Surface-stabilized Ferroelectric Liquid Crystal Cells Research Articles

Electrical properties of ferroelectric liquid crystals during thermal phase transition

The ferroelectric properties of ferroelectric liquid crystals (FLCs) during thermal phase transition in the surface-stabilized ferroelectric liquid crystal (SSFLC) cells were examined. The coercive electric field (Ec) and remanent polarization (Pr) changed with increasing temperature. These temperature-dependent variations of Ec and Pr were in agreement with the differential scanning calorimetry results. Moreover, the fatigue characteristic of the SSFLC cells was examined. No polarized fatigue was observed after 1 × 106 cycles of polarization switching, because FLCs may have the self-repair property for structural and interfacial defects.

Solitons in surface stabilized ferroelectric liquid crystals and the determination of the twist elastic constant

Excitation of solitary waves and their propagation in surface-stabilized ferroelectric liquid crystal cells under alternating external electric field is investigated both theoretically and experimentally. The effect of solitary waves on electro-optic response spectra is analyzed for different amplitudes of applied fields, temperatures, and sample thicknesses. It is shown that solitons can only be excited within narrow ranges of frequencies of the sufficiently strong electric fields. The minimal frequency, at which soliton waves appear in ferroelectric smectic liquid crystals, is found to be related to the material constants of these systems. It is proved that measuring this threshold frequency gives the possibility to determine one of the material parameters, if the others are known. In this way, the intra-smectic-layer elastic constant is found for systems with the chevron geometry.

The Impact of Static Disorder on Vibrational Resonance in a Ferroelectric Liquid Crystal

Vibrational resonance manifests itself in certain nonlinear systems that are subject to the action of a biharmonic force with very different frequencies. A proper frequency and/or amplitude of the high-frequency harmonic signal can lead to a resonantly enhanced response of the system with respect to the low frequency signal. In the present study we extent the concept of vibrational resonance to a thermotropic bistable surface-stabilized ferroelectric liquid crystal cell. Furthermore, we systematically investigate the impact of different origins of static disorder and reveal that different kinds of static disorder affect the resonant response in a qualitative different way.

Tailoring switching properties of dipolar species in ferroelectric liquid crystal with ZnO nanoparticles

The capacitance–voltage (C–V) characteristics of surface-stabilized ferroelectric liquid crystal (SSFLC) cells doped with zinc oxide nanocrystals (nc-ZnO) have been studied and fitted to the Preisach model to reveal the behaviours of dipolar species. To explain the experimental results, we propose a physical picture depicting an interaction of nc-ZnO dipole with surrounding SSFLC dipolar species. Our study shows that doping nc-ZnO into FLC not only increases the contrast ratio of the cells by 10 times but also enhances the dynamic polarization of the beneficial dipolar species while minimizing the polarization response of the species with counter effects.

Ferroelectric Liquid Crystal Cell With Phase Separated Composite Organic Film

We researched ferroelectric liquid crystal cells (FLCs) with a phase separated composite organic film (PSCOF) fabricated by photocuring a pre-polymer NOA-65. The conventional surface-stabilized FLC cell shows the so-called bistable hysteresis loop of electrooptical effect, whereas the PSCOF-FLC cell demonstrates monostable half-V shaped electrooptical characteristics. In this study, we tried to make this origin clear and fabricated some types of PSCOF-FLC cells under different photocuring conditions. As a result, it is found that NOA-65 has a minus electron charge and then the monostable FLC medium may be realized due to the bias electric field generated by NOA-65 film.

Alternating twist structures formed by electroconvection in the nematic phase of an achiral bent-core molecule

We report an unusual electroconvection in the nematic phase of a bent-core liquid crystal. In a voltage-frequency diagram, two frequency regions exhibiting prewavy stripe patterns were found, as reported by Wiant We found that these stripes never show extinction dark when cells were rotated under crossed polarizers. Based on the color interchange in between neighboring stripes by the rotation of the cells or an analyzer, twisted molecular orientation is suggested; i.e., the directors are alternately twisted from the top to the bottom surfaces with a pretilt angle in adjacent stripes, which is an analogue of the twisted (splayed) structure observed in surface-stabilized ferroelectric liquid crystal cells. The transmittance spectra calculated using the 4x4 matrix method from the model structure are consistent with the experimental observation.

Bistability in deformed helix ferroelectric liquid crystal

Bistability in deformed helix ferroelectric liquid crystal (DHFLC) has been investigated by electro-optical and dielectric methods and compared with the bistability phenomenon in surface stabilized ferroelectric liquid crystal (SSFLC). Bistability in DHFLC as well as SSFLC is based on different physical phenomena. In SSFLC, it occurs due to surface effect and in DHFLC, by the critical deformation of the helix, under the application of time delayed square voltage pulse of known magnitude and frequency. Memory behavior of DHFLC and SSFLC cells has been compared by optical, electrical, dielectric, and hysteresis methods and the difference has been analyzed. It has been observed that memory effect in DHFLC is independent of surface effect but critically depends on the applied voltage and frequency of time delayed square wave pulse.

Grating Based on Polymer-Stabilized Ferroelectric Liquid Crystal Films

A surface-stabilized ferroelectric liquid crystal (SSFLC) film, doped with a small amount of monomer, BAB-6, was fabricated. The alignment performance and electrooptical effect of SSFLC films polymerized under different polarities of the DC field were examined. A grating was then fabricated by curing a doped SSFLC cell with UV through a photomask. The grating thus formed was found to be a combination of a phase grating and an amplitude grating, between which the grating could be switched by applying a suitable voltage. The response time was in the order of 500–600 µs.

Profiles of the Director Across a Surface-Stabilized Ferroelectric Liquid Crystal Cell and the Corresponding Pyroelectric Current Spectra

The pyroelectric technique, called the laser-intensity-modulation method (LIMM), and a simple model of the surface-stabilized ferroelectric liquid crystal (SSFLC) structure were used for the investigation of the spatial distribution of the pyroelectric coefficient and the director profile in a ferroelectric liquid crystal (FLC) cell. The pyroelectric current from a cell, illuminated by the intensity-modulated laser beam, was measured as a function of the modulation frequency. The simulated pyroelectric current spectra were obtained from the spatial profiles of the pyroelectric coefficient calculated for SSFLC structures with different boundary conditions. The influence of the space charge of the free ions on the distribution of the azimuthal angle of the FLC director was investigated.

Effect of spontaneous polarization and polar surface anchoring on the director and layer structure in surface-stabilized ferroelectric liquid crystal cells.

We use the Landau-de Gennes model to study theoretically the effect of the magnitude of the spontaneous polarization (P(S)), the ratio (r) between the equilibrium layer tilt and the smectic cone angle, the thickness of the insulating alignment layers and the strength of the polar and nonpolar surface anchoring on the director and layer structure in surface-stabilized ferroelectric liquid crystal cells with the chevron structure of smectic layers. The system shows a surprising number of stable structures, accompanied by one or two metastable ones. At P(S) greater than the critical value only quasimonostable structures, which can exhibit the thresholdless (V-shaped) switching, exist at all r and both at weak and strong polar surface anchoring. At lower P(S) bistable and monostable structures can coexist. Bistable structures can be expected at high r, low P(S) and if polar surface anchoring is weaker than the nonpolar one. Lowering the ratio r and/or increasing the strength of polar anchoring promotes the stability of monostable structures. Thicker insulating alignment layers also drive the system into the monostable state. Polar surface anchoring induces a large surface electroclinic effect. As a result the nematic deformations close to the surfaces are very strong and the stress is relieved by bending of the smectic layers. This leads to the formation of a double chevron structure which is stable at very large polar anchoring and due to the surface electroclinic effect it is metastable also at lower values of polar anchoring.

Characteristics of Monostability in Asymmetrically Treated Surface-Stabilized Ferroelectric Liquid Crystal Cells

Surface-stabilized ferroelectric liquid crystal (SSFLC) with monostable state in asymmetrical cells was studied. We investigated some kinds of SSFLC cells which were treated asymmetrically with two substrate surfaces using some liquid crystal materials including isotropic–nematic–smectic A–smectic C (INAC) phase-sequence materials and short-pitch bistable ferroelectric liquid crystal (SBFLC). The C1-uniform state, the quasi-bookshelf state of INAC materials and SBFLC showed monostability in asymmetrical cells. We compared the characteristics of monostability in some asymmetrically treated cells which have an insulating layer only on one substrate. We also investigated the effects of spontaneous polarization on the monostability. We discuss the origins of monostability in asymmetrical cells referring to experimental results.

Measurement of the polarization profile across a surface-stabilized ferroelectric liquid crystal cell using the pyroelectric laser-intensity-modulation method

The pyroelectric technique, called the laser-intensity-modulation method, was used for investigations of the spatial distribution of the pyroelectric coefficient that gives information about the director profile in ferroelectric liquid crystal (FLC) cells. The pyroelectric current from a cell, illuminated by the intensity-modulated laser beam, was measured as a function of the modulation frequency. The Tikhonov regularization procedure was used for the deconvolution of the pyroelectric current spectrum into the spatial profile of the pyroelectric coefficient. It was found that in the surface-stabilized ferroelectric liquid crystal (SSFLC) structure with a splayed director profile, the flexoelectric contribution to the local polarization has comparable magnitude with the spontaneous one. The effective flexoelectric coefficient of the investigated FLC material and the flexoelectric polarization of the SSFLC structure were determined.

Structure and dynamics of ferroelectric liquid crystal cells exhibiting thresholdless switching.

Surface-stabilized ferroelectric liquid crystal cells are usually bistable, with the spontaneous polarization either up or down, perpendicular to the plane of the cell. In materials with high spontaneous polarization the polarization charge self-interaction leads to a monostable structure that exhibits a thresholdless, "V-shaped" switching of the optical response to an applied voltage. With a model that includes the electrostatic self-energy of the spontaneous polarization, we examine the transition from bistablity to monostability as a function of the polar and nonpolar anchoring energies, the liquid crystal elasticity, and the thickness of the alignment layers. Two bistable and one monostable structures are possible. The bistable state is only obtained when the thickness of the insulating alignment layer and the polar anchoring energy are small, or when the spontaneous polarization is small. From linear stability analysis we get analytical expressions for the limits of stability of the structures. Numerical calculations show that in some ranges of the parameters two structures can coexist. We also present the calculation of the polarization fluctuation modes. The lowest one becomes soft at the continuous transitions among the structures and at the limits of stability.

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.

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.

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.

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.