Ferroelectric Liquid Crystal Cells Research Articles

The Effects of Ionic Fields and Previous Switched States on Domain Formation in Pulse Addressed Ferroelectric Liquid Crystal Cells

Abstract We discuss the large extent to which ionic impuritics and previous states influence the switching of ferroelectric liquid crystal cells. In particular we see that the ionic field built up in response to the molecular dipoles causes switching to correlate with the previous state. This memory effect is not crasable even when the cell is fully reset into a uniform state. Also the strength of the reset pulse influences the ionic field and generally causes enhancement of partial switching in the opposite direction. There also exists an anomalous region where this enhancement doesn‘t occur. This is apparently due to variation in the elastic stability even when the cell appears to be in a uniform relaxed state.

Electrohydrodynamic Origin of Striped Texture in Ferroelectric Liquid Crystal Cells

Abstract Evidence is presented that the striped texture associated with quasibookshelf structure formation is electrohydrodynamic in origin. Results are presented and a model proposed in which this particular striped texture is formed as a result of electrohydrodynamic instabilities similar to Williams domains.

Degradation of Optical Bistability in Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) Cells

We investigate the degradation of optical bistability (bistability sticking) in FLC cells with alignment layers of different thicknesses using the static optical hysteresis technique. By storing th...

Mechanical Vibrations of Smectic Cells Under Fast Field Reversal

Abstract It is known that ferroelectric liquid crystal cells vibrate mechanically under the effect of ac fields. The vibrations are mainly due to the backflow and electroclinic effects. For small director oscillations the effect is mainly linear and goes with the frequency of the field. For larger oscillations higher harmonics appear too. We studied the practically interesting case when, due to rectangular excitation, the director switches fully between two stable positions. In the experiments bookshelf textures of (R)-MHPOBC and FLC6430 (from Hoffmann La Roche) were used. The accelerations of the cover plates due to the reversal of the field were detected in three orthogonal directions. The switches cause strong vibrations of the plates (lasting less than a millisecond,. acceleration ∼10m/s2). The vibration is largest at the SmCα*-Sm A phase transition indicating that the dominating effect is electroclinic in origin.

Ionic influences on the formation and evolution of domains in ferroelectric liquid crystal cells

In this paper the large extent to which ionic impurities within a ferroelectric liquid crystal cell can influence the outcome of a partial switching sequence is discussed. Both memorisation and enhanced switching effects are observed and are a direct result of the redistribution of ions within the cell. Memorisation of the areas switched in a pixel occurs when the ions are allowed time to drift and compensate the field set up by the molecular dipoles in switched or unswitched areas. Enhanced switching occurs when large reset pulses are applied before an opposite polarity partial switching pulse. Both of these effects persist even though the elastic state of the cell prior to the switching pulse always appears the same.

Two Dimensional Modelling of Domain Nucleation and Growth in Surface Stabilised Ferroelectric Liquid Crystals

Abstract In surface stabilised ferroelectric liquid crystal (SSFLC) cells driven by low voltages, typically below 10V, it is observed that domains of the switched state nucleate and grow, merging to fill the cell. A model of domain nucleation in SSFLCs is proposed, which considers a nucleation site as a local absence of the chevron interface torque. The model is implemented in a two-dimensional simulation of the SSFLC SCE-8 in a chevron configuration with C2 alignment. It is shown to behave realistically over the voltage range 2-10V by comparison with experimental results.

Pyroelectric properties of ferroelectric liquid crystal cells with chevron, bookshelf, and helical structures

The pyroelectric properties of ferroelectric liquid crystal (FLC) cells possessing helical and surface stabilized structures have been investigated. Pyroelectric properties of a FLC cell depend on the applied voltage, the internal smectic layer structure, and the ratio of the cell thickness to the material pitch d/p0. For surface-stabilized FLC (SSFLC) cells, hysteresis of the pyroelectric signal was observed for both the chevron and the bookshelf structures and is explained by the intermediate domain states that arise from a distribution in the pretilt azimuthal angle. For the chevron structure a reverse hysteresis loop is observed which is explained by a stronger competitive effect of the ionic field compared to the surface anchoring forces. Cells containing helical structures are studied and for cells with a large d/p0 ratio a linear response of the signal to applied voltage is found. Possible implications of the results obtained for SSFLC cells for which the change in the spontaneous polarization in the decreasing voltage cycle occurs for a voltage lower than obtained during a voltage increasing cycle are discussed.

Flow Coupled Switching Equations for Surface-Stabilized Ferroelectric Liquid Crystal Cells

Flow-coupled switching equations are derived for surface-stabilized ferroelectric liquid crystal devices, starting from Leslie, Stewart and Nakagawa's continuum theory for smectic C liquid crystals. Comparisons with nematic like dynamic equations are made.

The Optics of Ferroelectric Liquid Crystals

Abstract Ferroelectric liquid crystals (FLCs) have been under investigation for use in electro-optic devices for over a decade. In this review the development of the studies of the optics of FLCs over this period is discussed. Starting from the first electro-optic switching studies undertaken in 1980 by Noel Clark and Sven Lagerwall, and the structure in a FLC cell proposed to explain the effects, the process of developing understanding is followed. The review concentrates on the structure revealed to be present in a FLC filled cell, as this is of central importance in the development of optical devices using this material. Problems encountered in device development, and the approaches taken to overcome them are discussed, including special surface alignments, AC stabilization, and treatment of cells with large electric fields. FLC continuum theory is also discussed as this is needed for calculation of the liquid crystal director profiles, and hence the determination of optical performance. More recent de...

Dielectric response of surface stabilized ferroelectric liquid crystal cells

The low-frequency dielectric response of the surface stabilized ferroelectric liquid crystal (SSFLC) cells has been investigated. It was found that the dielectric response depends on the type of the smectic layer structure present and the cell thickness. An analytical solution to the equation of motion that describes the dynamics of SSFLC cells in the presence of a weak external field for both bookshelf and chevron geometries has been developed. This result is used to calculate the dielectric response of these SSFLC cells. Experimental values of the dielectric relaxation strength and the relaxation frequency show good agreement with those predicted by the theoretical model for the chevron structure. An observed discrepancy between the theory and the experimental results for the bookshelf structure is discussed.

Optical second-harmonic generation during switching in a ferroelectric liquid-crystal cell.

We report measurements of the temporal dependence of the second-harmonic generation from a 50 \ensuremath{\mu}m thick homeotropically aligned cell of ferroelectric liquid crystal (SCE9) (British Drug House, Ltd.) exposed to a symmetric square wave form electric field. The field switches the molecular configuration between the two unwound states with the director on the opposite positions of the tilt cone. The response of the second-harmonic signal measured in the phase matching geometry for one of these equilibrium states shows that a complete switching process takes several seconds. In experimental geometries away from the equilibrium phase matching geometry a strong transient second-harmonic signal is observed during the switching. It corresponds to the transient realization of the phase matching condition related to the intermediate molecular configurations. The study of this effect provides a precise time resolved monitoring of the molecular dynamics during the switching. Our results show that the director reorientation in a hometropic cell goes on within numerous domains of a size much smaller than the optical wavelength. On the basis of this assumption the temporal dependence of the director reorientation within the domains is calculated.

Soliton switching in ferroelectric liquid crystals and their transient electro-optic response

The switching behavior of ferroelectric liquid crystals (FLCs) is studied both experimentally and theoretically. Experimentally the electro-optic response of multiwavelength-thick FLC cells revealed transient peak and dip characteristics. This behavior could not be explained at all according to the standard picture of a switchable birefringent plate, nor according to transient light scattering associated with ferroelectric domain reversal. Solutions of a numerical model of the FLC electric and elastic torques, which results in the sine-Gordon equation, showed the propagation of kink-antikink pair solitons. The nonuniform director structures corresponding to these solitons are sufficient to explain the experimental behavior of the transient electro-optic response with the wavelength, the field, and the polarizer-cell-analyzer orientation. The electro-optic response is found to be highly sensitive to the fixed boundary dipole orientations.

120 × 100 pixel antiblooming array based on optically addressed ferroelectric liquid-crystal cells

We describe the principle and the operation of a 120 x 100 pixel antiblooming array. Each cell in the array is made up of a hydrogenated amorphous silicon photoconductor layer driving a ferroelectric liquid-crystal shutter. The device is designed to be coupled with a CCD camera to prevent high-blooming overload and image lag.

Electric Field Induced Domain Formation in Surface Stabilized Ferroelectric Liquid Crystal Cells

Two types of domains have been observed for S sub(C) ferroelectric liquid crystals in surface stabilized cells (SSFLC) by application of a high electric field with the smectic layers tilted by the amount of the chevron angle with respect to the normal of the rubbing direction in the substrate plane. The layer structure resembles that of a chevron configuration in the plane of the substrate similar to the recently reported stripe-shaped SSFLC structure. The two domain types 'appear' to switch in a reciprocal fashion when applying an AC electric field and observed between crossed polarizers. The temperature dependence of this effect has been investigated and an explanation proposed analogous to a striped texture model.

Switching on stray electric fields in ferroelectric liquid crystal cells

Abstract In liquid crystal dot-matrix displays light may leak through the display area between the pixels. To obtain sufficient contrast this non-pixel area has to be made non-transmissive. For ferroelectric liquid crystal (FLC) displays this may be done by switching the material in the gaps between the picture elements to a non-transmissive state by the stray electric fields that occur during application of voltages to the pixel electrodes. This is experimentally studied for test cells with an electrically modified smectic layer structure. The gap region considered is an asymmetric environment of the FLC material, as the transparent conductive coating has been removed on one substrate, whereas on the other substrate a conductor covers the glass. The FLC molecules in the non-pixel area prefer to direct their dipoles towards the covered substrate. To switch the FLC material with the stray electric fields, it is a prerequisite to outweigh this preference. We made spatially resolved observations for various gap widths and various applied voltages on 2 μm thick FLC layers. With bipolar voltage pulses of 64 μs width each, amplitudes of about 25 V are needed to switch the FLC in 3·2 or 4·0 μm wide gaps. It was found to be more difficult to switch gaps that, are 7 μm wide than was anticipated on the basis of the results for 4 μm gaps. This is attributed to the surface polarization charge due to the FLC permanent dipoles built up at the FLC-glass interface. Experimental results supporting this explanation are presented.

Influence of backflow on the reorientation dynamics of ferroelectric liquid crystals.

The elasto hydrodynamic equations for coupled reorientation and flow in surface-stabilized ferroelectric liquid-crystal cells have been solved numerically and the results are compared to the case in which the flow effects are neglected. Our simulations show that flow generally makes the switching faster in the case of the dipoles being in an unstable equilibrium state in an applied field. Our calculations show that it is important to include the backflow in the reorientation dynamics in order to have the correct switching picture.

Electrically controlled continuous tilt of optical axis in ferroelectric chiral smectic liquid crystal cell

A continuous tilt of an apparent optical axis caused by an applied ac voltage in a ferroelectric chiral smectic C liquid crystal cell is observed. The cell under crossed Nicols modulates the incident light with the voltage applied between −10 and 10 V. Dispersions of the optical modulation intensity and the dielectric constant have the same relaxation frequency (2 kHz). Observation of the dielectric behavior of the cell suggests that the relaxation in the optical modulation corresponds to that of the Goldstone mode. The deformation of the splay structure in the liquid crystal cell supposedly causes the continuous tilt of apparent optical axis.

Ferroelectric liquid crystal device using the τ-VMINmode

Abstract Ferroelectric liquid crystal (FLC) devices using the -Vmin mode were developed, combining FLC materials with negative dielectric anisotropy, the C2-uniform (C2U) orientational state and the “Malvern” drive schemes. The developed FLC materials in these devices showed the Vmin value under 35V, which is adequate to the drive voltage of ±40V. The C2U orientational state was found to be the preferred state for the -Vmin mode, and was obtained in FLC cells with medium pretilt aligning film. The FLC cells were driven by the Malvern drive schemes, achieving fast line address time (faster than 50μ sec/line), wide temperature range of driving (0∼40°C), high contrast (higher than 50:1) and drive voltage of ±40V.

Ferroelectric liquid crystal driving method for optical switching devices in communication networks

Abstract The requirements that must be met by ferroelectric liquid crystal (FLC) cells in long-wavelength optical switches in communication networks are clarified with respect to the differences from display technology. A new driving method in which low sustain voltage is stored in active-matrix FLC cell was presented and demonstrated experimentally. The memory ratio, extinction ratio and response time of cells 4∼5 μm thick were investigated to use at wavelength of 1.3∼1.6 μm. Thick cells driven with this new method show response time less than 100 μsec, extinction ratios greater than 30 dB and memory ratios about 98%. The initial aging characteristics of cells driven with the new method indicate that this will result in longer cell lifetime.

Coupled Director and Layer Reorientation in Layer Tilted Ferroelectric Smectic Liquid Crystal Cells

Abstract A reversible reorientation of smectic layers in surface stabilized ferroelectric liquid crystal (SSFLC) cells with layer tilted (chevron) geometries was found by electrooptical investigations. The reorientation of the layers superimposes the reorientation of the director along the S* c tilt cone and is suppressed, if the layers are irreversibly fixed in a bookself geometry by high electric fields. Based on a potential density expansion, a dynamical model of a coupled director and layer reorientation is proposed, which leads to excellent agreement with the electrooptical behaviour of the system investigated. The fit of the model to experimental data supplies a simultaneous determination of several material and cell constants. Basic electrooptical parameters, as contrast, switching times and optical overshoot, are reproduced correctly.