V-shaped Switching Research Articles

Spontaneous Polarization Influence of V-shaped Switching Ferroelectric Liquid Crystals on Pulse Driving Using Active Switch

The influence of spontaneous polarization on the active driving of V-shaped switching ferroelectric liquid crystals is discussed. Two ferroelectric liquid crystals with a high spontaneous polarization (200 nC/cm2) and a low spontaneous polarization (7 nC/cm2) are used in this study. The field-effect transistor used as an active switch and a storage capacitor is set parallel to a liquid crystal cell. In a high-spontaneous-polarization material, cell voltage increases, while the transistor is in the on-state. After the application of external driving voltage, cell voltage decreases owing to the discharging of the storage capacitor due to the changes induced by spontaneous polarization. On the other hand, in a low-spontaneous-polarization material, the decrease in cell voltage after the application of external driving voltage is very small. The low-spontaneous-polarization material, compared with the high-spontaneous-polarization material, is advantageous for controlling light intensity by active driving.

Near-lossless continuous phase modulation using the analog switching mode (V-shaped switching) in ferroelectric liquid crystals

The analog switching mode in ferroelectric liquid crystals, sometimes referred to as 'V-shaped switching,' has, thanks to its submillisecond switching capability, attracted much interest for future fast electro-optic displays where it is to be used for amplitude modulation. We have studied this mode for analog phase-only modulation. As V-shaped switching is based on a conical motion of the index ellipsoid this presents a challenging problem since both the orientation of the slow and fast axes, as well as the amount of birefringence varies in the switching process. We show theoretically, partly by means of Poincaré sphere analysis, that it is in fact possible to obtain near-lossless analog phase modulation between zero and pi radians in an ideal V-shaped switching cell through careful tuning of the polarization state of the input light. Furthermore, we were able to demonstrate this experimentally in a fabricated cell. Although this cell deviated slightly from the ideal conditions, e.g., the tilt cone half-angle was 38 degrees instead of the desired 45 degrees , we still obtained a continuous phase modulation between zero and 0.78pi rad with less than 2% modulation of the amplitude; the measured values agree very well with our numerical simulations of the real device.

Molecular Alignment Structure and Switching of a Ferroelectric Liquid Crystal Stabilized by a Polymer Network Created in the SmA Phase

ABSTRACT In order to realize monostability in surface-stabilized ferroelectric liquid crystals, polymer stabilization techniques have been proposed, and the polymer-stabilized ferroelectric liquid crystals (PSFLCs), which is fabricated by UV photocure of doped photocurable mesogenic monomers at a temperature where the FLC medium is in the SmA phase, has been reported to show V-shaped electrooptical characteristics. However, the reason why the monostable V-shaped switching can be realized has not been clear yet. In this research, the electrooptical characteristics, the microscopic textures, the tilt angle and the chevron angle of the PSFLCs photocured in the SmA phase were measured, and then the molecular alignment structure was discussed from these results.

The Effects of Introducing Ester Linking Groups on the Flexoelectro-Optic Properties of Symmetric Bimesogens

ABSTRACT In the chiral nematic phase, flexoelectricity can give rise to an interesting electro–optic switching effect, known as flexoelectro-optic switching. Flexoelectro-optic switching gives a fast v-shaped switching regime. Previous studies show that symmetric bimesogens are particularly suited for flexoelectro-optic switching. By introducing two ester linking groups into the molecular structure of a symmetric bimesogen, it was hypothesised that the flexoelectric properties will be enhanced significantly because of the resulting increase in the dipole moment of the molecules. This was found to be the correct; however, the inclusion of ester linking groups reduced the liquid crystallinity of the material.

Ferroelectric and antiferroelectric liquid crystals of chiral swallow-tailed materials with an ester group linked to achiral alkyl chains

Two homologous series of chiral materials, 3-pentyl (S)-2-{6-[4-(4′-alkanoyloxyphenyl)benzoyloxy]-2-napthyl}propionate, PmPBNP(m=7-11), and 2-ethyl-1-butyl (S)-2-{6-[4-(4′-alkanoyloxyphenyl)benzoyloxy]2-napthyl}propionate, EBmPBNP(m=7-11), derived from a chiral moiety, (S)-2-(6-methoxynaphthyl-2)propionic acid, in conjugation with swallow-tailed groups attached to the external side of the chiral centre, have been synthesized and their mesomorphic phases studied. Members of the PmPBNP series, with the exception of P7PBNP and P8PBNP, showed an enantiotropic antiferroelectric SmC*A whereas, members of the EBmPBNP series, with the exception of EB7PBNP, showed an enantiotropic ferroelectric SmC*A phase. The maximum spontaneous polarization for these materials was in the range 20–60 nC cm−2. The electro-optic responses measured for both series of materials in ferroelectric and antiferroelectric phases displayed thresholdless V-shaped switching.

Effect of the variation of swallow-tailed groups on the mesomorphic and electro-optical properties of chiral compounds derived from (S)-2-(6-hydroxy-2-naphthyl)propionic acid

Three homologous series of chiral swallow-tailed compounds, alkyl (S)-2-{6-[4-(4′-alkoxyphenyl)benzoyloxy]-2-naphthyl}propionates, (S)HNP(p,n,q) derived from (S)-2-(6-hydroxy-2-naphthyl)propionic acid in conjugation with a variety of swallow-tailed groups, attached to the external side of the chiral centre, have been synthesized and their mesomorphic and electro-optical properties studied. Both (S)HNP(p,1,2) and (S)HNP(p,1,3) exhibited an enantiotropic antiferroelectric SmC*A phase. This implys that the swallow-tailed groups in the molecules favour zigzag pairing of the molecules in the smectic phase. The maximum P S values of compounds (S)HNP(p,1,2) in the antiferroelectric phase were measured in the range 21–30 nC cm-2; those of compounds (S)HNP(p,1,3) were in the range 15–23 nC cm-2, indicating that these chiral compounds possess low polarity. The electro-optical response of the compounds in the antiferroelectric SmC*A phase displayed thresholdless V-shaped switching.

Simulations of energy and switching in AFLCDs with different boundary conditions

Using standard expressions for the various terms in the Gibbs free energy, the switching in antiferroelectric liquid crystal (AFLC) displays is simulated and the time evolution of various energy terms and of the liquid crystal director distributions are calculated. It is shown that when returning from a strong positive voltage to zero, one can reach two types of antiferroelectric state: the normal alternating state with the two bulk polarizations perpendicular to the electrodes and opposite to each other, and the alternative splayed symmetric state with two bulk polarizations parallel to the electrodes and again opposite to each other. The former case gives rise to tri-state switching characteristics, the latter to V-shaped switching. In general strong polar interaction with the alignment layer favours V-shaped switching while weak or no polar interaction give rise to tri-state switching characteristics. Since the V-shaped characteristic has so far only been demonstrated experimentally in ferroelectric liquid crystals (or antiferroelectric liquid crystals being in the ferroelectric state), the difference in AFLCs is discussed and the conditions for continuous switching are modelled. The simulations show that the switching characteristics of the antiferroelectric display can be controlled by the surface parameters.

Polymer-stabilized V-mode FLCDs and their application to color sequential fullcolor LCDs

Polymer-stabilized FLCDs exhibiting V-shaped switching called PS-V-mode FLCDs are fabricated using newly synthesized photocurable monomers and new FLC materials. These FLCDs show good temperature reliability and high enhancement in the optical throughput, and the reduction of the operating voltage. The device is defect-free and exhibits high contrast ratio 700:1 and further fast response with τ=300–500 μs with continuous grayscale operation. We demonstrate a full color LCD with resolution 254 ppi and 4 in. diagonal using color sequential tecnique.

Fabrication and electro-optic characteristics of polymer-stabilized V-mode ferroelectric liquid crystal display and intrinsic H-V-mode ferroelectric liquid crystal displays: their application to field sequential fullcolor active matrix liquid crystal displays

Shunsuke KobayashiJun XuHirokazu FurutaYuji MurakamiSatoru KawamotoMasafumi Oh-kouchiTokyo University of ScienceLiquid Crystal Institute and the GraduateSchool of Engineering ScienceYamaguchi, OnodaYamaguchi 756-0884, JapanHiroshi HasebeHaruyoshi TakatsuDainippon Ink. and Chemical Ind.Ina-machi, Kitaadachi-gunSaitama 362-8577, JapanAbstract. We fabricate ferroelectric liquid crystal display (FLCD) de-vices exhibiting a thresholdless V-shaped switching by adopting a me-sogenic liquid crystal polymer stabilization and also FLCDs exhibitinghalf-V-shaped switching using an undoped FLC material. These FLCDsare respectively called polymer-stabilization (PS)-V-mode FLCDs and in-trinsic H-V-mode FLCDs. By using especially developed polyimide ma-terials such as RN-1199 and RN-1411 series (from Nissan Chem., Ind.)for the surface alignment and together with a well-controlled rubbingtechnique, photoalignment, and ion-beam alignment, we succeed in fab-ricating zigzag defect-free FLCDs. In particular, the PS-V-mode FLCDfabricated in this way exhibits continuous operation with a high contrastratio of 700:1, and the intrinsic H-V-mode FLCD has good temperaturestability by using the combined alignment technique. Particularly, a field-sequential full-color (FS-FC) LCD using PS-V-mode FLCD shows goodlong-term stability for a continual operation. Using these FLCDs, we alsodevelop several prototype models of FS-FC active matrix LCDs with thespecification of video graphics arrays (VGAs) and extended graphicsarrays (XGAs), where we take advantage of their high-speed response(time constants 100 to 500 ms).

Modeling electrooptical effects in Ferroelectric Liquid Crystals. 2. V-Shape Switching in the SmC* phase

The thresholdless, hysteresis-free V-shape electrooptical switching in surface stabilized ferroelectric liquid crystal (FLC) cells has been modeled and the results compared with experimental data. High frequency V-shape switching (at f>100 Hz) was experimentally demonstrated and simulated numerically. The key role of the capacitance C of FLC aligning layers has been underlined: thin layers with large C promote the well known bistability effect, thick layers, on the contrary, are necessary for V-shape switching. The results of modeling are in quantitative agreement with experiment.

Energy Relations in Antiferroelectric Liquid Crystal Displays

In antiferroelectric liquid crystal displays either V-shaped switching or tri-state switching characteristics are possible. Both types of states may be locally stable at constant voltage. However if they exist in different region of a pixel separated by a wall, the lowest energy state will grow by wall motion. It is thus important to be able to calculate accurately the energy of stationary states. We present three ways of calculating the Gibb's free energy: by total simulation of non-stationary states, by partial simulation and calculation of the stationary states based on energy relations, and by analytical expressions of stationary V-shape states. This allows to check the accuracy of the simulations.

Switching of orthoconic antiferroelectric mixtures in PDLC system

Polymer-dispersed liquid crystals containing orthoconic antiferroelectric smectics were prepared by photopolymerization-induced phase separation. Mean droplet size allowed formation of antiferroelectric helix inside droplets. Bias electric field applied during phase separation aligned helical axes parallel to the cell plane. Obtained ellipsoidal droplets were optically uniaxial for perpendicular light beam. The cells were switched by square or triangle driving signal unwinding the helix. V-shape switching was found instead of threshold one observed in conventional cells of the same mixture. The switching time was crucially depended on temperature. Those effects were caused by anchoring conditions inside polymer cavities.

Molecular Alignment in SmLC Cells Having a Surface Layer with INS Phase Sequence

In smectic liquid crystals (SmLCs) having no nematic (N) phase in the phase sequence, such as antiferroelectric SmLCs and SmLCs with V-shaped electrooptical switching, it is hard to obtain the LC media which have a uniform molecular alignment structure. This is because the transition of isotropic (Iso) phase directly to Sm phase during the cooling gives rise to a non-uniform Sm layer structure due to the both occurance of molecular alignment and Sm layer formation at the same time. We propose an idea to fabricate defect-free SmLC cells in which the uniform Sm layer structure is realized by the use of an intervening surfacial LC layer with Iso-N-Sm (INS) phase sequence.

Switching Behaviour of Antiferroelectric Liquid Crystals

We present a model of antiferroelectric liquid crystals that explains a number of phenomena observed in their switching properties. These include the pretransitional effect, the thresholded transitions between the antiferroelectric and ferroelectric states, the frequency dependence of the hysteresis loop and the origin of the v-shaped switching observed in some antiferroelectric materials.

The effect of polymer networks on ferroelectric liquid crystals

Using a relative small amount of reactive mesogenic monomer and a quasi-bookshelf texture of ferroelectric liquid crystals (FLCs) we developed submicron-scale templated polymer fibrils from the two-dimensionally ordered hosts. The polymer fibrils capture the orientation of the host with thin polymer fibrils interweaving the smectic layers that act as additional surfaces for controlling the reorientation of FLC molecules. The SEM study shows the difference in morphology of the polymer fibrils depending on the polymerization conditions and FLC hosts. There are two distinctive electro-optical properties of polymer modified FLC such as the fast switching at a lower applied field and the voltage independence of switching time achieved by polymer stabilization using a mesogenic monomer. Further, we describe how the polymer network and the type of FLC enhance the realization of V-shaped switching behavior.

Hysteresis-free electro-optical switching in conductive ferroelectric liquid crystals: experiments and modelling

The hysteresis-free electro-optical switching, or so-called V-shaped, regime has been studied in a commercial ferroelectric liquid crystal (FLC) mixture having a smectic C* phase with a very small value of spontaneous polarization. The FLC was introduced into commercial EHC cells with thin aligning layers. In such cells V-shaped switching could be observed only at very low frequencies, less than 1 Hz. However, when the same material is strongly doped with a conductive impurity, its conductivity markedly increases and hysteresis-free switching is observed over a wide range of applied frequencies and voltages. Experimental results are in good agreement with computer modelling carried out as part of this work. The modelling takes into account all the important parameters of smectic C* FLC: non-polar anchoring conditions, possible bookshelf and chevron structures, the capacitance of the aligning layers and the conductivity of a FLC. The last two factors appear to be the most crucial for hysteresis-free switching in the smectic C* phase.

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.

Effect of Configuration of Diastereomers on the Electro-Optical Responses of the Antiferroelectric Phase

Two diastereomers, (R)-2-butyl (S)-2-{6-[4-(4‘-decyloxyphenyl)benzoyloxy]- 2-naphthyl}propionate, (R,S)-BDPBNP, and (S)-2-butyl (S)-2-{6-[4-(4‘-decyloxyphenyl)benzoyloxy]-2-naphthyl}propionate, (S,S)-BDPBNP, were prepared for investigating the effect of the configuration on mesomorphic and physical properties of the molecules. Both isomers displayed the same mesophase sequence viz. BPII-N*-TGBA*-SmA*-SmCAF*, and nearly the same magnitude of spontaneous polarization at any temperature below Curie point. However, the electrooptical responses of the isomers in the SmCAF* phase measured in 5-μm homogeneous cells coated with polyimide, showed that the (R,S)-isomer displayed a tristable switching, whereas the (S,S)-isomer displayed a V-shaped switching. These results suggested that V-shaped switching in antiferroelectric phase in the 5-μm cell bears no relation to the electrostatic effect of the bulk polarization but depends strongly on the absolute configuration of the diastereomers. The electrooptical respons...

Polymer-Stabilized Antiferroelectric Liquid Crystals

In smectic liquid crystals (SmLCs) with no nematic phase in the phase sequence, such as antiferroelectric SmLCs and SmLCs with V-shaped electrooptical switching, it is hard to obtain LC cells with a uniform molecular alignment structure. The direct transition from the isotropic phase to the Sm phase during cooling gives rise to a nonuniform Sm layer structure due to the dispersion of the layer normal. We propose a method of fabricating defect-free SmLC cells with a uniform Sm layer structure using polymer stabilization or the phase separation of the SmLC and the polymer.

Intrinsic aspect of V-shaped switching in ferroelectric liquid crystals: biaxial anchoring arising from peculiar short axis biasing in the molecular rotation around the long axis.

To clarify the intrinsic aspect of practically usable thresholdless V-shaped switching in ferroelectric liquid crystals, we have observed textures and measured polarized Raman scattering as well as optical transmittance in a thin homogeneous cell of a single compound by applying an electric field. The results indicate that the so-called surface stabilized ferroelectric states are destabilized, and that there exist rather stable two domains with broad and narrow molecular orientational distributions, both of which show the almost ideal V-shaped switching with considerably low transmittance at the tip of the V. We have concluded that the main cause of the V-shaped switching is the biaxial anchoring on the substrates coated with polyimide, which makes the most polarizable short axis normal to the substrates. It is in competition with the ordinary anchoring that favors the director parallel to the substrates, when the material has such a bulk intrinsic property that this short axis is parallel to the tilt plane. The competition makes the total anchoring energy almost independent of the azimuthal angle and gives rise to the V-shaped switching.