Study of optical parameters of two fluorinated isothiocyanato nematic liquid crystals exhibiting high birefringence

Abstract

Optical studies have been carried out on two fluorinated isothiocyanato nematic liquid crystal (LC) compounds 4′-butylcyclohexyl-3, 5-difluoro-4-isothiocyanatobiphenyl and 4′-pentylcyclohexyl-3, 5-difluoro-4-isothiocynatobiphenyl. Transition temperatures of the two samples were confirmed using a polarizing microscope. The two LC compounds were found to exhibit fairly high clearing temperatures. Measurements of refractive indices of the two compounds were done by using thin prism method with He-Ne laser beam of wavelength 630 nm. Birefringence of the two LC compounds was calculated from the measured refractive indices. Both the compounds are found to display fairly high values of birefringence. Validation of a modified four-parameter model, based on Vuks equation describing the temperature dependence of refractive indices of the two liquid crystals, is also presented in this paper. The model is validated by fitting the experimentally measured values of refractive indices, birefringence and average refractive indices of the two nematic LCs with the theoretical values. In this paper, the calculation of order parameters of the LCs is presented by using two methods: direct extrapolation method based solely on the birefringence data and by using modified Vuks method based on Haller’s extrapolation. As observed from the obtained results, this procedure of calculating order parameter gives very reasonable results.

Role of specific and nonspecific intermolecular interaction in electro-optical response of doped 6CHBT nematic liquid crystal with azo dyes

“…As it is seen in Fig. 4, at low concentration of dye, 0.1% and 0.5% wt A-dye doped samples, an increase in third order nonlinear susceptibility is seen, as compared to the pure sample (6CHBT), but χ (3) values for 1% and 3% wt A-dye doped samples have decreased with addition of dopants, same as Kerr constant. On the other hand, increment of alignment in LC matrix leads to high χ (3) values, at low concentrations of A dye, and a reduction in χ (3) values, in higher concentrations of dye in doped system, due to the loss of molecular ordering.…”

“…As a result, χ (3) values presented in Table 2 show that the doped system can be applied to optimize the performance of optoelectronic and photonic devices. Furthermore, with increasing the temperature the molecular ordering and χ (3) decreases in all samples. Fig.…”

“…Third order non-linear susceptibility, χ (3) , has been calculated by substituting the isotropic refractive index and the data obtained for Kerr constant, in Eq. (2).…”

“…Enhancement of the electro-optical properties of liquid crystal is dependent on the type, concentration and intrinsic characteristics of the dopants used [1][2][3]. Generally, various kinds of guest-host systems are used to enhance their optical and electro-optical properties and improve the performance of optoelectronic devices [4][5][6][7][8].…”

“…The Kerr constant also affects the third order non-linear susceptibility tensor, χ (3) , which is an important parameter for optical material, as can be seen by following Eq. [21]:…”