Infrared and Raman spectroscopy of the organic crystal DAST: Polarization dependence and contribution of molecular vibrations to the linear electro-optic effect

Abstract

We use infrared spectroscopy and Raman scattering to show that molecular vibrations considerably contribute to the linear electro-optic effect in the highly nonlinear optical organic crystal 4-N,N-dimethylamino-${4}^{\ensuremath{'}}\ensuremath{-}{\mathrm{N}}^{\ensuremath{'}}$-methyl stilbazolium tosylate (DAST). In addition to the almost negligible acoustic contribution (smaller than 2%), the vibrational contributions are estimated to reach 25% in DAST. Using infrared and Raman spectroscopy, as well as force-field calculations, the strongest vibrational modes contributing to the electro-optic effect are identified as in plane aromatic ring deformations and carbon-carbon stretching modes. In addition, we present experimental results on absorption coefficients and refractive indices in the infrared which have also an important impact on THz generation and detection using DAST crystals.