Terahertz and optical properties of nematic mixtures composed of liquid crystal isothiocyanates, fluorides and cyanides

Abstract

Several simple nematic eutectic liquid crystal mixtures composed of different two and three ring isothiocyanato compounds as well as mixtures consisting of analogous cyano and fluoro compounds were formulated. The optical properties of the thereby obtained mixtures, i.e. refractive indices, birefringence and absorption coefficients a e and a o are measured in the frequency range between 0.5 and 2.5 THz and in the visible range at 589 nm. Our results show that two mixtures can be identified, which seems particularly promising for applications in the terahertz (THz) range: those consisting only of isothiocyanates show the highest birefringence at THz frequencies while those consisting of isothiocyanates and cyanides have the smallest dichroic ratio over a broad THz range. We also found that in most cases, the absorption coefficients of the isothiocyanato compounds fulfill the relationship a e > a o while for cyano and fluoro compounds, a o > a e is observed as a dominating trend. Furthermore, we show that by carefully selecting constituents of both isothiocyanato and cyano compounds, we can achieve a mixture for which the values of a o are in the same order as for a e for THz frequencies below 2 THz.

High Birefringence Liquid Crystals

“…The level of absorption losses of mixtures composed of isothiocyanates, cyanides and fluorides (absorption coefficient α e and α o ) decreases as follows in THz region [133].…”

“…They exhibit not only reduced absorption losses but also a small dichroism because α e and α o of the component are averaged and levels them out [133].…”

High-birefringence nematic liquid crystal for broadband THz applications

“…It has been reported that three benzene rings connected directly or with bridge group exhibit larger birefringence than structures with fewer rings. [28] As shown in Figure 3, BL037 has a flat refractive index in THz spectrum and the birefringence value increases with frequency from 0.17 at 0.5 THz to 0.21 at 2.5 THz. However, the absorption in the ordinary states increases dramatically with frequency (44.6 cm −1 at 2.5 THz), while the extraordinary absorption slope is much less steep.…”

“…According to Reuter et al [28], LCs with NCS groups enable higher birefringence over a broad temperature range than CN groups and have rather low viscosity. This material has been studied by Trushkevych et al [29], but they did not use very efficient alignment schemes such as high voltage and the THz band they measured is limited to 0.5-1.6 THz.…”

“…A recent systematic study of the impact of the terminal groups on birefringence can be found in ref. [28]. Absorption in LC materials is mainly caused by torsion and reorientation movements in parts of the molecules such as rings, chains and polar substitutes.…”