Density Functional Theory Studies On The Optical Properties Of Liquid Crystals In Terahertz Band

Presently, terahertz(THz) technology is developing at a rapid pace. Liquid crystal(LC) is one of the most promising base materials to construct switchable devices in THz range because of its high optical anisotropy. However, the practical implementation of the devices is hampered by the relationships between birefringence, thickness and LC switching time. Due to the long wavelength, THz devices require a larger birefringence and thicker LC layer compared to the devices operated at optical frequencies. The thicker layer causes a higher absorption. Yet, in order to design an efficient switchable LC-THz devices, it is necessary to know precisely the absorption coefficient and refractive indices of LC and the dependency on the molecular structure. The prediction of the physical properties of the LC compounds, especially the optical refractive indices and its anisotropies, could save a considerable amount of the man-power and materials needed for the design or synthesis of new compounds. A priori screening of materials and the prediction of the optoelectronic properties would open vast opportunities for LC materials application.The main contents of the thesis are composed of three parts:(1)THz absorption spectra and potential energy distribution of LC. The THz absorption spectrum of 5CB、5OCB and PCH5 were simulated using DFT methods. An accurate assignment of the vibrational modes corresponding to absorption frequencies was performed using potential energy distribution(PED) in a frequency range of 0-3 THz. The influence of different core structures on the THz absorption spectra was discussed. The PED analysis showed that the different core structure leads to the different absorption features for LC. Compared with 5CB, the additional oxygen resulted in a blue shift of the absorption peak and increases the vibration intensity. Cyclohexane makes the molecule more flexible and consequently reduces the vibration intensity but adds a strong peak located below 1 THz. The results indicate that the scope of LC application must be considered in the LC-based THz device designing. The author expects to provide a useful suggestion on the design of novel LC material in THz wave.(2)Calculation on frequency and temperature properties of birefringence of LC in THz band. Using Vuks’ approximation and Haller’s approach in combination with DFT method, the frequency and temperature properties of refractive indices and birefringences of 5CB were calculated in THz range. A well agreement between the calculations and experimental values indicates that it is an applicable method to predict optoelectronic properties of LC materials in THz band. Taking into consideration of a reasonable compromise between accuracy and computational time, M06-2x /6-311+G(d,p) appears to be the most efficient and reliable method. The mentioned parameters of LC almost do not exhibit appreciable frequency dependence in THz band. The variation trends of refractive indices and polarizabilities with temperature of LC are identical.(3)A DFT study on the polarizability properties of LC in THz range. The frequency dependent molecule polarizabilities of PCH7、PCH5、5CB and 5OCB in THz range were calculated by DFT method. By means of plots of polarizability density analysis(PDA), the spatial contributions of electrons to the longitudinal polarizability were presented. The influence of alkyl chain and core structure on the microscopic polarizability of the LC molecule were investigated and interpreted using the finite field(FF) approach and PDA. The unsaturated group, such as benzene ring or cyanobenzyl, makes great contribution to the polarizability of LC. During the design process, the new type of LC molecules must be extended the length of π electron conjugated system, to reduce the energy gap between HOMO and LUMO, and hence improve LC molecules polarizabilty.In summery, the present work gives a useful guide to screen or design LC molecules for THz applications, and supplies an effective way to understand fundamental optoelectronic characteristic of LC materials in the THz frequency range.