Abstract-Terahertz Phonon Modes of Highly Efficient Electro-Optic Phenyltriene OH1 Crystals

Jongtaek Kim, Seung-Heon Lee, Seung-Chul Lee, Mojca Jazbinsek, Katsuhiko Miyamoto, Takashige Omatsu,Yoon Sup Lee, and O-Pil Kwon

J. Phys. Chem. C, Just Accepted Manuscript

DOI: 10.1021/acs.jpcc.6b07979

Publication Date (Web): October 4, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.6b07979

Understanding the origin of 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, 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 vibrational absorption is largest along the polar c-axis compared to the other two crystallographic axes. For comparison, vibrational absorption modes of the OH1 molecule in gas phase are also calculated. The calculated vibrational absorption spectrum of OH1 crystalline powder appears similar to that of the OH1 molecule in gas phase. However, the molecular vibrational motions in crystalline state are coupled motions of vibrational motions in gas phase. Interestingly, the vibrational mode of the torsion of the O-H bond with the largest absorption strength in gas phase is in crystal inhibited due to the crystal field effect. The origin of intense phonon modes of OH1 crystals is mainly related to relatively strong distortions of the push-pull -conjugated system including electron donor and acceptor groups.

Abstract-First-Principles Calculation of Terahertz Absorption with Dispersion Correction of 2,2′-Bithiophene as Model Compound

Jongtaek Kim †‡, O-Pil Kwon §, Mojca Jazbinsek ∥, Young Choon Park †, and Yoon Sup Lee *†

† Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea

‡ Department of Basic Science, Korea Air Force Academy (KAFA), Cheongju 363-849, Korea

§ Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Korea

∥ Institute of Computational Physics, Zurich University of Applied Sciences (ZHAW), 8401 Winterthur,Switzerland

J. Phys. Chem. C, Article ASAP

DOI: 10.1021/acs.jpcc.5b02661

Publication Date (Web): May 15, 2015

Copyright © 2015 American Chemical Society

*E-mail: yslee@kaist.edu. Tel.: +82-42-350-2821.

Terahertz absorption of organic materials is closely linked to molecular arrangements and their intermolecular interactions and is important for material identification as well as THz generation. Theoretical calculations of solid-state vibrations known as phonons help to understand intermolecular interactions responsible for THz absorption but frequently are of limited use without considering dispersion interaction. In this study, we have calculated the THz phonon modes of an organic model crystal 2,2′-bithiophene, considering dispersion intermolecular interactions assuming the fixed cell dimensions. Both energies and intensities of phonon modes at low frequencies were interpreted concentrating on the intermolecular level in conjunction with hydrogen bonds and showed an excellent agreement with the experimental results. This approach to identify the phonon modes responsible for strong THz absorptions and to interpret those modes in terms of intermolecular vibrations is also expected to be applicable to the field of THz generation using nonlinear optical organic crystals.