Surface-stabilized Ferroelectric Liquid Crystal Research Articles

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.

Spontaneous formation of horizontal chevrons in smectic-C* liquid crystals

We report an unusual structural formation observed in surface-stabilized ferroelectric liquid crystal cells with asymmetrically treated surfaces of 3-Glycidoxypropyl trimethoxysilane and nylon. On cooling the material W415 from the smectic-A* to the smectic-C* phase, the bookshelf smectic layers spontaneously rearrange to form a horizontal chevron structure, manifested optically as a periodic texture of uniform stripes. The polarization is perpendicular to the boundary plates and reverses direction from stripe to stripe, while the director is oriented in the plane of the cell and alternates symmetrically about the stripe direction. In an applied field the cells show a V-shaped (analog) electro-optic response.

Characterization of the Local Layer Structure of a Narrow Wall in a Surface Stabilized Ferroelectric Liquid Crystal Using Synchrotron X-Ray Micro-Diffraction

Synchrotron X-ray microbeam small angle diffraction experiments were carried out for the characterization of the local layer structure of the narrow wall of a zig-zag defect in a surface stabilized ferroelectric liquid crystal cell. A series of the rocking curve (ω) and the azimuthal (χ) intensity distribution profiles were measured as a function of the vertical position across the narrow wall. The local layer was deflected at the wall in the χ direction. For the inclined narrow wall making an angle of a few degrees to 10° with respect to the rubbing direction, the deflected layer bent in both ω and χ directions. At the narrow wall running parallel to the rubbing direction, the layer bent only in the ω direction. The local layer structure was discussed in relation to the surface anchoring effect and a uniformly bent layer structure was proposed to explain experimental results.

Influence of ions on the "V-shaped" electro-optic response of ferroelectric liquid crystals.

It has recently been shown that in surface-stabilized ferroelectric liquid crystal cells with high spontaneous polarization, the polarization charge self-interaction leads to a "V-shaped" optical response to an applied voltage. The presence of ionic free charges in the liquid crystal changes the internal electric field, and therefore also the spatial dependence of the polarization and optic axis orientation. We have numerically solved the nonlinear ion diffusion equation in the electric field due to external voltage, spontaneous polarization, and ions, and calculated the electro-optic response to a triangular applied voltage. When the period of the driving voltage is smaller than the zero-field diffusion time of the ions across the cell, and larger than the ion transit time in the applied field, an inverse hysteresis in the electro-optic response is obtained. At these time scales ions also cause characteristic changes in the shape of the electro-optic response.

Low-Frequency Relaxation Modes of Surface-Stabilized Ferroelectric Liquid Crystal

Very broad low-frequency dielectric data of a surface-stabilized ferroelectric liquid crystal (SSFLC) sample in the frequency (ω) domain was analyzed by the Tikhonov regularization method to derive three distinctive modes of relaxation in time (τ) domain spectra corresponding to the X-, D- and L-modes.

Low Frequency Dielectric Relaxations in Surface Stabilized Ferroelectric Liquid Crystal

Three relaxation modes are identified in the surface stabilized ferroelectric liquid crystal from the Tikhonov regularization analysis of the low frequency dielectric data. The field strength dependence of the relaxation frequency and mode strength for each relaxation mode was studied in detail so that we could help better clarifying for the characteristic relaxation mechanisms of the three low frequency modes recently found by various authors.

Evaluation of structural and adhesive properties of nylon 6 and PTFE alignment films by means of atomic force microscopy

Atomic force microscopy (AFM) has become a powerful technique for submicron investigation of surface properties. In this work we use the capability of this technique to investigate dielectric films used to align ferroelectric liquid crystals (FLC). In fact, the final performance of a surface stabilized FLC (SSFLC) flat panel display strongly depends on the alignment layer properties and quality. This work focuses on a comparison of two alignment films: the more conventional polyamide, nylon 6, and polytetrafluoroethilene (PTFE, commercially known as Teflon), only recently used as a new aligning material.

Temperature Dependence of Low Frequency Dielectric Constants in Surface-Stabilized Ferroelectric Liquid Crystal

We have studied the temperature dependence of two low-frequency dielectric relaxation modes in a surface-stabilized ferroelectric liquid crystal (SSFLC): space charge accumulation and X modes. From investigation of Cole–Cole plots the temperature dependence of dielectric strength Δε and relaxation frequency ωr for the two low-frequency modes was obtained. The relaxation frequency follows the Arrhenius relation in the respective asymptotic regions far from the phase transition temperatures. From the Arrhenius plots the activation energies for the space charge accumulation mode and the X-mode were obtained as 0.42 eV and 0.13 eV, respectively. Temperature dependence of rotational viscosity in the smectic C* (SmC*) phase conformed with the temperature dependence of the X-mode parameters in the effective Debye relaxation regime, which implicates a common origin of azimuthal molecular rotation for both the Goldstone mode and the X-mode. However, the X-mode is irrelevant to the symmetry breaking phase transition and continues to be observed at all temperatures far from the phase transition.

Mode analysis of low frequency dielectric losses in surface stabilized ferroelectric liquid crystal

Low frequency dielectric constant data of a surface stabilized ferroelectric liquid crystal (SSFLC) sample was analyzed by use of the Tikhonov regularization method to confirm three distinctive modes of relaxation corresponding to the X-mode, D-mode, and L-mode. The mode dependence on the probe field strength was studied in detail for two different cases of surface treatments: one with the indium tin oxide (ITO) coating only and the other with polyimide dressing and rubbing alignment on top of the ITO coating. Origins of the three relaxation modes were discussed on the basis of the observed effects of the different surface treatments.

Bistable switching in nematic cells with a ferroelectric alignment film

In this paper, it will be presented that a nematic cell with a ferroelectric liquid crystalline thin film in senes realises a bistable switching as seen in the surface-stabilised ferroelectric liquid crystal (SSFLC) display devices. Such a bistability is found to critically depend on the squareness parameter of the FLC film as well as such matenal properties of the nematics, e.g. the elastic and the spontaneous polarization constants. It is also found that there exists an appropriate elastic constant to improve the optical transmittance. The fundamental electro-optic properties of the presently proposed nematic cells imply the advantage beyond the conventional nematic display devices with no memory effect.

The effect of substrate surface smoothness on the chevron layer structure of surface-stabilized ferroelectric liquid crystals

For freedom from zigzag defects in SSFLCD, it is important to research in detail the effect of surface smoothness of substrate on the chevron layer structure. We have studied the effect of surface roughness of undercoating layer on the surface smoothness of alignment film and then the alignment structure of SSFLC. Furthermore, we suggest a model for explaining the freedom from defects in a SSFLC by investigating theoretically the free energy of Cl and C2 chevron structure.

High voltage multichannel wave form generator for liquid crystal research

The article describes a wave form generator designed primarily for experiments on addressing all kinds of liquid crystal displays. It can also be used in any application requiring several simultaneous sources of high-voltage arbitrary pulse trains. The instrument has eight channels capable of pulse amplitudes of ±100 V at a slew rate better than 300 V/μs. Its design differs significantly from a typical arbitrary wave form generator. First and foremost the wave forms are directly constructed from pulses with variable width as well as amplitude. Two interchangeable memory banks guarantee transient-free adjustments of generated wave forms. The generator is computer controlled with well integrated software and provides all functionality to assist in the creation of wave forms and experiments with liquid crystal addressing schemes in a straightforward and intuitive way. To point out its versatility we discuss a new mode of operation intended primarily for generation of analogue gray shades on a surface-stabilized ferroelectric liquid crystal display. The instrument creates conditions corresponding to driving a display of virtually any size with image frames changing at video rate.

Monostabilization of Surface-Stabilized Ferroelectric Liquid Crystal Using Polymer Stabilization

Surface-stabilized ferroelectric liquid crystals (SSFLCs) fabricated by adopting polymer stabilization in terms of the UV photocure of doped photocurable monoacrylates with mesogenic side chains at a temperature where the LC medium is in the SmC* phase under the simultaneous application of a monopolar electric field, exhibit monostable electrooptical characteristics with grayscale capability without threshold. We suggest a model for explaining the monostability of this polymer-stabilized SSFLC by investigating theoretically the free energy of the SSFLC before and after the photocure.

Fabrication of Zigzag Defect-Free Bistable SSFLCD by Rubbing Method: a Material Study and Physical Modelling

A surface-stabilized ferroelectric liquid crystal display (SSFLCD) fabricated using a developed polyimide (PI) alignment film (RN1199, Nissan chem. Ind.) demonstrates an excellent bistable memory capability with high contrast ratio owing to zigzag defect free. In order to clarify why PI-RN1199 film is so useful for fabricating defect-free SSFLCD, we have characerized PI-RN1199 film in comparison with other conventional PI films by observing the surface morphologies of rubbed PI films with an atomic force microscope (AFM). From the results of characterization of PI films, it is concluded that the formation of C2-uniform structure may be attributed to the smoothness of the surface of PI-RN1199 with a small pretilt angle.

Investigation of the thickness mode in surface stabilized ferroelectric liquid crystal cells

Dielectric properties of surface stabilized ferroelectric liquid crystal (SSFLC) cells have been investigated both experimentally and theoretically. The thickness mode is found to consist of two relaxation processes. A decomposition of an analytical solution of the complex dielectric permittivity epsilon(omega) in terms of these processes is given. The experimental results show agreement with theory and lead to the conclusion that the thickness mode can be separated into two processes. It has been found that the ratio of the dielectric strengths for the two processes depends on the cell thickness, and that the ratio of their frequencies is approximately equal to 10. The lower frequency process is assigned to the relaxation within the bulk and the higher frequency process is assigned to the fluctuations of molecules at the two surfaces of the cell. The latter process is seen only in thicker cells for the reason that anchoring at surfaces predominantly controls the behaviour of thinner cells.

Electro-optic characteristics of surface-stabilized ferroelectric liquid crystal devices with the stripe-shaped domain structure

The stripe-shaped domain (SSD) structure has been obtained in the initial ferroelectric liquid crystal (FLC) alignment after the doping of a naphthalocyanine compound into rubbed polyimide films. Pre-tilt angles on the aligning films have been measured. The memory capability of aligned SSFLC cells has been enhanced nearly to completion, and contrast ratios reached 86:1 with the appearance of the SSD structure. Thus fabricated, 64 64 electrically controlled FLC spatial light modulator has better electro-optic characteristics and stability than a non-doped device.

Simultaneous addressing of two rows in SSFLC displays

A new addressing technique for driving Surface-Stabilized Ferroelectric Liquid Crystal (SSFLC) displays by selecting two rows at a time is proposed. It combines the ideas from fast addressing modes of SSFLC with multiline selection of rms responding devices. While fast addressing schemes need just a fraction of response time to drive a display line, the frame refresh rate can be further doubled by simultaneous addressing of two rows. The technique has been verified experimentally and the results are presented.

Optical Switching of a Ferroelectric Liquid Crystal Spatial Light Modulator by Photoinduced Polarization Inversion

Surface-stabilized ferroelectric liquid crystals(FLCs) have enabled the development of spatial light modulators (SLMs) with fast response times, high viewing contrasts, and orientational bistability, which allow digital information to be stored in a FLC-SLM film. It is shown that optical switching of an FLC-SLM can be achieved intrinsically with a visible light source using the chiral thioindigo dopant shown in the Figure.

A combined method of multiple-reflection and optical-wave-guide Raman scattering spectroscopies for studying the electric-field-induced reorientation of a ferroelectric liquid crystal cell

A combined method of multiple-reflection and optical-wave-guide Raman spectroscopies was proposed for studying the electric-field-induced reorientation of a surface-stabilized ferroelectric liquid crystal (FLC) cell. The method allowed us to measure selectively Raman scattering (RS) spectra from the FLC in a bulk phase and those from the FLC near an SiO x alignment layer. Time-resolved RS spectra from the two parts were measured by applying step-wise external electric fields, indicating that the FLC near the alignment layer exhibits a retarded response to the field compared to that in the bulk phase. The results proved the usefulness of the proposed method.

The Effect of Polymer Stabilization on the Alignment Structure of Surface-Stabilized Ferroelectric Liquid Crystals

It has been shown that a polymer network, formed in the SmA phase by UV photocure of photocurable liquid crystalline monoacrylates that are doped in a surface-stabilized ferroelectric liquid crystal (SSFLC), eliminates zig-zag defects in the SmC* phase, Since a chevron layer structure is transformed into a quasi-bookshelf layer structure by the polymer main chain. In this study, we have observed the electrooptical (EO) effect of this polymer-stabilized SSFLC (PS-SSFLC) with the aim of exploring the effect of the liquid crystalline polymer network on the alignment structure of FLC molecules. As experimental results, it was found that liquid crystalline side chains bond strongly with a main chain and attract FLC molecules in the rubbing direction. Furthermore, we propose and demonstrate a new PS-SSFLC fabricated also by UV photocure of doped monoacrylates particularly at a temperature where the LC medium is in the SmC* phase under the simultaneous application of a monopolar electric field. This device exhibits monostable EO characteristics with a high contrast ratio owing to being defect free, having grayscale capability without threshold, and a fast response speed.