太赫兹波段液晶分子极化率的理论研究

近年来,太赫兹(THz)波段电磁辐射的研究引起科学技术界广泛的关注.液晶(LC)材料具有宽带可调的特性且拥有成熟的工业技术基础,在基于液晶设计的太赫兹可调器件研究中显示了巨大的应用潜力.因此,为了快速发展实用的LC-THz调制器件,对液晶材料在太赫兹频率范围内的光电特性进行系统的了解是至关重要的.分子极化率是表征分子中电荷分布的重要物理量.采用密度泛函理论方法对液晶分子PCH5,5CB和5OCB在太赫兹波段的极化率性质进行计算研究,从电子结构的角度,利用极化率密度分析方法考察了分子不同区域对极化率数值的贡献,详细探讨了尾链、核心结构和极性取代基等不同基团对极化率及其各向异性的影响.

Theoretical study on the polarizability properties of liquid crystal in the THz range

Terahertz (THz) technology developed rapidly in recent years. Liquid crystals (LCs) are one of the most promising base materials to construct switchable devices in THz range because of their high optical anisotropies. However, the practical applications of the devices are hampered by the relationships between birefringence, thickness and LCs switching time. Due to the long wavelength, THz device requires a larger birefringence LC than the device operated at optical frequencies. Yet, in order to design an efficient switchable LC-THz device, it is crucial to find or synthetize LC material which will still display a useful birefringence at THz frequencies. The birefringence properties of LC are determined by the molecular polarizability of the relevant material. Knowledge of the LC molecular polarizability and its dependence on the molecular structure is important for designing LC molecules with desired THz properties. The prediction of the photoelectric characteristics could save a considerable quantity of the man-power and materials needed for the design or synthesis of new LC compounds. A priori screening of materials and the prediction of the optoelectronic properties would make a vast opportunity for expanding the LC material application scope. Hence, the main purpose of the present work is to provide a theoretical method of calculating and analyzing the THz polarizability properties of LC single compounds for LC-THz device applications. In this work, the frequency dependent molecule polarizability values of liquid crystal PCH5, 5CB and 5OCB in THz range are calculated by the density functional theory method. The geometries of the studied LCs are optimized at B3 LYP levels with the standard 6-311G(d) basis set. From the optimized geometries the molecule THz polarizabilities of LCs are calculated by the M06-2x functional with 6-311++G(2d, p) basis set, and they are found to be in good agreement with experimental data. By plotting the polarizability density analysis (PDA), the spatial contributions of electrons to the longitudinal polarizability are presented. The influences of alkyl chain and core structure on the microscopic polarizability of the LC molecule are investigated and explained by using the finite field approach and PDA. The results show that the unsaturated group, such as benzene ring or cyanobenzyl, makes great contribution to the polarizability of LC. In the design process, the new type of LC molecule must be extended the length of πup electron conjugated system, to reduce the energy gap between HOMO and LUMO, and hence improving LC molecule polarizabilty. We hope that the present work could give a useful guide in screening or designing LC molecules for THz applications, and offer an effective way to understand fundamental optoelectronic characteristic of LC materials in the THz frequency range.