Indices Of Nematic Liquid Crystals Research Articles

40.2: Programmable topological waveguide via nematic liquid crystals

With the rapid development of display technology, liquid crystals have gained significant attention. The unique optical properties of liquid crystals, such as birefringent optical performance and adaptive stimulus-responsiveness, provide a soft material platform for optical communication, biosensing, spatial light modulation, and topological photons. In this paper, based on the tunable refractive index of nematic liquid crystal and the robust transmission characteristics of topological photonic crystals, a valley photonic crystal based on liquid crystal (LCs-VPC) is proposed. The LCs-VPC has graphene-like lattice structure, and the liquid crystal molecules with different refractive indexes are confined in cylinders and hopingly arranged at six vertices of the lattice to break the space inversion symmetry. The topological non-trivial state is generated by adjusting the LCs’ refractive index via bias voltages. Topological waveguides with valley-protected edge modes are designed by stacking different states of lattice cells in the plane. The programmability of the waveguide is achieved by applying an external voltage to encode the state of the LCs in each cylinder. Based on the programmable topological waveguide, a programmable beam splitter has been designed. Our proposed LCs-VPC devices are beneficial for the development of integrated topological photonic circuits.

MgO nanoparticle effect on nonlinear refractive index of nematic liquid crystal doped with Sudan black B dye using Z-scan method

In this paper, by capitalizing on z-scan technique and using cw He–Ne laser at 632.8 nm, we investigated the impact of added MgO nanoparticles on nonlinear optical properties of 6CHBT liquid crystal which was doped with Sudan black B dye. The applied cell had homogeneous alignment. MgO nanoparticles were added to 6CHBT liquid crystal which was doped with 0.3 wt.% Sudan black B dye at three concentrations of 0.1, 0.3 and 0.6 wt.%. It was found that the added MgO nanoparticles did not significantly change the nonlinear refractive index of the compound at 0.1 and 0.6 wt.% concentrations; however, they enhanced the nonlinear refractive index of the compound at 0.3 wt.% concentration. The accumulation of nanoparticles in high concentrations causes high scattering of light which can reduce the intensity of transmitted light and, as a result, nonlinear effects.

A direct assessment of refractive indices of nematic liquid crystals at broad VIS - MWIR range

ABSTRACTWe developed an interference method for determination refractive indices of nematic liquid crystals in a broad electromagnetic spectrum. Values of ordinary and extraordinary refractive indices were measured in VIS (from 0.4 to 0.8 μm), NIR (from 0.8 to 1.4 μm), SWIR (from 1.4 to 3.0 μm) and at an edge of MWIR (from 3.0 to 4.2 μm) regions. The main task of this work was to develop the efficient and accurate interference method. The absolute error of these measurements is not higher than 0.02. The method was tested on two newly designed mixtures. The first one was a high birefringence liquid crystal mixture for the laser rangefinder, while the second one is intentionally prepared for breathalyser application. The mixture does not possess high absorption bands at 3.4 μm. To measure a dispersion of the refractive indices cells with various thicknesses and dielectric mirrors were prepared. The paper presents a theoretical discussion and experimental results.

INVESTIGATION OF THE INFLUENCE OF ELECTRIC FIELD ON THE ANISOTROPY OF NEMATIC LIQUID CRYSTALS BY MEANS OF X-RAY INTERFEROMETRIC METHOD

The refractive index of nematic liquid crystal (NLC) E7 in the X-ray spectral range using the X-ray interferometric method, was measured experimentally. It was shown that NLC E7 is anisotropic medium for X-rays. This method is sensitive to reorientation of the molecules of liquid crystals and allows to measure the refractive index of homogeneously aligned NLC layer, when the molecules are reoriented under the influence of external electric field.

Properties analysis of tellurite photonic crystal fiber filled with nematic liquid crystal

We theoretically investigate the modes, birefringence, dispersion, confinement loss, nonlinear coefficient and the temperature properties of tellurite photonic crystal fiber filled with nematic liquid crystal. The birefringence caused by the high index difference between ordinary and extraordinary refractive indices of nematic liquid crystal makes it possible for single-mode single-polarization guiding. The influence of rotation angle of the director of liquid crystal on the modal properties is also demonstrated. The results demonstrate that the dispersion curve of tellurite photonic crystal fiber filled with nematic liquid crystal along fast axis is more flatten than that along slow axis the fast axis. The tellurite photonic crystal fiber filled with nematic liquid crystal has low confinement loss and high nonlinear coefficient. When temperature is increased from 15°C to 45°C, the birefringence of tellurite photonic crystal fiber filled with nematic liquid crystal gradually decreases and the dispersion along slow axis gradually becomes flat, which is very useful for making compact and diverse polarization-manipulating devices.

PA51 干渉法を用いた常光屈折率と異常光屈折率の波長分散の同時測定(液晶フォトニクス・光デバイス,ポスター発表,2015年日本液晶学会討論会)

PA51 干渉法を用いた常光屈折率と異常光屈折率の波長分散の同時測定(液晶フォトニクス・光デバイス,ポスター発表,2015年日本液晶学会討論会)

The Study of Refractive Indices of Liquid Crystal Mixtures

In the present study the ordinary refractive indices of Nematic liquid crystal (LC), Cholesteric liquid crystal and their mixtures were measured by multiple wavelength refractometer in the visible range with temperature range from 20°C to 80°C. As the wavelength increases, the refractive indices may decrease. As the temperature increases, the refractive index may decrease. The ordinary refractive index decreases or increases depending on the crossover temperature of the Liquid Crystal employed. The wavelength- and the temperature-dependent refractive indices are fundamentally interesting and practically important for optimizing the display performances and other photonic devices employing LCs.We also tried to investigate factors affecting the temperature gradient of ordinary refractive index (dno/dT).Liquid crystal should exhibit a large temperature gradient refractive index to reduce the required laser intensity for triggering the nonlinear optical effects. The two crucial parameters for achieving large dno/dT are high birefringence and low clearing point. However, these two requirements are often conflicting each other. The temperature at which dno/dT = 0 is called the cross-over temperature To. If Tc, the clearing temperature of liquid crystal material is much higher than the room temperature then the cross-over temperature would be relatively high and dno / dT < 0 at room temperature. From all the investigations it is observed that the values of refractive index change with temperature, wavelengths as well as with the mixture. Some of the liquid crystal mixtures show excellent index matching property.

Determination of ordinary and extraordinary refractive indices of nematic liquid crystals by using wedge cells

AbstractA new accurate and fast interference method for determining ordinary and extraordinary refractive indices of nematic liquid crystals is presented and discussed. The method relies on microscopic measurements of distances between interference fringes appearing in polarised parallel coherent monochromatic light beam transmitted normally to the surfaces through a wedge cell filled with a nematic. Both glass plates confining the cell are coated with a partly transparent thin film of metal which is deposited by evaporation in vacuum. Owing to the multiple reflections between the surfaces and a small edge angle, the interference fringes observed near the wedge apex edge are sharp and equidistant. To apply this method one needs only small amount of an investigated liquid crystal. Basic mathematical formulae and results of an experiment are briefly discussed.

Polarization dependent guiding in liquid crystal filled photonic crystal fibers

A theoretical study of nematic liquid crystal filled photonic crystal fibers (LCPCFs) is presented. Detailed investigations including the polarization dependent bandgap formation and the modal properties are given for LCPCFs, in which alignment of the molecules could be controlled by external static electric field. The polarization dependent bandgap splitting caused by the high index difference between the ordinary and the extraordinary dielectric index of nematic liquid crystals provides the possibility of single-mode single-polarization guiding. A polarization operation diagram is proposed to describe the guiding behavior of LCPCFs. The influence of rotation angle ϕ of the director of liquid crystals on the modal properties is investigated. It is shown that the polarization axis of the guided mode is determined by the rotation angle ϕ, which could be controlled by external electric field.

Design of tunable bandgap guidance in high-index filled microstructure fibers

Tunable photonic bandgap (PBG) microstructure fibers, which were filled nematic liquid crystals (NLC), were theoretically investigated based on bandgap theory. By means of the modified plane-wave method, it is found that PBGs shift to the longer wavelength with increasing refractive index of NLC [ny(θ)] for y-polarized light. Fundamental modes are found in these PBG reigns, whose effective mode area, leakage loss and group velocity dispersion (GVD) have been calculated by using the full-vector finite-element method with anisotropic perfectly matched layers. The mode fields become larger with the increase of ny(θ), whereas the leakage loss varies slightly. Moreover, GVD is strongly dependent on ny(θ) and wavelength, which is much larger than the material dispersion of silica.

Self‐Assembled Cylindrical Lipid Tubules with a Birefringent Core

Optical anisotropic fibers with straight, zigzag, and loop shapes were fabricated by filling hollow cylindrical lipid tubules with nematic liquid crystals (see Figure). The optical anisotropy of the filled lipid tubules was examined by a polarizing optical microscope. Due to the tunable refractive index of nematic liquid crystals, tubules with liquid-crystal cores have potential application in sensors, photonic devices, and optical communications.

Evaluation of the Refractive Indices and their Wavelength Dispersion of Liquid Crystal by using Renormalized Transmission Ellipsometry

Renormalized Transmission Ellipsometry (RTE) was applied to a precise measurement of anisotropic refractive indices of nematic liquid crystals. It is quite important for the evaluation to use a monodomain cell with high quality. To realize the good alignment, the alignment film was coated. In this study, the influence of the alignment film thickness and its refractive indices on the accuracy of the measurement is reported. From the numerical analysis and several experiments, it is shown that if ne(λ) > no(λ)≈ nglass(λ) is satisfied and nglass(λ) is nearly equal to nal(λ), the evaluated results of no(λ) and ne(λ) are not influenced by the film thickness dal. From the results, RTE can be the powerfull tool to measure no(λ) and of ne(λ) for the LC substances with easy operation.

Renormalized Ellipsometry for Determining the Anisotropic Refractive Indices of Nematic Liquid Crystals

Renormalized transmission ellipsometry was applied to the precise measurement of anistropic refractive indices of nematic liquid crystals. It is quite beneficial for electrical industry that the conventional sandwich-type liquid crystal cell becomes an applicable sample and experimental measurement can be carried out using a conventional instrument for transmission ellipsometry. A simplified measurement procedure can provide continuous values of refractive indices within the visible wavelength range. At the same time, the cell gap and twist angle of the liquid crystal cell can be estimated by this measurement precisely.

On the Consistency and Applicability of Internal Field Models in Nematics

Abstract Information on anisotropy of the local field of light wave in nematics is obtained mainly from refractivity measurements. Such investigations, supplemented by those concerning thermal dependence of density ρ(T), allow for determining optical polarizability anisotropy of nematogenic molecules, as well as their orientational order parameter. This paper discusses, on the basis of extensive experimental data, the following problems:•applicability of approximate relations between refractive indices of nematic liquid crystals (NLCs) and their molecular parameters for describing optical properties of nematic mesophase has been estimated;•anisotropy of the local field factor Fk has been investigated, as well as its influence on calculated anisotropy of molecular polarizability. Influence of various extrapolation procedures on calculated molecular parameters of the NLCs has been investigated.

Wavelength and Voltage Dependences of Refractive Indices of Nematic Liquid Crystals

Abstract Refractive indices of a mixture of nematic liquid crystals as functions both of wavelength and applied voltages were measured. The measured results are compared with a theoretical calculation in which first the distributions of tilt angle of the molecules were computed by solving a reorientation equation, then, the effective refractive indices were computed from this distribution using an index ellipsoid. It is shown that the agreement between the experimental measurements and the calculated values are fairly good within the experimental error.