Liquid Crystal Director Research Articles

Laser-driven microflow-induced bistable orientation of a nematic liquid crystal in perfluoropolymer-treated unrubbed cells

We demonstrate laser-driven microflow-induced orientational change (homeotropic to planar) in a dye-doped nematic liquid crystal. The homeotropic to planar director alignment is achieved in unrubbed cells in the thermal hysteresis range of a discontinuous anchoring reorientation transition due to the local heating by light absorption in dye-doped sample. Various bistable patterns were recorded in the cell by a programmable laser tweezers. The width of the patterns depend on the scanning speed of the tightly focussed laser beam and the minimum width obtained is approximately equal to 0.57μm which is about 35 times smaller than the earlier report in the rubbed cells. We show that the motion of the microbeam spot causes local flow as a result the liquid crystal director is aligned along that direction.

Photoalignment Layers for Liquid Crystals from the Di-π-methane Rearrangement

Photoalignment of nematic liquid crystals is demonstrated using a di-π-methane rearrangement of a designed polymer. The alignment mechanism makes use of the strong coupling of the liquid crystal directors to dibenzobarrelene groups. The large structural changes that accompany photoisomerization effectively passivate segments of the polymer, allowing the remaining dibenzobarrelene groups to dominate the director alignment. Photoisomerization requires triplet sensitization, and the polymer was designed to have a uniaxially fixed rigid structure and rapid triplet energy transfer from the proximate benzophenone units to the dibenzobarrelene groups. The isomerization was observed to be regiospecific, and thin films showed alignment.

Study of light-induced reorientation of the nematic liquid crystal director by birefringence dynamics

It is proposed to apply the birefringence method to measure the threshold of the light-induced Freedericksz transition and the nonlinearity enhancement factor of nematic liquid crystals by director relaxation dynamics in the light field. A birefringence change is recorded by interference of ordinary and extraordinary components of the light beamaffecting the liquid crystal director orientation. The Freedericksz transition thresholds and nonlinearity enhancement factors are measured for the samples doped with comb-shaped polymers with various degrees of polymerization. The determined thresholds are in agreement with the results obtained by the aberrational self-action method.

Constant-pressure simulations of Gay–Berne liquid-crystalline phases in cylindrical nanocavities

Applications of liquid crystals (LCs) are based on controlling the orientational and translational order of the medium. One important way of control is via confinement. In this work, uniaxial thermotropic LCs confined to nanosized cylindrical cavities are studied using isobaric parallel tempering (PT) Monte Carlo (MC) simulations. The LCs are modeled using the Gay-Berne (4.4, 20.0, 1, 1) (GB) potential in long, smooth-walled cavities. The chosen particle-wall interaction favours homogeneous planar anchoring - the alignment of molecules along the wall. We report the results for the phase structure appropriate to three different cavity sizes as well as comparison to the results of bulk simulations. Ensemble averages for orientational and translational order parameters as well as their local behavior as a function of the distance to the cavity wall is calculated by reweighting results from all the simulated temperatures. We find that the LC director tends to align strongly with the axis of the cylindrical cavity. The orientational order is enhanced and translational order suppressed by the walls of the cavity. Hence, there are notable differences between the local order close to the wall and near the cylinder axis. The position-dependent distributions of the order parameters result in smooth phase transitions in their respective system-wide averages. Particularly, the nematic-isotropic (N-I) transition is replaced by a continuous nematic-paranematic (N-PN) transition. This is caused by the core region of the cavities becoming isotropic at high temperatures, whereas near the wall the LC retains nematic order. In contrast to previous NVT ensemble simulations, we find the effect of confinement on the smectic (Sm) layering to be weak. Also, Sm-N and N-PN transitions are found to be both sharper and residing at higher temperatures than in the constant-volume simulations. At temperatures where the bulk LC is a solid, we observe a wall-induced density wave in the confined systems, which outweighs the self-organization of the LC to hexagonal in-plane order.

A Preconditioned Nullspace Method for Liquid Crystal Director Modeling

We present a preconditioned nullspace method for the numerical solution of large sparse linear systems that arise from discretizations of continuum models for the orientational properties of liquid crystals. The approach effectively deals with pointwise unit-vector constraints, which are prevalent in such models. The indefinite, saddle-point nature of such problems, which can arise from either or both of two sources (pointwise unit-vector constraints, coupled electric fields), is illustrated. Both analytical and numerical results are given for a model problem.

Indirect Magnetoelectric Coupling in Mixtures of Magnetite and Ferroelectric Liquid Crystal

The existence of an indirect magnetoelectricity in a soft composite material prepared as a mixture of magnetite ferromagnetic nanoparticles (NPs) and the ferroelectric liquid crystal (FLC) Felix 015/100 was verified by using a SQUID-based magnetometer. It was observed that the orientation of nanoparticles is directly coupled to the liquid crystal director field. Such a coupling allows the possibility of indirect coupling between the magnetic and ferroelectric order. This gifts the mixtures of FLC and magnetic nanoparticles (MNPs) with a great potential for soft indirect magnetoelectrics.

Spatial and orientational control of liquid crystal alignment using a surface localized polymer layer

We present an alignment method for the surface contacting liquid crystal (LC) director. This method allows complete control of the polar pretilt angle as a function of position in a liquid crystal device, and has the potential of controlling the azimuthal orientation of LC. Important considerations of this method are to form a thin layer of reactive monomers at the LC cell interior surface, and to control the deleterious effects of flow due to polymerization induced concentration gradients. To achieve these, the voltage and frequency of the applied electric field and the UV intensity during the polymerization process are significant.

Characterization of the dynamics of surface stabilized ferroelectric liquid crystal under electric field by full optical snapshot matrix Mueller polarimeter

We investigated the layer dynamics of a conventional surface-stabilized ferroelectric liquid crystal (SSFLC) using a full-optical snapshot Mueller matrix polarimeter (SMMP) based on wavelength polarization coding. Time-resolved polarimetric measurements were performed with different SSFLC samples, and a strong correlation between the polarimetric parameters and the SSFLC under electric field at different exposure times was found. It has been shown that the SMMP polarimeter is able to determine the evolution of the trajectory of the liquid crystal director between the two addressed states, the reversible motion of the smectic layer while switching, as well as the irreversible transition from chevron to bookshelf texture.

Concentrating Photovoltaic System Using a Liquid Crystal Lens

A concentrating photovoltaic (CPV) system with a steady electric output adopting an electrically tunable concentration ratio of the liquid crystals (LC) lens is demonstrated. The distribution of the LC directors of the LC lens can be controlled by an applied voltage in order to adjust the number of incident photons in an area. As a result, the CPV system adopting an LC lens can be operated with a static and maximum output power density under different ambient illuminations because the LC lens with an electrically tunable concentration ratio helps to increase the photocurrent at a low illumination and prevent the effect of the series resistance at a high illumination. The detailed operating principles are discussed, and the experimental results are performed. We believe this letter can help enhance the output power density of CPV systems by using active optical elements whose concentration ratio is tunable.

Stress-induced state transitions in flexible liquid-crystal devices

This work studies the stress–strain dynamics for the transient optoelectronic characteristics of flexible liquid-crystal (LC) devices. Due to the fast response of LC directors, the configuration of the LC is assumed to be in quasi-equilibrium during the process of elastic deformations of the flexible structures. The LC medium hence can be treated effectively as a thin-film layer and can approximately follow the strain–stress mechanism in the solids. Relevant theoretical algorithms are studied in this work, and numerical results present the stress-induced state transitions in the π cell.

Tunability of optical memory in ferroelectric liquid crystal containing polyvinylpyrrolidone capped Ni nanoparticles for low power and faster device operation

We demonstrate here tunability of optical memory and faster electro-optic response in ferroelectric liquid crystal dispersed with ferromagnetic nickel nanoparticles. Minute concentration of Ni induced an optical memory at a DC field as small as 2 V owing to the magnetoelectric effect of spin dependent screening. Memory retention period could be tuned by varying DC field and the concentration of nanoparticles as well. Furthermore, there is a remarkable improvement in response time (80%) and rotational viscosity of ferroelectric liquid crystal-Ni mixtures which is attributed to the coupling of strong intrinsic field of Ni nanoparticles with liquid crystal molecular director field.

Magnetite nanorod thermotropic liquid crystal colloids: Synthesis, optics and theory

We have developed a facile method for preparing magnetic nanoparticles which couple strongly with a liquid crystal (LC) matrix, with the aim of preparing ferronematic liquid crystal colloids for use in magneto-optical devices. Magnetite nanoparticles were prepared by oxidising colloidal Fe(OH)2 with air in aqueous media, and were then subject to alkaline hydrothermal treatment with 10moldm−3 NaOH at 100°C, transforming them into a polydisperse set of domain magnetite nanorods with maximal length ∼500nm and typical diameter ∼20nm. The nanorods were coated with 4-n-octyloxybiphenyl-4-carboxylic acid (OBPh) and suspended in nematic liquid crystal E7. As compared to the conventional oleic acid coating, this coating stabilizes LC-magnetic nanorod suspensions. The suspension acts as a ferronematic system, using the colloidal particles as intermediaries to amplify magnetic field–LC director interactions. The effective Frederiks magnetic threshold field of the magnetite nanorod–liquid crystal composite is reduced by 20% as compared to the undoped liquid crystal. In contrast with some previous work in this field, the magneto-optical effects are reproducible on time scales of months. Prospects for magnetically switched liquid crystal devices using these materials are good, but a method is required to synthesize single magnetic domain nanorods.

Twisted nematic liquid crystal polarization grating with the handedness conservation of a circularly polarized state

We propose a liquid crystal (LC) polarization grating that conserves the polarization state of incident light, wherein the variation range of the twist angle is 2π. The design scheme for theoretically 100% diffraction efficiency of the first-diffraction order is derived, and a prototype LC grating is evaluated. Under zero voltage, the fabricated LC grating exhibits high efficiency of the first-order diffraction, validating the proposed design scheme. The high efficiency of the second-order diffraction can also be achieved under a high voltage so that the LC director in the midplane is vertical to the substrate plane. The circular polarization sense of the second-order diffraction is identical to that of the incident light as in the case of the first-order diffraction. This grating functions as a beam deflector, steering the input beam in three different directions (zeroth-, first-, and second-order diffractions) by adjusting the applied voltage.

Light scattering from liquid crystal director fluctuations in steady magnetic fields up to 25 tesla

We report on homodyne dynamic light scattering measurements of orientational fluctuation modes in both calamitic and bent-core nematic liquid crystals, carried out in the new split-helix resistive magnet at the National High Magnetic Field Laboratory. The relaxation rate and inverse scattered intensity of director fluctuations exhibit a linear dependence on field-squared up to 25 tesla, which is consistent with strictly lowest order coupling of the tensor order parameter Q to field (Q(αβ)B(α)B(β)) in the nematic free energy. However, we also observe evidence of field dependence of certain nematic material parameters, an effect which may be expected from the mean field scaling of these quantities with the magnitude of Q and the predicted variation of Q with field.

Highly efficient twisted nematic liquid crystal polarization gratings achieved by microrubbing

We demonstrate twisted nematic liquid crystal (LC) polarization gratings (TNPGs) with a high first-order diffraction efficiency (>90%) that have the twist angle as constant, whereas the azimuthal angle of the mid-plane liquid-crystal director varies linearly in space. The high diffraction-efficiency design scheme is derived by means of the Jones matrix method. The designed first-order diffraction efficiency theoretically reaches 100% for the thick LC layer in which the retardation equals (n + 1/2)λ (n is an integer and λ is the wavelength). We also present a simple model for the grating period- and twist angle-dependent threshold voltage of the Freedericksz transition in TNPGs.

Director Distribution in Cano-Grandjean Wedge Influenced by Surface Anchoring

Experimental and theoretical studies of the chiral liquid crystal (CLC) director distribution in a wedge shape cell with a weak surface anchoring as well in a planar layer with a gradient of the pitch are performed. The theory predicts that the director distribution in individual Cano-Grandjean zones as well in narrow walls dividing the zones are dependent on the strength and shape of the anchoring potential. The performed calculations for a wedge and a planar layer of variable thickness show that the experimentally distinguishable details of the director distribution in the wedge area between two consequent walls (Cano-Grandjean zone) allow one to obtain information on the shape of the surface anchoring potential and especially favorable for the measurements are several first individual Cano-Grandjean zones. The measurements of the director orientation at the wedge surface with weak surface anchoring versus the coordinate along wedge surface carried out by means of optical polarized transmission spectra were used to reconstruct the director distribution at this surface for the second Cano-Grandjean zone. The relevance of the obtained director distribution to the different model surface anchoring potentials is discussed.

A polarization-independent liquid crystal phase modulation using polymer-network liquid crystals in a 90° twisted cell

A polarization-independent liquid crystal phase modulation using polymer-network liquid crystals in a 90° twisted cell (T-PNLC) is demonstrated. T-PNLC consists of three layers. Liquid crystal (LC) directors in the two layers near glass substrates are orthogonal to each other and those two layers modulate two eigen-polarizations of an incident light. As a result, two eigen-polarizations of an incident light experience the same phase shift. In the middle layer, LC directors are perpendicular to the glass substrate and contribute no phase shift. The phase shift of T-PNLC is electrically tunable and polarization-independent. T-PNLC does not require any bias voltage for operation. The phase shift is 0.28 π rad for the voltage of 30 Vrms. By measuring and analyzing the optical phase shift of T-PNLC at the oblique incidence of transverse magnetic wave, the pretilt angle of LC directors and the effective thickness of three layers are obtained and discussed. The potential applications are spatial light modulators, laser beam steering, and micro-lens arrays.

First-order light-induced orientation transition in nematic liquid crystal in the presence of low-frequency electric field

Director deformation in a planar nematic liquid crystal (NLC) with negative optical nonlinearity (director rotates away from the light field) has been studied. First-order orientation transition and a wide hysteresis loop were observed at changing the light intensity in the presence of an ac field that is non-perpendicular to the undistorted director of liquid crystal due to pretilt. The theory of light beam interaction with NLC under ac field was constructed.

Four-domain twisted nematic structure with enhanced liquid crystal alignment stability and fast response time

In this paper, we propose a stable four-domain twisted nematic structure with wide viewing angle characteristics and a fast response time. A stable structure was realized by introducing a high pre-tilt angle approach incorporating stacked planar and vertical alignment layers. The response time and thermal stability of the proposed device were dramatically improved by the addition of UV-curable reactive mesogen (RM) mixed with a vertical alignment material. Polymerized RMs along the liquid crystal (LC) director increased the polar anchoring energy and reduced back-flow phenomena. A wide viewing angle and fast response time characteristics were achieved over all gray levels.

On liquid crystal diffractive optical elements utilizing inhomogeneous alignment

Formation of a desired liquid crystal (LC) director distribution by the use of inhomogeneous anchoring and pre-tilt angle for electrically controlled diffractive optical elements (DOE) is studied. Such LC DOE can have high periodicity and diffraction efficiency. At the same time they are free of constructive regularities, e.g. a periodic arrangement of the electrodes or thickness deviations, which have undesired impact on diffractive characteristics of LC DOE of other types. We focus on evaluation of potential functional abilities of LC DOE with inhomogeneous alignment. The reasons causing restriction of the LC DOE diffraction efficiency and periodicity are considered. Approaches for improvement of characteristics of the LC DOE are discussed.