Structure Of Ferroelectric Liquid Crystals Research Articles

Effect of hydrophobic aerosil dispersions on the Goldstone mode dynamics in ferroelectric liquid crystal near the point of polarisation sign reversal

ABSTRACTBroadband dielectric spectroscopy in the frequency range from 10−2 Hz to 107 Hz was used to study the effect of hydrophobic aerosil nanoparticles on the dynamics of the Goldstone mode in a ferroelectric liquid crystal near the point of spontaneous polarisation sign reversal. The obtained results show that an important role in the modification of the dynamics of the Goldstone mode and the phenomena at the point of spontaneous polarisation sign reversal is played not only by the presence of the nanoparticles themselves in the ferroelectric liquid crystal structure, but also the state of the surface of these nanoparticles. Similarly to the hydrophilic aerosil, the polarisation sign reversal temperature slightly decreases with the increase of the hydrophobic aerosil concentration. For small concentrations of hydrophobic aerosil, only slight changes in the dynamics of the Goldstone mode over the entire temperature range are detected. The observed deviations are due to the different influence of aerosil nanoparticles on the structure and dielectric properties of the tested composite caused by the specific surface treatment. The presence of the Goldstone mode above and below the point of polarisation sign reversal seems to confirm the model of two conformers with different structure and opposite dipole moments.

Impact of twisted alignment on the smectic layer structure of ferroelectric liquid crystal

The twisted aligned cell of ferroelectric liquid crystal is assessed through electro-optical and dielectric spectroscopy and a comparison is made with antiparallel planar aligned cells of the same thickness around 8 μm. The study has been carried in two types of twisted aligned samples, one is natural cooled sample in the heating chamber and another is slow cooled at the rate of 0.05 °C/min. The influential distinct results have been observed in natural cooled twisted cells. In natural cooled cells, the helicoidal structure at the surface and in the bulk of the cell is distinct and contributes separately to the response of the cell. On the other hand, at the same cooling condition in twisted cell, the smectic layers are in a twisted state resulting into a non-uniform helical structure, therefore, the partial helicoidal unwound structure at the surface and in the bulk of the cell are not observed contributing to the dielectric permittivity separately unlike in planar aligned cell. The twisted cell is found to induce the restoring force within the sample. However, the slow cooling cycles on the twisted cell changes to the strain free equilibrium state, whereas the behavior of the planar sample cell remains unchanged. The studies are significant for the display devices based on twisted alignment of FLC cells.

Mechanism of electrooptic switching time enhancement in ferroelectric liquid crystal/gold nanoparticles dispersion

ABSTRACTIt is well established that incorporation of nanoparticles (NPs) in the structure of ferroelectric liquid crystals (FLCs) leads to a decrease in their electrooptic response time. Several approaches have been suggested to explain this effect (decrease in rotational viscosity of FLCs, ions enhanced localised electric field, dipole–dipole interaction among NPs and FLC molecules, FLC ordering). In this article, we will report the role of the voltage divider formed by the structural elements of a FLC cell based on ferroelectric liquid crystal/gold nanospheres (FLC/GNSs) dispersion in enhancement of the switching time. Using the impedance spectroscopic measurements, it was demonstrated that the dispersing of GNSs leads to the increase in the voltage drop on FLC/GNSs layer in comparison with the pristine FLC one. Consequently, the electrooptic response time of the FLC/GNSs cell is faster than that of the pristine one. However, the rotational viscosity of the FLC does not depend on the presence of the GNSs.

NONLOCAL EFFECT IN LIQUID CRYSTALS

In this paper, we investigate the role of nonlocal inter- action energy on nucleations of periodic solutions in a one-dimensional problem arising in smectic liquid crystals. In this article, we analyze the structure of ferroelectric liquid crystals with long-range interactions of polarizations. We assume that the liquid crystals are of smectic type, possess spontaneous polarization. The vast majority of the nematogens are polar compounds but the absence of nematic ferroelectric behavior indicates that there is equal probability of the dipoles pointing in either direction. Because of this it is generally assumed that the permanent dipolar contribution to the orientational order is small enough to be negligible. However, it has been known that the interaction between neighboring dipoles is signiflcant compared with dispersion forces. It is possible to construct a model that accounts for permanent dipoles but it is still consistent with the non-polar character of the medium. When dealing with such type of interactions, the concept of near-neighbors becomes relevant. We start with the spin-1=2 Ising model with interaction Hamiltonian

Effect of Polymer Stabilization on Molecular Alignment of Ferroelectric Liquid Crystal in Wide-Gap Cell

We have investigated the molecular alignment structure of ferroelectric liquid crystals (FLCs) in a wide-gap cell for application to optical devices, such as an electrically controlled lens. The transformation of the smectic layer structure has been observed under an electric field, particularly under a low electric field in the wide-gap cell. We have attempted to suppress this transformation, which considerably degrades the optical performance of FLCs, by the polymer stabilization technique. As a result, we have found that the layer transformation can be suppressed successfully. However, the disarray of the FLC molecular alignment may occur owing to the deformation of polymer networks accompanied by FLC molecular reorientation caused by the application of an electric field.

Light driven optical switching of the surface stabilized antiferroelectric liquid crystals

A holographic grating's recording onto liquid crystal (LC) cells filled with nematic mixtures was widely discussed over the last decade (Sutkowski et al., 2006) [1]. Recently, the optical recordings using polar liquid crystals – synclinic SmC ⁎ and anticlinic SmC A ⁎ phases at the surface stabilized ferroelectric liquid crystal structures (SSFLC) were reported (Sutkowski et al., 2009) [2]. Among the others, SSFLCs provide such attractive properties like submicrosecond switching and the easily induced memory effect, which are desired at contemporary re-writable holography applications. Using SS(A)FLC the micrometer size gratings are also generated (Clark and Lagerwal, 1980) [3]. Here we present a study of the performance of recording of a phase grating of 20 micron resolution using custom fabricated measuring cells with photosensitive orienting layers. The photo- and electro-optical properties of the selected synclinic SmC ⁎ and anticlinic SmC A ⁎ structures have been examined and presented. Various operating modes of studied structures are shown and described. The charge transfer mechanism of the optically driven switching is proposed and discussed.

Molecular Alignment of Ferroelectric Liquid Crystal in Wide-Gap Cell for Liquid Crystal Lens

The high-speed response of ferroelectric liquid crystals (FLCs) is advantageous for optical devices such as an electrically controlled lens. We have investigated the molecular alignment structure of FLC in a wide-gap cell for application to the lens. As a result, it has been found that a zigzag defect-free FLC medium can be obtained by utilizing alignment films, which possess a smooth surface and give rise to the defect-free situation in a narrow-gap cell, and by cooling from the nematic to the smectic phase at a low rate of <1 °C/min. However, the transformation of the smectic layer structure has been observed under an electric field, particularly under a very low electric field in the wide-gap cell. We have attempted to suppress this transformation, which considerably degrades the optical performance of FLC, by the polymer stabilization technique.

Light Scattering of Short Helix Pitch Ferroelectric Liquid Crystal

Two new light scattering modes have been discovered in short helix pitch (p0 ≅ 350 nm) ferroelectric liquid crystal (FLC). One of the modes arises because of a special kind of the FLC structure non-uniformity that originates inherently as a new phenomenon if the helix pitch is very small. Another mode relates to the helix twisting after the driving voltage switching off.

Holographic Recordings Using Bistable SmC* Structures

Numerous methods of re-writable optical recordings using nematic mixtures for holographic media have been presented. A new approach to this problem is possible by using surface stabilized ferroelectric liquid crystal structures (SSFLC). This is very promising from the theoretical point of view because the surface stabilized the structure of ferroelectric smectic SmC* material inherently consists of a memory effect while driving by external electric field. Moreover, due to the layered structure of the surface stabilized ferroelectric liquid crystal there is a possibility of obtaining high resolution of the written gratings. In the paper, some theoretical basis regarding possibility of optical addressing of SSFCL cells are discussed. Methods of preparation of the optically addressed SSFLC cells in room temperature bistable SmC* structure are presented. Experiments with these cells in simple holographic two-beam set-up (DWTM) are described.

Accurate calculation of reflectance spectra for thick one-dimensional inhomogeneous optical structures and media: stable propagation matrix method

Numerical instability is usually observed when the propagation matrix method is used to calculate the reflectance and transmittance spectra for the thick one-dimensional inhomogeneous optical structures and media. To remove this numerical instability we applied two procedures, the normalization and the singular-value decomposition, for the propagation matrix and the matrix involved in calculating the matrix of reflection coefficients, respectively. Examples of a cholesteric liquid crystal and a helical structure of ferroelectric liquid crystals with a twist defect show that the modified propagation matrix method is able to accurately calculate the reflectance spectra for thick structures.

The Effect of Polymer Doping on the Formation of Helical Structure in Ferroelectric Liquid Crystals

The effect of polymer stabilization on the helical structure of ferroelectric liquid crystal (FLC) and the deformation of the helical structure under the application of an electric field has not been cleared. In this study, conoscope observations for polymer-stabilized (PS) FLCs were performed, and we try to examine the polymer anchoring strength from the comparison between the experimental and the computer simulation results. It is found that the helical structure can be stabilized by introducing polymer networks, and the elastic constant of PSFLC may increase in different classes even in a few polymer concentration. Furthermore, it is found that the polymer stabilization can suppress the temperature variation of the elastic constant.

Molecular Alignment in Ferroelectric Liquid Crystal Display Having Microscopically Patterned Electrodes

Ferroelectric liquid crystals (FLCs) are one of the promising materials for next-generation liquid crystal displays (LCDs) because of fast response, microsizing of pixel and so on. Since the molecular alignment structure of FLC is strongly affected by the surface roughness of substrate, it is important to investigate the influence of the microscopically patterned electrodes on the alignment of FLC molecules. In this study, we have researched the influence of the unevenness of the electrodes on the surface of the polyimide alignment film and the alignment structure of FLC, and then tried to obtain a defect-free uniform alignment even in FLCD having microscopically patterned electrodes. As a result, it was obvious that the way of wetting of the polyimide solution is largely different even in the small difference of the surface unevenness in the patterned electrodes. A uniform alignment of FLC molecules was obtained by using the combination of a flat surface of polyimide film and a relatively even surface of the glass substrate with microscopically patterned electrodes.

Pyroelectric investigations of a hydrogen bonded ferroelectric liquid crystal gel by LIMM

AbstractThe laser intensity modulation method (LIMM) is employed to determine spatially resolved polarization distributions in sandwich cells containing a hydrogen‐bonded ferroelectric liquid crystal (FLC) gel. At no external electric fields, contributions to the distributions at the surface of the FLC layer are dominating in all the samples with different concentration of gel former. These are attributed to non‐vanishing polarization due to surface interaction. In this case, the effect of hydrogen‐bonded network on the polarization distribution is not visible. In external electric fields, additional contribution to the resulting distribution caused by the induced polarization due to unwinding the FLC helix has been observed. Furthermore, the influence of hydrogen‐bonded network on the polarization distribution is also detected when the gel former content is increased up to 5.0 wt%. Therein the shape of the measured pyrospectra is completely different to other FLC gel samples with lower gel former concentration, where their maximum distributions still locate at the surface of FLC layer which is comparable to the initial field‐free state. These result indicate that the helical structure and orientation director of FLC are able to be stabilized effectively by the gel network even under strong external electric field. Copyright © 2004 John Wiley &amp; Sons, Ltd.

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.

Phase transitions in ferroelectric liquid crystals probed by optical second harmonic generation

Phase transitions in thin (1.5–3 μm) planar cells of a ferroelectric liquid crystal (FLC) mixture are studied using the optical second harmonic generation (SHG) technique. This method is well known because of its high sensitivity to the structural and electronic properties of surfaces, interfaces and thin films, as well as to the appearance of the ferroelectric state of a material. It is shown that the studied FLC mixture retains a highly regular structure up to about 80 °C, when a sharp decrease of SHG intensity, associated with the transition of the LC to the nonpolar (N*) phase, is observed. SHG intensity measurements show that the transition from chiral smectic C (SmC*) to chiral smectic A (SmA*) phase which takes place at about 45 °C for the bulk FLC mixture does not influence the ferroelectric state of subsurface FLC layers. At the same time, SHG interferometry measurements show that definite changes in the FLC structure happen above the bulk transition temperature from SmC* to SmA* phase. A DC electric field induced switching of the FLC cells is also observed.

MEASUREMENT OF ELECTRORHEOLOGY EFFECT OF FERROELECTRIC LIQUID CRYSTAL USING A SHEAR-HORIZONTAL WAVE DELAY LINE OSCILLATOR

An electrorheology (ER) effect of a ferroelectric liquid crystal (FLC) is described on the basis in numerical analysis and experiment. Propagation characteristics of a shear-horizontal (SH) wave in a liquid crystal layer between a piezoelectric ceramic thin plate and a glass thin plate is numerically analyzed. The phase velocity of the SH wave depends predominantly on the viscosity of the FLC. The ER effect is measured using a SH wave device under the construction of delay line oscillator. The observed DC electric field dependence of oscillation frequency under the use of a helical structure of FLC is related to the observed microscopic result. This method has the advantage of simplicity and a small amount of liquid sample for evaluating the ER effect of the FLC.

Spatial light modulator based on hydrogenated amorphous silicon/deformed-helix ferroelectric liquid crystal structure: Influence of dielectric mirror

The paper presents results on the comparative study of the two modes of the Optically Addressed SLMs based on the amorphous hydrogenated silicon-ferroelectric liquid crystal structure: the transmissive mode SLM and the reflective mode SLM with dielectric mirror. The FLC was used in DHF-mode.

Nonlinear optical properties of ferroelectric liquid crystal

Nonlinear optical phenomena such as second harmonic generation (SHG) have been studied in various ferroelectric liquid crystals. SHG efficiency depends on the molecular structure of ferroelectric liquid crystal and can be controlled by applied voltage. Phase matching of SHG has been well realized in these ferroelectric liquid crystals. SHG in polymeric ferroelectric liquid crystals, ferroelectric liquid crystal-conducting polymer composites, thin ferroelectric liquid crystal films and liquid crystals which exhibit flexoelectric effect have also been discussed.

Chevron structures in surface-stabilized ferroelectric liquid crystals

The computation of director profiles and energy parameters in surface-stabilized ferroelectric liquid crystal structures requires the evaluation of many terms accounting for different contributions to the general solution. This evaluation is usually performed using several simplifications and assumptions adapted to the specific phenomenon under study. In this work, a comprehensive expression including all possible terms not avoiding a computable solution is presented. The effect of every term on the general expression is analyzed.

Photochemical switching of polarization in ferroelectric liquid crystals: Effect of structure of host FLCs

Abstract A novel principle for photochemical control of optical properties of ferroelectric liquid crystal (FLC) is presented, which is based on photochemically induced switching of polarization of FLC triggered by photoisomerization of azobenzene derivative doped in FLC. At the irradiated sites of the azobenzene/FLC mixtures, flip of polarization was induced, causing the change of the optical properties. The effects of the structures of host FLCs on the photochemical flip of polarization are discussed.