Terahertz Phonon Modes of Highly Efficient Electro-optic Phenyltriene OH1 Crystals

Understanding the origin of the phonon modes of highly efficient electro-optic crystals is very important for designing materials and for optimizing their photonic applications. Here we investigate the origin of phonon modes in the 0.1–15 THz range of the benchmark electro-optic OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)­malononitrile) crystal, which is interesting due to its large electro-optic coefficient and high THz-wave generation efficiency. The phonon modes (and vibrational absorption properties) of OH1 crystals are evaluated theoretically by periodic density functional theory and also experimentally by THz absorption spectroscopy. The theoretical calculations are well-matched with experimental results. The THz absorption properties are highly anisotropic; the amplitude of the vibrational absorption is the largest along the polar c-axis compared to the other two crystallographic axes. For comparison, the vibrational absorption modes of the OH1 molecule in the gas phase are also calculated. The calculated vibrational absorption spectrum of OH1 crystalline powder appears similar to that of the OH1 molecule in the gas phase. However, the molecular vibrational motions in the crystalline state are coupled motions of vibrational motions in the gas phase. Interestingly, the vibrational mode of the torsion of the O–H bond with the largest absorption strength in the gas phase is in the crystal inhibited due to the crystal field effect. The origin of the intense phonon modes of OH1 crystals is mainly related to relatively strong distortions of the push–pull π-conjugated system including electron donor and acceptor groups.