Liquid Crystal Director Research Articles

Numerical Modeling of the Static Electric Field Effect on the Director of the Nematic Liquid Crystal Director

A two-dimensional model of the Frederiks effect is used to investigate the static electric field effect on the orientation of the nematic liquid crystal (LC) director in a side-electrode cell. The solutions are obtained by the standard finite-difference methods. The programs for the numerical solution of a two-dimensional parabolic partial differential equation are developed both in FORTRAN and C/C++. The Frederiks transition threshold for the central part of the cell and the dependences of the director’s orientation distribution on a high electric field are obtained. The results of the calculation are compared with the experimental data.

Hybrid photosensitive structures based on nematic liquid crystals and lithium niobate substrates

Abstract Liquid crystal cells based on lithium niobate substrates have recently been proposed as good candidates for optofluidic devices and for light-induced controlled generation of defects in liquid crystal films. The peculiarity of these structures lies in the possibility of using the bulk photovoltaic effect of lithium niobate to obtain an optically induced dc field able to affect the molecular liquid crystal director. Reversible fragmentation and self-assembling of liquid crystal droplets driven by the lithium niobate pyroelectric properties have also been reported. We review the basic results obtained so far with the aim of making the point and seeing what else can be done in the framework of the realization of hybrid structures combining lithium niobate with the electro-optical and nonlinear optical properties of liquid crystals.

Surface-induced transition of nematic liquid crystals on graphene/SiC substrate

A nematic liquid-crystal director aligns along the armchair direction of graphene grown on a SiC substrate. The temperature-dependent textural change in the nematic phase is different from the usual texture on the alignment layer. The isotropic-nematic phase transition temperature decreases over time after cell fabrication. Tiny domains occur with the phase transition and appear to be merged from a critical temperature with decreasing temperature in the nematic phase. This is thought to be due to the transition occurring at the interface. On the other hand, the nematic liquid crystal in graphene grown on a Cu foil does not show textural changes, and the phase transition temperature does not decrease even after a long time has elapsed. X-ray fluorescence measurements indicate that silicon atoms exist in the liquid crystal possibly extracted from the SiC substrate. A model of first-order phase transition on the graphene surface has been proposed. This transition is accompanied by an inhomogeneous distribution of scalar order parameters and the transformation from a small multi-grained structure to a large domain distribution in the director field. This structural transformation takes place at a temperature different from that corresponding to the bulk transition. Such behavior may be explained by the adsorption process of silicon atoms in contact with the SiC-graphene interface.

Helical pitch-dependent electro-optics of optically high transparent nano-phase separated liquid crystals.

Feeble light leakage in a dark state of conventional optically isotropic liquid crystal (OILC) device has a strong impact on the contrast ratio of a liquid crystal (LC) device. In order to overcome such intrinsic problem, we proposed an OILC in which the LC directors inside droplets are twisted by introducing chirality. The light leakage is effectively suppressed by matching the refractive indices between LC and polymer matrix; consequently, we achieved a high contrast ratio, 1:1401. Interestingly, the on-state transmittance is enhanced by ~49% compared to conventional OILC. The response time was also improved and the hysteresis was suppressed to be negligible. The improved electro-optic performances of the proposed OILC device would give diverse applications in upcoming flexible display and various photonic devices.

Liquid Crystal-Templated Synthesis of Mesoporous Membranes with Predetermined Pore Alignment.

We demonstrate that polymeric films templated from liquid crystals (LCs) provide basic design principles for the synthesis of mesoporous films with predetermined pore alignment. Specifically, we used LC mixtures of reactive [4-(3-acryloyoxypropyloxy) benzoic acid 2-methyl-1,4-phenylene ester (RM257)] and nonreactive [4-cyano-4'-pentylbiphenyl (5CB)] mesogens confined in film geometries. The LC alignment was maintained by functionalization of the surfaces contacting the films during polymerization. Through photopolymerization followed by extraction of the unreacted mesogens, films of area in the order of 10 cm2 were obtained. We found that, when restricted to an area either through a mechanical or a configurational constraint, open and accessible pores were incorporated into the films. The average direction of the pores could be determined by the LC director during polymerization, and the average diameter of the pores can be tuned in the range of 10-40 nm by varying the reactive monomer concentration. The polymeric films synthesized here can potentially be used for the ultrafiltration purposes. We demonstrated successful separations of proteins and nanoparticles from aqueous media using the polymeric films. The films exhibited 2 orders of magnitude higher flux when the pores were aligned parallel to the permeate direction compared to the perpendicular direction. Overall, the outcomes of this study provide basic tools for the synthesis of porous polymeric films with predetermined pore directions that can potentially be suitable for separations, drug delivery, catalysts, and so forth.

Tunable transmission of a nematic liquid crystal as defect in a 1D periodic structure of dielectric materials by orientation and re-orientation of liquid crystal molecules.

In this paper, we investigate tunable transmission characteristics of a one-dimensional periodic structure (1DPS), designed with periodic dielectric materials containing a nematic liquid crystal (NLC) as a defect layer, on the basis of orientation and re-orientation of LC molecules. The nonlinear differential equation for the director of the liquid crystal under the light field is solved numerically. The relation between the liquid crystal director and the intensity of the electromagnetic wave (EMW) is derived. Transmittances of the liquid crystal defect layer in the 1DPS are calculated with the variation of the intensity of the incident wave and liquid crystal director tilt angles. By varying the director tilt angle of the liquid crystal molecules as well as the incident angle of the EMW, the shifting of the transmitted defect mode wavelengths is studied. Such study is helpful to understand how orientation and reorientation of the molecules affect the transmittance of the considered periodic structure when the EMW interacts with an embedded liquid crystal as a defect layer in the 1DPS of dielectric materials. Such photonic structure of dielectric materials with the liquid crystal layer as a defect layer can be used to fabricate bistable switches, optical filters, feedback lasers, etc.

Liquid Crystals under Confinement in Submicrometer Capsules.

Liquid crystal (LC) ordering and phase transition behavior under confined conditions have attracted extensive attention and enabled many applications. However, the ordering and phase transition behavior of LCs in submicrometer capsules have seldom been studied, primarily due to the lack of proper capsulizing and visualization approaches to such small LC microcapsules. Herein, we achieve submicrometer LC capsules with the sizes down to 100 nm by using emulsion-based interfacial sol-gel reaction. The behavior of LCs under the submicrometer confinement conditions is investigated while the sizes and chemical composition of the microcapsule shell surface are tuned in a controllable way. The phase transition temperatures of LCs in the submicrometer capsules shift from those of bulk LCs due to the surface-induced ordering of LCs under the strong confinement conditions, which causes formation of topological defects and alters the order parameter. Using nonlinear optical imaging technology, we explore the structures of director field of LCs that arise as a result of the competition between the surface boundary conditions and LC elasticity. The results show that the nanoscale encapsulation can significantly influence the structural configurations of the director and phase transitions of LCs under various confinement conditions.

Ultrafast laser induced nanostructured ITO for liquid crystal alignment and higher transparency electrodes

Femtosecond laser nanostructured indium tin oxide (ITO) coated glass is shown to act both as a liquid crystal (LC) alignment layer and as an electrode with higher transparency. The nanopatterns of the 120 nm period were created using ultrashort laser pulses directly on ITO films without any additional spin coating materials or lithography process. Nine regions of laser-induced nanostructures were fabricated with different alignment orientations and various pulse energy levels on top of the ITO confirming the follow-up of the LC director to the line orientation. The device interfacial anchoring energy was found to be ∼1μ J/m2, comparable to the anchoring energy of nematic LC on photosensitive polymers. The transparency as an electrode was found to improve due to the better antireflection and lower absorption expected from a nanostructured surface.

Liquid crystal metasurfaces on micropatterned polymer substrates.

Formation of photonic liquid crystal metasurfaces on rubbed polyimide substrates patterned by focused ion beam is demonstrated. Modulation of the surface anchoring conditions with periods from 1 to 6 micrometers gives rise to periodic deformation of the nematic liquid crystal director field. The exact periodicity is confirmed by the light diffraction measurements. Distinct colors originating from the specific zero-order diffraction spectra are observed and qualitatively explained in terms of an analytical model within the one-constant approximation. Quantitatively accurate optical spectra are obtained by the full scale numerical simulations taking into account all relevant material parameters. The results pave the way for hybrid liquid-crystal-based metasurfaces with tunable optical transmission, diffraction, and lasing.

Using liquid crystals to control surface plasmons

ABSTRACTWe propose using a liquid crystal (LC) layer placed on top of graphene ribbons or a graphene monolayer to control surface plasmons. The plasmon frequency depends on the LC director orientation which can be controlled by an applied voltage. The surface plasmons excitation is sensitive to the light polarisation and can be manipulated using the orientation properties of LCs. We further suggest using periodically anchored LCs instead of a relief grating to excite and control the plasmons in monolayer graphene. The suggested scheme can be used as the basis for a tunable optical filter or a modulator.

Chiral Polymeric Nanocapsules and Their Use for Conformational Deracemization of Liquid Crystals

We present the first preparation and properties of chiral nanocapsules. The chiral shell, a polyurea derivative, was obtained by interfacial emulsion polymerization of l-lysine with polymethylene polyphenyl isocyanate. The chirality of these nanocapsules was manifested by its ability to induce conformational deracemization of a liquid crystal. This induced chirality was measured using the “Raynes’ experiment”, in which the boundary conditions of cells impose a ±90° rotation of the liquid crystal director from one surface to the other. Both left- and right-handed director twist domains appear on cooling from the isotropic to the nematic phase. Owing to the weak induced chirality of the liquid crystal, one sense of director rotation is energetically more favorable and its domain size expands, resulting in curvature of the domain walls. The curvature was measured as a function of capsule concentration and serves as a metric of the induction of chirality in the surrounding liquid crystal.

Block preconditioners for saddle point systems arising from liquid crystal directors modeling

We analyze two types of block preconditioners for a class of saddle point problems arising from the modeling of liquid crystal directors using finite elements. Spectral properties of the preconditioned matrices are investigated, and numerical experiments are performed to assess the behavior of preconditioned iterations using both exact and inexact versions of the preconditioners.

Electrically tunable VCSEL with intra-cavity liquid crystal: Design, optimization, and analysis of polarization- and mode-stability

Tunable vertical-cavity surface-emitting lasers (VCSEL) provide broad wavelength tuning range at very high modulation speed, but usually employ micro electromechanical systems, which are sensitive to vibrations. In this paper we propose a tunable 1550 nm VCSEL design with enclosed intra-cavity liquid-crystal (LC) cell. The laser is electrically pumped and the wavelength selectivity is obtained by varying LC’s refractive index through electro-optical tuning of the LC director.By performing one-dimensional transfer-matrix optical calculations, supplemented with a fully-vectorial three-dimensional self-consistent modeling of thermal, electrical and optical phenomena, we optimize the design and show the dependency of the emitted wavelength on LC cell thickness and the LC director orientation. The analysis is concluded with a possibility of achieving 68.5 nm tuning range in a continuous-wave operation regime and without polarization or transverse mode switching.

Voltage-Controllable Guided Propagation in Nematic Liquid Crystals

Voltage-controllable guided channels are formed in a planar nematic liquid crystals cell. The director of liquid crystals can be aligned by applying external voltage, which results in a difference of the refractive index between two adjacent channels; therefore, the incidence beam can be coupled from one channel to another. First, we discussed the propagation of the beam and the self-focusing in a single channel; then we discussed the propagation of the beam and the coupling effect in the two channels. The results showed that the propagation of the beam can be selected in each channel by applying voltages in the two individual electrode channels.

Optics of light scattering caused by liquid-crystal-director fluctuations

ABSTRACTWe have applied the light-scattering equation caused by liquid-crystal (LC)-director fluctuations as derived by De Gennes [1] to the case where the polarization direction of the incident light is at an arbitrary angle with respect to the LC directors within the cell. Based on the De Gennes’s equation, we have used an out-of-plane (OPR) cell-rotation method to measure the pretilt angles of a tilted-homogeneously aligned LC cell. Our measured pretilt angles are in good agreement with that obtained by the published OPR-rotation method [2] based on a different mechanism. Our method is simple in setup and requires no complicated data-fitting calculations when the pretilt angles are below about 30°. In addition, there is no need to know cell parameters except the ordinary and extraordinary refractive indices of the LC medium at the wavelength of incident light.

81‐3: Effects of Flexoelectricity and Ion on the Flicker of Fringe Field Switching Liquid Crystal Display

Although LCDs are the leading technology for flat panel displays, their energy efficiency is low. One way to improve the energy efficiency is to decrease the driving frequency when static images are displayed. As the driving frequency is decreased, the transmittance of the display may vary with time, known as flickering. We carried out both experimental and simulation studies to investigate the origins that cause the flickering problem. Our results show that flexoelectric effect caused by non‐uniform liquid crystal director configurations and ions in the liquid crystal are the main factors responsible for the flickering. We quantitatively analyzed the flickering caused by the two factors.

Formation of radial aligned and uniform nematic liquid crystal droplets via drop-on-demand inkjet printing into a partially-wet polymer layer

This paper investigates the drop-on-demand inkjet printing of a nematic liquid crystal (LC) onto a variety of substrates. Achieving both a well-defined droplet boundary and uniformity of the LC director in printed droplets can be challenging when traditional alignment surfaces are employed. Despite the increasing popularity of inkjet printing LCs, the mechanisms that are involved during the deposition process such as drop impact, wetting and spreading have received very little attention, in the way of experiments, as viable routes for promoting alignment of the resultant LC droplets. In this work, radial alignment of the director and uniformity of the droplet boundary are achieved in combination via the use of a partially-wet polymer substrate, which makes use of the forces and flow generated during droplet impact and subsequent wetting process. Our findings could have important consequences for future LC inkjet applications, including the development of smart inks, printable sensors and lasers.

Numerical Analysis of H-PDLC Using the Split-Field Finite-Difference Time-Domain Method.

In this work, an accurate numerical modeling of the diffraction properties of transmission holographic polymer dispersed liquid crystal (H-PDLC) gratings is presented. The method considers ellipsoid geometry-based liquid crystal (LC) droplets with random properties regarding size and location across the H-PLDC layer and also the non-homogeneous orientation of the LC director within the droplet. The direction of the LC director inside the droplets can be varied to reproduce the effects of the external voltage applied in H-PDLC-based gratings. From the LC director distribution in the droplet, the permittivity tensor is defined, which establishes the optical anisotropy of the media, and it is used for numerically solving the light propagation through the system. In this work, the split-field finite-difference time-domain method (SF-FDTD) is applied. This method is suited for accurately analyzing periodic media, and it considers spatial and time discretisation of Maxwell’s equations. The scheme proposed here is used to investigate the influence on the diffraction properties of H-PDLC as a function of the droplets size and the bulk fraction of LC dispersed material.

Electroclinic effect in the chiral lamellar α phase of a lyotropic liquid crystal.

In thermotropic chiral Sm-A^{*} phases, an electric field along the smectic layers breaks the D_{∞} symmetry of the Sm-A^{*} phase and induces a tilt of the liquid crystal director. This so-called electroclinic effect (ECE) was first reported by Garoff and Meyer in 1977 and attracted substantial scientific and technological interest due to its linear and submicrosecond electro-optic response [S. Garoff and R. B. Meyer, Phys. Rev. A 19, 338 (1979)0556-279110.1103/PhysRevA.19.338]. We now report the observation of an ECE in the pretransitional regime from a lyotropic chiral lamellar L_{α}^{*} phase into a lyo-Sm-C^{*} phase, the lyotropic analog to the thermotropic Sm-C^{*} phase which was recently discovered by Bruckner etal. [Angew. Chem. Int. Ed. 52, 8934 (2013)1433-785110.1002/anie.201303344]. We further show that the observed ECE has all signatures of its thermotropic counterpart, namely (i) the effect is chiral in nature and vanishes in the racemic L_{α} phase, (ii) the effect is essentially linear in the sign and magnitude of the electric field, and (iii) the magnitude of the effect diverges hyperbolically as the temperature approaches the critical temperature of the second order tilting transition. Specific deviations between the ECEs in chiral lamellar and chiral smectic phases are related to the internal field screening effect of electric double layers formed by inevitable ionic impurities in lyotropic phases.

Control of liquid crystal alignment using surface-localized low-density polymer networks and its applications to electro-optical devices

We developed an alignment method for control of the polar pretilt angle at the boundaries of a liquid crystal (LC) layer in a continuous range 2–90° by introducing a low-density polymer network. The network was formed as a result of the electric field-induced segregation of a photo-reactive monomer added in concentrations of 0.5–1% to the LC host and subsequent in-situ UV-light-induced polymerization. LC director was “frozen” within a less than a micron-thick layer in the vicinity of the cell substrates at high average tilt angles anchoring the LC in the bulk similarly to a high-pretilt alignment layer. The resultant pretilt angle was determined by the voltage applied before and during polymerization and the duration of voltage application before UV-light exposure. The desired pretilt angle could be set over a small area of a sample which allowed for the fabrication of LC devices with spatially variable optical retardation. Using this method, we fabricated high-pretilt splay and bend LC cells, optically recorded converging lenses and bi-prisms with electrically controlled optical characteristics, and phase diffraction gratings with resolution better than 50lines/mm.