Surface-stabilized Ferroelectric Liquid Crystal Research Articles

Inverse switching phenomenon of surface-stabilized ferroelectric liquid crystals under a high voltage region

A study of the electro-optie response of a surface-stabilized ferroelectric liquid crystal device has been made for the high drive voltage region. We find an inverse switching memory effect in the polarity to be built up after the trailing edges of the applied voltage pulses. The inverse switching memory effect is observed by applying higher and longer voltage pulses. The effect is considered to be produced by the internal electric field produced by ions, and the internal electric field strength depends on the thickness of the orientational film.

Effect of spontaneous polarization on smectic C*-smectic A* phase transition temperature and the thickness dependence of the spontaneous polarization of ferroelectric liquid crystal

The influence of spontaneous polarization on the smectic C*-smectic A* phase transition was studied theoretically by means of Landau theory. calculations that the transition temperature from a non-chiral to chiral smectic C phase is shifted due to the effect of bilinear and biquadratic couplings. A qualitative relationship has also been derived between spontaneous polarization and cell thickness which supports the experimental observations of the increase of spontaneous polarization with the increase of cell thickness for surface-stabilized ferroelectric liquid crystals. It was observed from the theoretical

Dynamic Molecular Reorientational Behavior Induced by Polarization Inversion of Ferroelectric Liquid Crystal

Abstract In order to clarify the rotational direction of molecular reorientation induced by a polarization inversion of ferroelectric liquid crystal (FLC), an electric current along the direction perpendicular to both an electric field and the smectic layer normal was measured for the first time for the FLC cell with a thickness of 50 μm. The electric current curve against time upon the application of an a.c. electric field with a form of triangular wave exhibited a peak attributed to a polarization inversion. The sign of the electric current peak value was inverted by the polarity inversion of an electric field, indicating that the LC molecules rotate in the same direction on the tilt cone upon polarity inversion, that is one-way rotation. The same result was given by polarizing optical microscope observation of the director change during the polarization inversion in a free-standing film. This result differs from that for a surface-stabilized FLC cell.

Novel liquid crystal mixtures for a surface-stabilized ferroelectric LCD

We synthesized novel fluorine-substituted chiral compounds having a fluorine atom at an asymmetric carbon and a difluorinated biphenyl ring as a core of a liquid crystal molecule by means of an original method. The ferroelectric mixtures were obtained by mixing the chiral compounds with the non-chiral liquid crystal mixture with a wide SmC temperature range between 29 C (crystallized) and 78 C. They show a large P s greater than 10 nC cm -2 at 5 wt% of the chiral compound. Two types of cell, called 'parallel' and 'antiparallel', were fabricated according to the relative direction of the rubbing direction on the substrates. The parallel cells filled with the FLC mixtures show the usual bistable SSFLC (surface-stabilized ferroelectric liquid crystal). The fast response time of 60mus (22V pp pulse width 250mus, at room temperature) was obtained. The apparent cone angle was 45.2 in the switching state and 40.5 in the memory state. On the other hand, the antiparallel cells show an unusual monostable behaviour, i.e. the director falls back to the original configuration when the applied voltage is switched off. The surface-stabilized monostable cells show very attractive characteristics for application for a TFT-active matrix LCD; a high contrast of 81 :1, a fast response time (of the order of 1ms), and an analogue-grey scale with excellent linearity within the low voltage range below 4 V.

Threshold Properties of SSFLC in Terms of Polarization Structures

Bistability of the surface stabilized ferroelectric liquid crystal (SSFLC) is one of the most important factors of display perfomances. The electrooptical switching is based on the bistability which provides threshold characteristics to the SSFLC display device. The threshold properties of the SSFLC device which has a layer structure of liquid crystal molecules and provides an in-plane molecular switching are assumed to be governed by not only a surface anchoring but also a bulk structure itself. We show some experimental results which suggest a contribution of ferroelectric liquid crystal's bulk structure to the electrooptical threshold properties. Layer structures: the bookshelf and the chevron geometries are one of the main bulk factors. Our experimental results clarified that the polarization switching behavior was intrinsically different between the bookshelf and the chevron structures. The bookshelf geometry also shows a possibility of gray shades.

Switching of pixels in surface-stabilized ferroelectric liquid crystal cells

Switching of pixels in surface-stabilized ferroelectric liquid crystal cells is studied. We have performed numerical calculations in two spatial dimensions on an adaptive grid to determine solutions for the scalar fields of the director twist angle and the electric potential. In the bulk we employ the full Frank-Oseen energy (i.e. no one-constant approximation) with the addition of electric terms represented by a uniaxial dielectric tensor and the spontaneous polarization. On the surface the director is anchored by a Rapini-Papoular potential. An idealized bookshelf-configuration with fixed tilt angle of the director is assumed. During both switching on and off disclination pairs are created on the surface. These disclinations form the ends of Bloch walls which bound the pixel. In the range of medium voltages after switching on an internal wall remains within the pixel which leads to an inhomogeneous director field and complicates switching off.

Alignment of Surface Stabilized Ferroelectric Liquid Crystal Cell on Obliquely Evaporated Silicon Monoxide Film

An alignment method for obtaining uniform surface stabilized ferroelectric liquid crystal (SSFLC) cell is presented. The uniform alignment is realized by ultrathin (<80Å) silicon monoxide (SO) film evaporated at 80° and the treatment of A. C. electric field. The alignment effects of ultrathin SiO film, thick SiO film(> 1000Å) both evaporated at 80°C and the SiO film evaporated at 60° on the FLC are described. The layer direction of FLC cell with ultrathin SiO film at 80° is perpendicular to that with thick film at the same deposition angle. The advantage of the ultrathin film is that the SSFLC cell aligned by it can be easily treated to perform perfect bistability, excellent contrast and fast switching speed. An possible orientation mechanism on surface of oblique evaporation film is discussed.

Numerical study of electromagnetic wave propagation in twisted birefringent layers by the spectral moments method

This paper presents a first attempt at using the spectral moments method (SMM) to solve Maxwell's equations in twisted anisotropic media in the presence of defects. This numerical method, previously developed in condensed matter physics, allows computation of Green functions for very large systems. The dynamic matrix of the discretized system is built from the medium parameters. Green functions, calculated for a given source, representing a point source at infinity and given receiver, are developed as a continued fraction whose coefficients are related to the moments and directly computed from the dynamic matrix. In this study we compute the light transmitted through thin surface-stabilized ferroelectric liquid crystal cells with a chevron structure and a twisted director distribution. The efficiency and accuracy of the method are analysed by comparing the results obtained by SMM with the analytical solution obtained using the Jones matrix formalism. Finally, we apply SMM to compute the transmitted light with different director configurations. We show, by comparisons with experimental data, that the simplest director configuration is certainly the most probable.

Light-induced director axis reorientation in a dye-doped ferroelectric liquid crystal and its application to self-aligned spatial filtering

The light-induced director axis reorientation in a surface-stabilized ferroelectric liquid crystal is investigated experimentally. Several-order-of-magnitude enhancement in light sensitivity and response speed of the director axis reorientation is achieved by incorporation of dichroic dyes into the liquid crystal. A novel scheme for self-aligned spatial filtering by use of the dye-doped liquid crystal is also proposed and is demonstrated with milliwatt laser powers and millisecond updating times.

Self-screening of the ferroelectric polarization in smectic C* cells in the bookshelf geometry

Surface stabilized ferroelectric liquid crystal (SSFLC) displays made of chiral smectic C* liquid crystals often present screening effects generally discussed in terms of the depolarizing field produced by ions attracted close to the electrodes. Here, we analyze the screening properties of distortions in the director field that form close to the electrodes because of the anchoring conditions. We show that the ferroelectric polarization effectively measured in such SSFLC cells is smaller than the real one, and depends on the spacing distance between the plates. Corrections are, therefore, necessary for samples of thickness &amp;lt;10 μm. This screening mechanism, which moreover increases the bistability of the cell, is as fast as the polarization reversal itself contrarily to the screening due to ion migrations.

Generalized dynamic domain shape calculation in ferroelectric liquid crystals.

We have developed an analytical algorithm analogous to the Wulff construction for calculating the spatial evolution of growing two-dimensinal (2D) domains as a function of time for arbitrary starting shapes. Applying the method to polarization domain growth in surface-stabilized ferroelectric liquid crystals, we can derive the full 2D shapes of polarization domains with curved walls from the directional variation of the velocity of planar walls as measured by computer simulations in only 1D. It is shown that the domain shape is determined in the present model by the anisotropic nematic elastic properties of the material. \textcopyright{} 1996 The American Physical Society.

Tolane cinnamates—a new class of stable, high birefringence dopants

Abstract Ferroelectric smectic C* liquid crystals have been shown to exhibit high speed, electro-optic switching when incorporated into the surface stabilized ferroelectric liquid crystal (SSFLC) light valve. Certain applications utilizing IR modulation, waveguide, and phase modulation, and certain fiber-optic devices, can benefit from materials with a birefringence higher than that typically seen in FLC materials (Δ≊ 0.15). A new class of compounds, the tolane cinnamates, was developed to help meet this need. They were made by attaching a cinnamate moiety to the well known high birefringence tolane core. This resulted in two improvements: first, the birefringence increased from 0.23 to over 0.3; and second, the tolane cinnamate system was significantly more stable to UV light than the parent tolane system. The new materials showed smectic C as well as nematic phases and demonstrated good compatibility with both nematic and FLC hosts.

Molecular orientation and color alternation of surface stabilized ferroelectric liquid crystal cell

Molecular orientation configurations across zig-zag defects and thin-line defects of the surface stabilized ferroelectric liquid crystal (SSFLC) cell are presented. The transmission spectra of the SSFLC cell of non-uniform states are numerically calculated using the asymmetric chevron structure model. The calculations agree well with experimental results, which confirms the continuity of the N-director across the chevron interface. The color alternation of the SSFLC cell between crossed polarizers is well explained.

Pretilt angle inducing mechanisms and surface-stabilized ferroelectric liquid crystals

Abstract We have investigated the role of fluorine containing groups and of alkyl side chains in the generation of pretilt angles. From X-ray photoelectron spectroscopy it was concluded that the fluorine-containing groups force the polyimide to adapt a certain conformation and that this conformation governs the pretilt angle. The effect of alkyl side chains was studied by modification of the surface of poly(vinylalcohol) with alkanoyl chlorides. The pretilt angle was found to depend on the length and surface density of the grafted chains. A uniform and tilted orientation could only be obtained by grafting long alkyl chains. The influence of the pretilt angle on the effective cone angle in ferroelectric liquid crystals has been calculated and was found to agree reasonably well with the experimental values. Angles between the memory states of 40° have been obtained leading to brightness levels comparable to those of the twisted nematic effect.

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

The local layer structure of the broad wall of a zig-zag defect in a thin-surface stabilized ferroelectric liquid crystal cell was characterized using a synchrotron X-ray microbeam of less than 5 µm spatial resolution. By using a rocking curve measurement at the broad wall, multiple or broad peaks were observed between a pair of peaks due to a chevron structure. These new peaks are clear evidence of a modified pseudo-bookshelf structure at the wall. For 1.5 µm thick cells, a bookshelf layer is relatively flat, but is accompanied by small areas of inclined layer connecting the bookshelf and the chevron structures. For 10 µm thick cells, the pseudo-bookshelf structure bends or undulates both perpendicular and parallel to the rubbing direction. No appreciable change in the layer spacing was observed in the modified pseudo-bookshelf structure. The temperature dependence of the broad wall layer structure was also measured.

Computational Calculations on the Chevron Structures in Ferroelectric Liquid Crystals

Abstract In this paper we present numerical results for the chevron structure in a Surface Stabilized Ferroelectric Liquid Crystal (SSFLC)1 sample. Using a phenomenological expression of the Sc bulk free energy and the numerical relaxation method, we calculate the spatial distribution of the director and the tilt of the layer in the chevron for twisted and uniform states for different applied voltages. In particular the results confirm the asymmetrical twist that has been observed in recent experiments.2,3 The calculations take into account specific boundary conditions for both parameters with an applied electric field.

The relationship between the polar anchoring strength and the memory capability of surface-stabilized ferroelectric liquid crystals on rubbed polyimide surfaces

Abstract The relationship between the polar (out-of-plane tilt) anchoring strength and the memory capability of surface-stabilized ferroelectric liquid crystal displays (SSFLCD) with rubbed polyimide (PI) surfaces as alignment layers has been investigated. The polar anchoring strength of nematic liquid crystals (NLC) on rubbed PI surfaces with low polymer concentration is weak compared to that on PI surfaces with high polymer concentration. However, we have found that the memory capability of SSFLC on rubbed PI surfaces with low polymer concentration is high compared to that on rubbed PI surfaces with high polymer concentration. It is expected that the weak anchoring strength is attributed to the good memory capability of the SSFLCD on rubbed PI surfaces.

Electric Field Effect on Layer Structure in Surface-Stabilized Ferroelectric Liquid Crystals

Electric field dependence of the layer structure in surface-stabilized ferroelectric liquid crystals (SSFLCs) was investigated by means of an X-ray diffraction experiment. With increasing amplitude of the applied electric field (square wave, 20 Hz), in the SmC* phase, the layer structure in our SSFLCs changed from the chevron layer structure to bookshelf layer structure. In the SmA phase (high temperature than the SmC* phase), the tilted bookshelf layer structure changed to a nontilted bookshelf one. The layer structure in the SmC* phase changed from an “asymmetric” chevron layer structure to “symmetric” chevron layer structure when the samples were cooled from the SmA phase. The application of electric field to SSFLCs in the SmA phase induced the well-oriented bookshelf layer structure, and it is also useful to obtain the symmetric chevron layer structure in the SmC* phase.

Computer simulation of domain growth in ferroelectric liquid crystals.

The electro-optic response of a surface-stabilized ferroelectric liquid crystal (SSFLC) cell is determined largely by the dynamics of polarization reversal at the chevron interface. This process involves the nucleation, growth, and ultimate coalescence of polarization reversal domains. These domains are generally faceted, being either polygonal or characteristically boat shaped. We have performed computer simulations in two dimensions (2D) of field-driven domain growth at the chevron interface in SSFLCs. By including elastic anisotropy and the orientational binding of the chevron in the equation of motion, we get anisotropic domain growth and, for a range of applied field strengths E, partially faceted domain shapes. The measured growth rates have the same field dependence as is seen experimentally, the domain area increasing as A\ensuremath{\propto}(Et${)}^{2}$. We have also developed an analytical algorithm similar to the Wulff construction for calculating the spatial evolution of 2D domains as a function of time for arbitrary starting shapes. In the present case of ferroelectric domain growth, we can derive full 2D domain shapes from the directional variation of the velocity of planar walls as measured by computer simulations in only 1D.

Study of Molecular Orientational States and Dielectric Properties of Surface Stabilized Ferroelectric Liquid Crystals

Abstract The study of the molecular orientational states and the dielectric properties of surface stabilized ferroelectric liquid crystals (SSFLCs) was reviewed. The molecular orientational models were presented and they gave us a total understanding of the orientational states which appear in SSFLCs with parallel rubbing. Using the molecular orientational models, the dielectric properties of SSFLCs were studied. Three dielectric permittivities were measured for each of two samples, the permittivity of the homeotropic cell and the permittivity of the planar homogeneous cell with and without the DC bias, and then the dielectric tensor components were calculated based on the molecular orientational models. Results for Merck SCE-8 are reported. The novel frequency dependence of the dielectric biaxiality was found. The sign of the dielectric biaxiality inverted around 1kHz, being negative at low frequencies and positive at high frequencies. The relationship between the dielectric behavior of SSFLC cells and t...