Antiferroelectric Liquid Crystal Cells Research Articles

Switching performance of a polymer-stabilized antiferroelectric liquid crystal based on bent-core molecules

Mixtures of a non-reactive and a photo-reactive liquid crystal, both consisting of bent core molecules, were used to prepare polymer-stabilized antiferroelectric liquid crystal cells by in-situ photo-polymerization. The mixtures exhibit a larger temperature range of the antiferroelectric phase than the pure compounds. The polymer network improves the ability of uniform alignment and thus enhances the switching contrast. In addition, the polymer-network reduces the switching times, in particular the relaxation time from the field-induced state to the ground state.

Solitary wave propagation in antiferroelectric liquid crystal cells and the quadrupolar term in the interlayer interaction

Planar antiferroelectric liquid crystal (AFLC) cells exhibit propagating solitary waves. By measuring the retardation and the apparent tilt angle, we find that the conventional model for the solitary wave propagation in AFLCs is not generally applicable. The model being proposed here shows that the solitary wave propagation both along and normal to the smectic layer plane can be explained by a double well potential energy governing the relative orientations of the c directors in the adjacent smectic C layers with an energy barrier between the synclinic and the anticlinic states. The quadrupolar term of the interlayer interaction is found to be significantly large in a pure AFLC.

Modifying electrooptics of orthoconic antiferroelectric liquid crystal cells by manufacturing procedures

AbstractOrthoconic antiferroelectric liquid crystals (AFLCs) having 45° tilt angle have been proposed to overcome the problem of reduced contrast usually exhibited by regular antiferroelectric displays. However, the pitch of the helix induced by chirality is customarily short in existing orthoconic materials, making it difficult to achieve surface stabilized configurations when the material’s pitch is smaller than the cell thickness.In this work, the influence of different manufacturing procedures on the electrooptical behaviour of orthoconic AFLCs is studied. Using the same orthoconic AFLC mixture, aligning surfaces and manufacturing protocol, we have observed two dissimilar responses, true orthoconic behaviour, and orthoconic V-shape response. The electrooptical response depends ultimately on the rubbing strategy adopted in either case.

Asymmetric Response Induced by Low Energy Surfaces in Antiferroelectric Liquid Crystal Cells

New antiferroelectric liquid crystal (AFLC) cells having a uniquely asymmetric hysteresis response were obtained by using dissimilar surface anchoring on the cell glass plates. In particular, a low surface energy fluorinated block copolymer was deposited by spin-coating on one side and Nylon6 was cured and rubbed on the opposite side. These asymmetric AFLC displays featured an unusually large shift of the hysteresis curve and a fairly stable greyscale when driven with previously designed waveforms. The contrast ratio in both quasi static and dynamic conditions was especially high (ca. 100) for AFLCs. The new findings can shine new light on the current proposed models of asymmetric hysteresis, based on ion currents and charge trapping onto the alignment surfaces.

Electrical Response of Antiferroelectric Liquid Crystal Cells with Asymmetric Response

Liquid crystal microdisplays are heavily used in applications ranging from static and quasi-static images to video-rate displays. Antiferroelectric liquid crystals with asymmetric response exhibit good properties to be employed in this kind of devices, for example in mobile telephones. Asymmetric response arises from dissimilar surface conditioning onto the cell glass plates. The resulting asymmetric antiferroelectric devices feature multiplexed analogue grey scales with very simple driving schemes. In this work, a comparative study of properties of antiferroelectric liquid crystal cells with symmetric and asymmetric response is carried out. Asymmetric cells give rise dissimilar current peaks depending on the polarity of the applied waveform. This effect may stem from various contributions, such as ionic conductance and free charges accumulated superimposed to the symmetric antiferroelectric response.

The Role of Alignment Layers on the Induced Relaxation of Passively Multiplexed Antiferroelectric Liquid Crystal Displays

ABSTRACT We have investigated the static and dynamic electrooptical effects of adding an alignment layer derived from a chiral liquid crystal having SmC* and N* phases to the standard polymer alignment layer in antiferroelectric liquid crystal cells. Our results show that the added layer reduces the relaxation time of the liquid crystal, the threshold voltage and the dynamic range required to generate a full greyscale. The electrooptical response of materials was tested using a 7 level waveform, and the relaxation was studied using a novel memory test.

Addressing Waveforms for Asymmetric Antiferroelectric Liquid Crystal Displays

Antiferroelectric Liquid crystal cells having a SiOx on one side and a conventional alignment layer on the other side have been prepared. This manufacturing process allows the antiferroelectric cell to be driven as a ferroelectric cell. This behavior opens the possibility of driving the cell with simple bipolar pulses, allowing some interesting effects such as video-rate multiplexed analogue gray scale.

Dual-antiferroelectric liquid crystal display with high transmittance efficiency and no dispersion

When an antiferroelectric liquid crystal (AFLC) cell is set between crossed polarizers for optical modulators or displays, it has low transmittance efficiency. This happens because AFLC materials have inherent tilt angles of typically 18 to 38 deg, not 45 deg. To improve the low transmittance efficiency, we propose a new optical configuration for antiferroelectric liquid crystal displays or devices, which also improves color dispersion. It consists of two crossed antiferroelectric liquid crystal cells. The characteristics of this dual-antiferroelectric liquid crystal display are simulated and experimented.

Optimal design of optical performance of reflective ferroelectric‐ and anti‐ferroelectric liquid‐crystal displays

Abstract— In an in‐plane optical geometry, such that the average optic axis lies on the plane parallel to both substrates, the optical properties of a reflective ferroelectric liquid‐crystal (FLC) or antiferroelectric liquid‐crystal (AFLC) cell were studied within the framework of the 2 × 2 Jones matrix formalism. To obtain good achromaticity and high brightness, the cell parameters such as the molecular rotation angle and the effective phase retardation of the AFLC layer were optimized. The device performances of the AFLC cell were experimentally demonstrated in this geometry.

P‐100: Driving Technique for Grayscale in Dual‐Antiferroelectric Liquid Crystal Display

AbstractWhen an antiferroelectric liquid crystal (AFLC) cell is set between crossed polarizers for optical modulators or displays, it has low transmittance efficiency. This happens because AFLC materials have inherent tilt angles of typically 18°∼38°, not 45°. To improve the low transmittance efficiency, we propose a new optical configuration for antiferroelectric liquid crystal displays, which also improves color dispersion. We compare the Dual‐AFLCD with a conventional AFLCD by simulation and experiment with the fabricated cells, and grayscale characteristics of this Dual‐antiferroelectric liquid crystal display are estimated from the experiments in this paper.

Tilt plane orientation in antiferroelectric liquid crystal cells and the origin of the pretransitional effect.

The optic, electro-optic, and dielectric properties of antiferroelectric liquid crystals (AFLCs) are analyzed and discussed in terms of the local tilt plane orientation. We show that the so-called pretransitional effect is a combination of two different electro-optic modes: the field-induced antiphase distortion of the antiferroelectric structure and the field-induced reorientation of the tilt plane. In the presence of a helix, the latter corresponds to a field-induced distortion of the helix. Both electro-optic modes are active only when the electric field has a component along the tilt plane. Thus, by assuring a horizontal surface-stabilized condition, where the helix is unwound by surface action and the tilt plane is everywhere parallel to the cell plates, the pretransitional effect should be suppressed. We also discuss the dielectrically active modes in AFLCs and under which circumstances they contribute to the measured dielectric permittivity.

Fully leaky guided mode study of an orthoconic antiferroelectric liquid crystal cell deviating from perfect horizontal surface stabilization

The optical properties of an antiferroelectric liquid crystal (AFLC) cell slightly deviating from the ideal orthoconic condition, i.e., 45° tilt angle and fully horizontal surface stabilization (the molecular tilt plane parallel to the glass plates), have been investigated using the fully leaky guided mode (FLGM) technique. In contrast to standard optical investigative methods, such as polarization microscopy, the FLGM data give information about the orientation of the index ellipsoid or the molecular tilt plane, not only in the plane of the cell but in three dimensions. The sensitivity of the FLGM technique thus allows a detailed optical characterization on a mesoscopic scale and, in particular, has allowed us to discriminate, in the investigated cell, a tilt out of the substrate plane in the anticlinic state, thus a deviation from the horizontal surface stabilization condition. Orthoconic surface-stabilized AFLCs are, in the ideal case, uniaxial negative with the optic axis perpendicular to the glass plates and therefore provide a perfect dark state between crossed polarizers even if the direction of the smectic layer normal is not homogeneous in the plane of the cell. Small deviations from the ideal case lead to the appearance of a very small birefringence in the cell plane which, however, should not compromise the attractive optical properties of this class of AFLCs. Using laser diffraction we have verified that, even with the deviation from the horizontal surface stabilization detected by the FLGM study, a horizontal chevron geometry, clearly visible in the synclinic state, is not optically detectable in the anticlinic state of orthoconic AFLCs. We conclude that the attractive optical properties are preserved in a reasonably large interval around the ideal orthoconic conditions allowing for the practical exploitation of these materials as key components in liquid crystal displays and other electro-optic devices.

Electrical Model for Thresholdless Antiferroelectric Liquid Crystal Cells

The equivalent circuit for a thresholdless antiferroelectric liquid crystal (TLAFLC) cell has been derived using an electrical model. The model may be used to predict the TLAFLC dynamic behavior. The electrical response in the time domain of TLAFLC cells upon certain hypothesis has been obtained. Optical characteristics related to these electrical features have been derived as well. The equivalent circuit model reasonably reproduces the voltage across the cell, switching currents and the optical hysteresis obtained by experimental methods. The procedure for determining the voltage-dependent capacitance due to induced spontaneous polarization is shown. On the basis of the proposed model, we also suggest a number of clues to optimize some electrical parameters of liquid crystal materials as well as driving schemes.

Electrical Model for Thresholdless Antiferroelectric Liquid Crystal Cells

The equivalent circuit for a thresholdless antiferroelectric liquid crystal (TLAFLC) cell has been derived using an electrical model. The model may be used to predict the TLAFLC dynamic behavior. The electrical response in the time domain of TLAFLC cells upon certain hypothesis has been obtained. Optical characteristics related to these electrical features have been derived as well. The equivalent circuit model reasonably reproduces the voltage across the cell, switching currents and the optical hysteresis obtained by experimental methods. The procedure for determining the voltage-dependent capacitance due to induced spontaneous polarization is shown. On the basis of the proposed model, we also suggest a number of clues to optimize some electrical parameters of liquid crystal materials as well as driving schemes.

Asymmetric Electro-Optic Response in Antiferroelectric Liquid Crystals

A new manufacturing procedure producing asymmetric hysteresis curves in surface-stabilized antiferroelectric liquid crystal cells has been developed. It is based on the use of dissimilar surface conditionings on either side of the cell. The central relaxed AFLC state is not located at zero volts. The AF→F transitions on either side, consequently, are not produced at the same switching voltage. This behavior opens the possibility of driving the cell with several new modes, allowing some interesting effects such as video-rate multiplexed analogue grayscale with simple bipolar pulses, and longterm multistability.

Dynamic Layer Response under Electric Field in Antiferroelectric Liquid Crystal Cells Measured by Synchrotron Microbeam Time Resolved X-Ray Diffraction

Direct observation of the local layer response of an antiferroelectric liquid crystal to the step form electric field has been carried out by a time resolved synchrotron X-ray micro diffraction measurement. When an electric field was changed from high voltage to OV, corresponding to the ferroelectric to antiferroelectric phase transition, the local layer transformed from the bookshelf to the quasi-bookshelf structure within 0.3 ms. The horizontal chevron structure was found in both the phases, though the decrease in the horizontal chevron angle was observed during a period of 0.2 ms after turning off the electric field. In the antiferroelectric to ferroelectric phase transition process (from OV to high voltage), the layer structure transformed to the bookshelf within 0.04 ms.

A Study of New Operation Mode for High Contrast Ratio and Fast Switching Time in Antiferroelectric Liquid Crystal(AFLC)

A new method of switching mode in AFLC cell is proposed for faster switching time and higher contrast ratio. In this mode the ″dark″ state is obtained by applying negative full voltage while the ″bright″ state is achieved by applying positive full voltage to the cell. The switching time is reduced to 100 <TEX>$mutextrm{s}$</TEX> for the cell whose switching time is 22 ms when operated in conventional mode. The contrast ratio is also improved vastly with this method. The possibility of achieving gray scale was shown in this mode of operation.

Dynamic Behaviour of the Local Layer Structure of Antiferroelectric Liquid Crystals under a High Electric Field Measured by Time-resolved Synchrotron X-Ray Microbeam Diffraction

The local layer structure response to a triangular electric field in an antiferroelectric liquid crystal cell has been measured using synchrotron X-ray diffraction with 3 ms time resolution and a few µm spatial resolution. The initially coexisting vertical and horizontal chevron structures are irreversibly transformed to the layer structure with a rearranged molecular orientation at the surface (so-called vertical bookshelf structure). After the irreversible transformation, the rearranged layer structure shows a reversible transition between the horizontal chevron (high field, ferroelectric state) and the combination of the modified vertical and horizontal chevron (low field, antiferroelectric state) associated with the field-induced antiferroelectric-ferroelectric transition. The reversible layer structure has a smaller horizontal chevron angle (a few degrees) than that in the initial state (about 17°). The detailed microbeam diffraction revealed that the layer structure at a low electric field consists of a broad vertical chevron with a small chevron angle and a bent bookshelf in combination with a horizontal chevron, depending on the analyzing position. The stripe texture is related to the modified horizontal chevron structure.

Field-driven helix unwinding in antiferroelectric liquid crystal cells.

The interaction between the antiferroelectric helix and an electric field applied perpendicular to its axis is investigated, both experimentally and theoretically. A two-stage switching process is observed: (i) in the pretransitional regime the helix distorts and then unwinds to form a nonhelical antiferroelectric state, with the plane of the molecules parallel to the applied field; (ii) at higher fields switching to the ferroelectric state occurs. The mechanism for unwinding is the interaction of the applied field with a polarization that is induced by a change to the anticlinic ordering.

Asymmetric switching of antiferroelectric liquid-crystal cells

Antiferroelectric liquid-crystal cells usually show symmetric electro-optic switching response from the antiferroelectric state to two opposite ferroelectric states. Intermediate transmission levels (analog gray scale) can be stabilized, applying a constant dc bias voltage after switching. Modifying the manufacturing process and using narrow dynamic-range antiferroelectric materials, a fully asymmetric response has been achieved. This asymmetric switching allows the antiferroelectric cell to be driven as a ferroelectric cell, reducing or eliminating the bias voltage, and ultimately leading to analog optical multistability, i.e., devices whose optical transmission may be arbitrarily set and maintained without power supply.