Phonon Modes of Organic Electro-Optic Molecular Crystals for Terahertz Photonics

We investigate theoretically and experimentally the phonon modes of organic electro-optic crystals for THz-wave photonics. THz phonon modes significantly influence the characteristics of THz-wave generation and detection through the THz absorption and refractive-index dispersion. For a model as THz generation material, electro-optic PNP (2-(N-prolinol)-5-nitropyridine) molecular crystal containing a relatively small numbers of atoms in the unit cell is chosen for phonon-mode calculation. THz phonon modes with their frequencies and intensities are calculated by periodic density functional theory, and the resulting spectrum exhibits a good agreement with the experimental absorption spectrum measured by THz time-domain spectroscopy. THz-wave generation experiment is performed with PNP as THz generator and the organic electro-optic OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile) crystal as THz detector. Considering the phase matching condition and THz absorption, which are both influenced by the THz phonon modes of THz generation and detection materials, the calculated THz generation spectrum also matches well with the experimental THz spectrum obtained by optical rectification at the pump wavelength of 1300 nm. Therefore, the phonon mode identification is very important for optimizing and designing new THz source and detection materials as well as for THz spectroscopy.