the high-frequency electro-optic properties of DAST.