Linear optical switching in a FLC/waveguide composite device

A fast electrooptic modulation in a polymer waveguide using a ferroelectric liquid crystal has been proposed. In this device, the surface stabilized ferroelectric liquid crystal and the soft mode ferroelectric liquid crystal are used as an active material on the passive polymer waveguide, and electrooptic switching is realized by controlling the total reflection at the polymer waveguide-liquid crystal interface. The response time is of the order of several microseconds. The analogue electrooptic modulation in the waveguide is realized using the field induced linear molecular tilt of the electroclinic effect in the soft mode ferroelectric liquid crystal.     

Linear optical switching in a FLC/waveguide composite device

Abstract

A fast electrooptic modulation in a polymer waveguide using a ferroelectric liquid crystal has been proposed. In this device, the surface stabilized ferroelectric liquid crystal and the soft mode ferroelectric liquid crystal are used as an active material on the passive polymer waveguide, and electrooptic switching is realized by controlling the total reflection at the polymer waveguide-liquid crystal interface. The response time is of the order of several microseconds. The analogue electrooptic modulation in the waveguide is realized using the field induced linear molecular tilt of the electroclinic effect in the soft mode ferroelectric liquid crystal.     

Polarization and viscosity measurements of ferroelectric liquid crystalline monomer polymer mixtures

Measurements of spontaneous polarization, tilt angle and response time on mixtures of a ferroelectric side chain polyacrylate with a low molecular mass liquid crystal of a very similar structure are reported. From the obtained values the rotational viscosity for the ferroelectric switching process is calculated. All mixtures exhibit an Arrhenius-like temperature dependence of the rotational viscosity, the values for the polymer being three orders of magnitude higher than for the low molecular mass compound.     

Dye guest-host effects in ferroelectric liquid crystals

Here we present the optic and electro-optic characteristics of ferroelectric dye guest-host devices incorporating different dye and S*_C host materials. We demonstrate fast response time (< 100 μs) single polarizer and zero-polarizer device configurations for high order parameter dichroic dyes and single polarizer fluorescent dye devices. The dynamic data include measurements of response times as functions of voltage, temperature and dye concentration, from which we conclude that the inclusion of certain dyes does not adversely affect the ferroelectric phase. The contrast ratios of these novel guest-host devices are also presented and are shown, as expected, to be functions of tilt angle (and therefore temperature). However, we demonstrate that the high tilt angle of 45° is not necessary for high contrast devices, and in fact for certain devices configurations θ = 22½° is optimum. These results are discussed.     

Modelling monopolar and bipolar switching in ferroelectric liquid crystal devices using a three variable theory

The switching process in ferroelectric liquid crystal devices takes place through the formation and evolution of domains, which can be modelled using a three variable approach. Here we discuss details of the model which are necessary in order to reproduce the domain nucleation and switching times as a function of applied voltage for both monopolar and bipolar pulses. We show that in order to reproduce data correctly the model must incorporate a complex empirical elastic stress term.     

Measurement of the rotational viscosity of ferroelectric liquid crystals based on a simple dynamical model

Rotational viscosity and spontaneous polarization are the most important properties of a ferroelectric liquid crystal with regard to its switching time in surface stabilized or a.c. field stabilized displays. Whereas there is an abundant literature about spontaneous polarization, only a few attempts have been made to determine the rotational viscosity. We set up a model for the electric response of a ferroelectric liquid crystal cell on application of an electric field. For the application of a triangular wave voltage we derive a relation between the rotational viscosity, the spontaneous polarization, the tilt angle, the maximum induced polarization current and the electric field strength. Experiments are carried out on several ferroelectric liquid crystals and the derived relation was used to determine the rotational viscosity. The relation between the rotational viscosity and the polarization on the one hand and the optical switching time on the other hand is discussed in some detail.     

Ferroelectric liquid crystal siloxane homo and copolymers

Several new chiral liquid crystal siloxane homopolymers and copolymers have been synthesized and studied as to their mesomorphic and ferroelectric behaviour. Almost all of them exhibit chiral smectic C phases over wide temperature ranges. DSC and X-ray measurements are presented; spontaneous polarizations, response times and tilt angles are also given. Some of the spontaneous polarizations are very high, more than 100 nC/cm², and the response times are found to be among the fastest for liquid-crystalline polymers, less than 1 ms.     

Contrast ratio and switching of zigzag defect-free surface stabilized FLCD by photoinduced alignment

The contrast ratio and electro-optical switching behaviours of surface stabilized ferroelectric liquid crystal cells with alignment layers of a photodegradable polyimide prepared by polarized ultraviolet light irradiation have been investigated. The higher contrast ratio was obtained for a zigzag defect-free sample obtained by the photo-induced alignment method than by the rubbing method. The switching behaviours of the FLC, such as spontaneous polarization and response time, were measured. Higher spontaneous polarization and faster response times were obtained with cells prepared by the photoinduced alignment method. In addition, response times of the order of microseconds, which are fast enough to realize high quality moving images with video-frame rate, were obtained.     

Contrast ratio and switching of zigzag defect-free surface stabilized FLCD by photoinduced alignment

The contrast ratio and electro-optical switching behaviours of surface stabilized ferroelectric liquid crystal cells with alignment layers of a photodegradable polyimide prepared by polarized ultraviolet light irradiation have been investigated. The higher contrast ratio was obtained for a zigzag defect-free sample obtained by the photo-induced alignment method than by the rubbing method. The switching behaviours of the FLC, such as spontaneous polarization and response time, were measured. Higher spontaneous polarization and faster response times were obtained with cells prepared by the photoinduced alignment method. In addition, response times of the order of microseconds, which are fast enough to realize high quality moving images with video-frame rate, were obtained.     

Light modulation characteristics of a single-polarizer electro-optical cell based on polymer dispersed ferroelectric liquid crystals

Features of the light passing through a single-polarizer electro-optical cell based on a uniaxially oriented film of a polymer dispersed ferroelectric liquid crystal have been studied. The optical response as a function of the applied electric field is shown to depend on the cell geometry. A detailed analysis is presented of the dependence of the maximum light transmission, modulation depth and contrast ratio on the angle between the light polarization and the film orientation direction, on its optical anisotropy and on the molecular tilt angle. An approximate formula is proposed to estimate the highest attainable contrast. Parameters under study have been measured for PDFLC films varying in optical anisotropy value. The experimental results prove to be in good agreement with theoretical estimations.     

Light modulation characteristics of a single-polarizer electro-optical cell based on polymer dispersed ferroelectric liquid crystals

Features of the light passing through a single-polarizer electro-optical cell based on a uniaxially oriented film of a polymer dispersed ferroelectric liquid crystal have been studied. The optical response as a function of the applied electric field is shown to depend on the cell geometry. A detailed analysis is presented of the dependence of the maximum light transmission, modulation depth and contrast ratio on the angle between the light polarization and the film orientation direction, on its optical anisotropy and on the molecular tilt angle. An approximate formula is proposed to estimate the highest attainable contrast. Parameters under study have been measured for PDFLC films varying in optical anisotropy value. The experimental results prove to be in good agreement with theoretical estimations.     

Fast switching and luminance-controlled fringe-field switching liquid crystal device for vehicle display

ABSTRACT

Liquid crystal displays (LCDs) for vehicle displays should exhibit a fast response time in wide temperature range and wide-viewing angle in horizontal and downward directions without grey-scale inversion but limited brightness in the upward direction because the display images can be reproduced in the front window glass of a vehicle, affecting driver’s front visibility. Currently, fringe-field switching (FFS) liquid crystal device is widely commercialised for high resolution and wide-viewing-angle LCD; however, it needs to improve response times and limit the display brightness in the upward direction. As a solution, we propose a homogeneously aligned liquid crystal device in which liquid crystal director does tilt as well as twist deformation in a confined area by both vertical- and fringe-electric fields, exhibiting about two times faster decay response time than that of conventional FFS mode with suppressed luminance in the upward direction. The proposed liquid crystal device can be applied to LCDs for vehicle displays.     

Electro-optic properties of thiol-ene polymer stabilized ferroelectric liquid crystals

Electro-optic properties of polymer stabilized ferroelectric liquid crystal (PSFLC) systems are examined as a function of varying concentrations of either a linear or crosslinked thiol-ene polymer. The thiol-ene method of polymer stabilization is a drastic change from previous studies designed to avert the problem of polymer phase separation. FLC rise time and tilt angle measurements were used to determine the effects of the polymer network on the optical properties of the system. The addition of monomer impurities to both systems demonstrated a reduction in tilt angle, which translated into decreased switching speeds in both systems prior to polymerization. The crosslinked thiol-ene system showed increased switching times due to the creation of polymer in the interlayer spacing of the FLC, but exhibited minimal increase in the rotational viscosity of the system. In addition, the crosslinked polymer systems resulted in an increase in the liquid crystalline order, which produced an increase in the contrast ratio of the system. The linear polymer system showed drastically different results as compared with the crosslinked system. The rise time and tilt angle measurements decreased upon polymerization of the linear thiol-ene and the rotational viscosity and contrast ratio values also decreased. We suggest that the linear thiol-ene polymer phase separation from the interlayer spacing leads to a microscopic misalignment of the FLC molecules, causing a decrease in the optical properties of the LC.     

The effect of phase transitions on the response times of ferroelectric liquid crystal devices

The response times of ferroelectric liquid crystal devices are of great importance, and are known to be related to various physical properties of the chiral smectic C* phase, such as the spontaneous polarization, the cone angle and the rotational viscosity. We report measurements of the effect of the smectic phase transitions on the response times and show that these are influenced by the smectic C* to smectic A transition temperature.     

Electro-optic properties of thiol-ene polymer stabilized ferroelectric liquid crystals

Electro-optic properties of polymer stabilized ferroelectric liquid crystal (PSFLC) systems are examined as a function of varying concentrations of either a linear or crosslinked thiol-ene polymer. The thiol-ene method of polymer stabilization is a drastic change from previous studies designed to avert the problem of polymer phase separation. FLC rise time and tilt angle measurements were used to determine the effects of the polymer network on the optical properties of the system. The addition of monomer impurities to both systems demonstrated a reduction in tilt angle, which translated into decreased switching speeds in both systems prior to polymerization. The crosslinked thiol-ene system showed increased switching times due to the creation of polymer in the interlayer spacing of the FLC, but exhibited minimal increase in the rotational viscosity of the system. In addition, the crosslinked polymer systems resulted in an increase in the liquid crystalline order, which produced an increase in the contrast ratio of the system. The linear polymer system showed drastically different results as compared with the crosslinked system. The rise time and tilt angle measurements decreased upon polymerization of the linear thiol-ene and the rotational viscosity and contrast ratio values also decreased. We suggest that the linear thiol-ene polymer phase separation from the interlayer spacing leads to a microscopic misalignment of the FLC molecules, causing a decrease in the optical properties of the LC.     

Fast switching ferroelectric liquid-crystalline polymers

The principles of ferroelectricity, especially in liquid crystals (FLC) and polymers, are briefly presented. In a liquid-crystalline side chain polyacrylate electro-optical switching with response times of 200–400 μs was measured. Besides the ferroelectric switching an even shorter electroclinic switching process was detected. The influence of the spacer length and the molecular weight on the ferroelectric properties is discussed. The kind and number of chiral centers and their location in the side group strongly affects the electrooptical properties. In one polymer a change of the optical contrast caused by a sign reversal of the spontaneous polarization was found. In another FLC polymer three switching states (antiferroelectric behaviour) were detected. With coloured and fluorescent FLC polymers, obtained by mixing or by copolymerization new promising applications in electrooptical devices are expected.     

Modelling of switching in surface-stabilized ferroelectric liquid crystal cells using a three-variable model in one dimension

The switching process in ferroelectric liquid crystal devices takes place through the formation and evolution of domains, which is modelled here using a three-variable approach. This approach includes variation of the director profile in one dimension, through the thickness of the cell. Here we discuss details of the model which are necessary in order to reproduce the domain nucleation and switching times as a function of applied voltage for both monopolar and bipolar pulses. We show that the three-variable modelling of SSFLC switching in one dimension produces excellent comparisons with experimental data for both bipolar and monopolar pulses.     

Are short-pitch bistable ferroelectric liquid crystal cells surface stabilized?

Recent papers have described the short-pitch bistable ferroelectric liquid crystal effect (SBF) attributing its bistability to its stripe layer texture [1]. We have studied the bistability of this SBF effect for the short-pitch ferroelectric liquid crystal (FLC) mixture ZhKS-76 using thin planar aligned cells. Both the SBF texture (the stripe texture) and a uniform texture (uniform tilt layer structure with the extinction direction along the layer normal) were observed in different regions of a given cell when the cell was cooled down slowly from the isotropic phase to the chiral smectic C phase. Upon applying external fields, both regions are characterized by the formation of helix unwinding lines. The stripe area showed zig-zag unwinding lines and the uniform area exhibited straight unwinding lines, both running parallel to the layers. The bistability study shows a similar hysteresis curve and threshold behaviour on switching for both the SBF texture area and the uniform area, although the uniform area gcve better contrast. These facts strongly indicate that as in the long pitch FLCs, the surfaces rather than the layer stripe texture hinder the formation of the helix in the cell, and this produces dynamic bistable switching.     

Are short-pitch bistable ferroelectric liquid crystal cells surface stabilized?

Abstract

Recent papers have described the short-pitch bistable ferroelectric liquid crystal effect (SBF) attributing its bistability to its stripe layer texture [1]. We have studied the bistability of this SBF effect for the short-pitch ferroelectric liquid crystal (FLC) mixture ZhKS-76 using thin planar aligned cells. Both the SBF texture (the stripe texture) and a uniform texture (uniform tilt layer structure with the extinction direction along the layer normal) were observed in different regions of a given cell when the cell was cooled down slowly from the isotropic phase to the chiral smectic C phase. Upon applying external fields, both regions are characterized by the formation of helix unwinding lines. The stripe area showed zig-zag unwinding lines and the uniform area exhibited straight unwinding lines, both running parallel to the layers. The bistability study shows a similar hysteresis curve and threshold behaviour on switching for both the SBF texture area and the uniform area, although the uniform area gcve better contrast. These facts strongly indicate that as in the long pitch FLCs, the surfaces rather than the layer stripe texture hinder the formation of the helix in the cell, and this produces dynamic bistable switching.     

Modelling of switching in surface-stabilized ferroelectric liquid crystal cells using a three-variable model in one dimension

The switching process in ferroelectric liquid crystal devices takes place through the formation and evolution of domains, which is modelled here using a three-variable approach. This approach includes variation of the director profile in one dimension, through the thickness of the cell. Here we discuss details of the model which are necessary in order to reproduce the domain nucleation and switching times as a function of applied voltage for both monopolar and bipolar pulses. We show that the three-variable modelling of SSFLC switching in one dimension produces excellent comparisons with experimental data for both bipolar and monopolar pulses.