Alignment of polymer network-stabilized ferroelectric liquid crystal using a magnetic field

Polymer network-stabilized ferroelectric liquid crystals with homogeneous alignment have been produced in cells without a surface alignment layer. In this technique, a crosslinkable monomer is mixed into a ferroelectric liquid crystal and polymerized in a magnetic field to form a polymer network that will stabilize the alignment of the ferroelectric liquid crystal. The concentration of the monomer is an important factor in achieving alignment of the ferroelectric liquid crystal. Both the morphology of the final composite layer and the molecular alignment of the host FLC are affected by the curing temperature at which the UV curing of the sample is started.     

Ferroelectric polymer nanocomposite alignment layer in twisted nematic liquid crystal devices for reducing switching voltage

Twisted nematic liquid crystal device (TNLCD) was fabricated using a ferroelectric zinc oxide (ZnO)-doped polyimide alignment layer. The ferroelectric nanoparticle can produce a local electric field, which can trigger the orientation of liquid crystal molecule and reduces the switching voltage. The uniform dispersion of ferroelectric ZnO nanoparticles in the alignment layer was studied using field emission scanning electron microscopy and atomic force microscopy. The ferroelectric property of ZnO-doped polyimide was investigated using dynamic contact electrostatic force microscopy. An increased local electric field due to the presence of nano ZnO was confirmed with the help of scanning tunnelling microscopy. An augmentation of capacitance was observed with an increase in concentration, which substantiates the reduction of switching voltage of TNLCD with the modification of ferroelectric nanoparticle-doped alignment layer.     

A sub-hertz frequency dielectric relaxation process in a ferroelectric liquid crystal material

Dielectric studies of the first order phase transition of a ferroelectric liquid crystal material having the phase sequence chiral nematic to smectic C* have been performed using thin (2.5 mum) cells in the frequency range 0.01 Hz to 12 MHz. For planar alignment, one of the cell electrodes was covered with a polymer and rubbed. Optically well defined alignment was obtained by applying an a.c. field below the N*-SmC* transition. Charge accumulation was enhanced by depositing a thick polymer aligning layer for the alignment of the liquid crystal molecules. A sub-hertz frequency dielectric relaxation process is detected in smectic C*, in the chiral nematic and a few degrees into the isotropic phase, due to the charge accumulation between the polymer layer and the ferroelectric liquid crystal material. The effect of temperature and bias field dependences on the sub-hertz dielectric relaxation process are reported and discussed.     

Optic mode determination of the relaxed state configuration in a ferroelectric liquid crystal cell

Here the method of propagation of optic modes to examine the alignment in a ferroelectric liquid crystal layer is fully discussed. A sample geometry which allows the excitation of optic modes in a ferroelectric liquid crystal cell is presented and the data obtained in terms of reflectivity. The data are analysed by modelling the expected reflectivity from such a cell for a selection of proposed models for the optic tensor configuration. Models where the smectic layering lies perpendicular to the cell surfaces, or bends slowly across a cell are seen to be incorrect. It is found that a discontinuity in the smectic layering in the middle of the cell must be included to model the data well. This is consistent with the chevron structure seen in X-ray scattering work, and leads to an optic tensor alignment which is largely uniform across the ferroelectric layer. The relation between this and observation of conventional ferroelectric cells is discussed.     

A sub-hertz frequency dielectric relaxation process in a ferroelectric liquid crystal material

Dielectric studies of the first order phase transition of a ferroelectric liquid crystal material having the phase sequence chiral nematic to smectic C* have been performed using thin (2.5 mum) cells in the frequency range 0.01 Hz to 12 MHz. For planar alignment, one of the cell electrodes was covered with a polymer and rubbed. Optically well defined alignment was obtained by applying an a.c. field below the N*-SmC* transition. Charge accumulation was enhanced by depositing a thick polymer aligning layer for the alignment of the liquid crystal molecules. A sub-hertz frequency dielectric relaxation process is detected in smectic C*, in the chiral nematic and a few degrees into the isotropic phase, due to the charge accumulation between the polymer layer and the ferroelectric liquid crystal material. The effect of temperature and bias field dependences on the sub-hertz dielectric relaxation process are reported and discussed.     

The effect of thermal treatment of the alignment layer on the electro-optical response of SSFLCs

Annealing the polyimide alignment layer affected the IR spectra of the alignment layer and correspondingly the electro-optic effect of surface stabilized ferroelectric liquid crystal displays. It was found that with proper annealing a smectic C* liquid crystal with a spontaneous polarization as high as 30 nC/cm² can show good stability towards disturbing pulses. The optimum annealing conditions consisted of a rather narrow temperature band. A model where amorphous regions in an otherwise crystalline polymer act as islands of instability is proposed.     

Nano P(VDF-TrFE) doped polyimide alignment layers for twisted nematic liquid crystal devices

ABSTRACT

The rapid development of consumer electronics and portable devices imposes a great demand for energy efficient information display systems. Among the information display devices, liquid crystal display (LCD) devices stands in the front. The fabrication of energy-efficient LCD systems demands new material and techniques. In this work, the conventional polyimide alignment layer of twisted nematic liquid crystal device (TNLCD) was replaced with ferroelectric polymer nanoparticle doped alignment layer. Morphology of the alignment layer was analysed using field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The ferroelectric nature of the polymer alignment layer was studied using dynamic contact electrostatic force microscopy (DC-EFM). TNLCD cells are fabricated with this modified alignment layer and the switching characteristics are compared with the conventional TNLCD devices. The TNLCD with modified alignment layer has shown a reduction of 50% in threshold (V_thr) and 47% reduction in saturation voltage (V_sat).     

Analytical approach to V-shaped characteristics in antiferroelectric liquid crystal displays

Not only in ferroelectric liquid crystal displays (LCDs), but also in antiferroelectric LCDs, grey levels are possible by actively addressing the ‘continuous director rotation mode’. For ferroelectric LCDs this was shown qualitatively and quantitatively in previous articles. In this article it is shown that an exact analytical approach is also possible for antiferroelectric LCDs. In two consecutive layers the director orientations are symmetric, and at zero voltage they are in a splayed state. The conditions on alignment layer thickness and interaction coefficients are related to those of ferroelectric liquid crystal but are easier to fulfill.     

Fabrication of homogenously self-alignment fringe-field switching mode liquid crystal cell without using a conventional alignment layer

We propose a novel method to fabricate a uniaxially homogeneous alignment of liquid crystal (LC) molecules without using a conventional alignment layer such as polyimide film. The method produces the polymer alignment layer (PAL) by polymerisation of the monomer including in the LC layer above the T_NI of the LC material. The fringe-field switching (FFS) mode LC cell with the PAL (FFS-PAL-LC cell) produced from the monomer 4,4?-di-mehacryloyl-oxy chalcone (4,4?-DMOCh) exhibited enough level of alignment state and electro-optical property compared with the FFS-LC cell having the conventional polyimide-type alignment layer. We can expect that the FFS-PAL-LC cell is useful for next-generation displays such as flexible liquid crystal displays (LCDs) because the method does not need high-temperature process of over 200°C.     

Aliphatic polyimides as alignment layers for surface stabilized ferroelectric smectic C liquid crystal cells

Eight aliphatic polyimides have been synthesized and tested as alignment layers for surface stabilized ferroelectic liquid crystals with a number of room temperature ferroelectric mixtures. The cone angles are used as a quantitative measure of the bistability obtained in these cells. The effect of the structure of the polyimides on the cone angles obtained is discussed. It is shown that the structure of the polymer alignment layer strongly influences the cone angle found between the relaxed states in a surface stabilized ferroelectric liquid crystal cell. Highly crystalline polymers give a larger cone angle than less crystalline homologues.     

Analytical approach to V-shaped characteristics in antiferroelectric liquid crystal displays

Not only in ferroelectric liquid crystal displays (LCDs), but also in antiferroelectric LCDs, grey levels are possible by actively addressing the 'continuous director rotation mode'. For ferroelectric LCDs this was shown qualitatively and quantitatively in previous articles. In this article it is shown that an exact analytical approach is also possible for antiferroelectric LCDs. In two consecutive layers the director orientations are symmetric, and at zero voltage they are in a splayed state. The conditions on alignment layer thickness and interaction coefficients are related to those of ferroelectric liquid crystal but are easier to fulfill.     

Polymer and azo-dye composite: a photo-alignment layer for liquid crystals

In this article, we disclose a method to fabricate polymer-stabilised azo-dye photo alignment layers for liquid crystal. The idea includes the introduction of polymer network in the alignment layer, in optimal concentration, followed by two-step irradiation. The stabilised photo-alignment layer has been explored for different aspects of the display-related parameters, viz. anchoring energy, stability for various display-related environments as a function of concentration of monomer and irradiation, residual DC and voltage holding ratio. The composite photo-alignment layer offers well-suited parameters for the liquid crystal alignment and therefore could find application in a variety of modern photonic and display devices.     

Smectic layer structure of ferroelectric liquid crystal formed between fine polymer fibres

This paper describes the alignment of ferroelectric liquid crystal (FLC) structures formed between aligned polymer fibres, where the FLC smectic layers are determined by polarising microscopy and X-ray diffraction. The FLC/polymer composite films were formed from a nematic phase FLC/monomer solution using a photopolymerisation-induced phase separation method. It was found that bending of the FLC smectic layers was induced in both the film plane and the cross-sectional plane at the phase transition from smectic A to chiral smectic C of the FLC material. The light transmittance properties of the composite film between crossed polarizers was analysed by light propagation simulation in several optical anisotropic media, based on the evaluated smectic layer model.     

Smectic layer structure of ferroelectric liquid crystal formed between fine polymer fibres

This paper describes the alignment of ferroelectric liquid crystal (FLC) structures formed between aligned polymer fibres, where the FLC smectic layers are determined by polarising microscopy and X-ray diffraction. The FLC/polymer composite films were formed from a nematic phase FLC/monomer solution using a photopolymerisation-induced phase separation method. It was found that bending of the FLC smectic layers was induced in both the film plane and the cross-sectional plane at the phase transition from smectic A to chiral smectic C of the FLC material. The light transmittance properties of the composite film between crossed polarizers was analysed by light propagation simulation in several optical anisotropic media, based on the evaluated smectic layer model.     

Domain manipulation of ferroelectric BaTiO3 thin films: application to the local reorientation of liquid crystal

A method is reported for local alignment of nematic liquid crystal (NLC) molecules. It consists in the poling of small areas of ferroelectric thin films using scanning probe microscopy. A liquid crystal deposited onto such a surface is aligned via a dipole-dipole interaction. The ferroelectric films are first characterized using X ray diffraction, atomic force microscopy and ellipsometry. The domain manipulation and local poling of the film are achieved and characterized using an electrostatic microscopy type set-up. A hybrid nematic liquid crystal cell (NLC-OFC: nematic liquid crystal - oxide ferroelectric cell) is then constructed and its alignment inspected using polarizing microscopy (in reflection mode). The reorientation is explained by invoking a simple interaction between the dipole moment of the LC and the surface electric field generated by the poled area. In addition, a complimentary experiment is performed to determine the depth to which the poled area affects the liquid crystal alignment. This consists of measuring the deflection of a collimated beam (optical soliton) propagating across the poled area.     

In situ self-assembled photo-switchable liquid crystal alignment layer using azosilane monomer-liquid crystal mixture system

Self-assembled monolayers (SAMs) are a unique approach for the liquid crystal (LC) alignment in electro-optical applications such as displays. Herein, a new methodology for photo-switchable LC alignment layer using an azosilane monomer and LC mixture system in the absence of any other foreign alignment layer is presented. The azosilane monomer spontaneously separated from the host LCs, and formed a stable monolayer network on the substrate surface. Data from X-ray photo-electron spectroscopy (XPS), spectroscopic elipsometry (SE), water contact angle and LC alignment studies confirmed that, in the azosilane and LC mixture system, azosilane makes an in situ SAM that is capable of photo-switchable LC alignment layer on glass and indium tin oxide (ITO) substrates. The LCs are aligned with respect to change in the photo-isomerisation of the azo molecule.     

The optical tensor configuration in a surface stabilized ferroelectric liquid crystal determined by using half leaky guided modes

The optic tensor configuration in a surface stabilized ferroelectric liquid crystal cell is investigated using optical excitation of half leaky guided modes. A thin ferroelectric liquid crystal layer is confined between a high index pyramid, with an index greater than the maximum of the liquid crystal, and a glass substrate having an index less than the minimum of the liquid crystal. Using standard attenuated total reflection experimental procedures, over a small angle range a series of sharp resonant peaks are recorded in the s-polarized reflectivity using p-polarized incident light. These peaks are extraordinarily sensitive to details of the optical tensor configuration within the cell. Fitting theoretically modelled reflectivities from multilayer Fresnel theory to the data allows determination of near surface alignment, bending of the chevron, surface tilt angle and biaxiality. To give a clear physical explanation for the great sensitivity of the technique, the electromagnetic field component distributions in the cell are also presented and analysed. The results confirm that the half leaky guided mode method has enormous potential for the study of the optic tensor configuration in liquid crystal layers.     

Photorefractive effect in composites of ferroelectric liquid crystal and photoconductive polymer

The photorefractive effect in composites of a ferroelectric liquid crystal (FLC) and several photoconductive polymers was investigated. The photorefractivity of mixtures of photoconductive polymers and an FLC (polymer/FLC), as well as that of photoconductive-polymer-stabilized ferroelectric liquid crystals (PPS-FLCs) was examined. The polymer/FLC samples exhibited two-beam coupling gain coefficients of about 6∼12 cm^-1 in a 5 µm gap cell. The photopolymerization of a methacrylate monomer in the FLC medium established a polymer-stabilized state in which the alignment of FLC molecules was mechanically stabilized. The noise in a two-beam coupling signal was reduced significantly in the PPS-FLC samples.     

Fast switching beam steering based on ferroelectric liquid crystal phase shutter and polarisation grating

We report on fast-switching and high-efficient optical beam steering based on a polymerisable liquid crystal polarisation grating (PG) in combination with ferroelectric liquid crystal (FLC) phase shutter. The PG was fabricated in a convenient single-step holographic exposure process using photo-sensitive azo-dye material as alignment layer for liquid crystal (LC) director. A binary electro-optical FLC was employed for circular polarisation selection, which enables the electro-tunable steering of the combined system. The efficiency of 95.7% with 82 μs switching time is obtained for 1064 nm laser. This work provides a versatile candidate for non-mechanical beam steering devices.     

Use of mode mixing to determine the optic tensor configuration of a thin ferroelectric liquid crystal layer

It is possible to probe directly the optic tensor profile within a thin ferroelectric liquid crystal layer by the propagation of prism-coupled leaky guided modes. Plane parallel monochromatic radiation of single polarization is made incident through a prism into a planar ferroelectric liquid crystal layer. Monitoring the reflected signal as a function of incident angle results in a series of sharp dips at angles corresponding to the excitation of modes in the layer. For a ferroelectric layer in which there is in-plane or out-of-plane tilt, conversion of one linear polarization to the orthogonal polarization may occur, by detecting this conversion it has been possible to show the existence of thin boundary regions at the two surfaces of the thin ferroelectric liquid crystal layer.