Surface-stabilized Ferroelectric Liquid Crystal Research Articles

Impact of the concentration in polymer on the dynamic behavior of Polymer Stabilized Ferroelectric Liquid Crystal using Snap-shot Mueller Matrix Polarimetry

Experimental results are presented related to the dynamic behaviour of Polymer Stabilized Ferro-electric Liquid Crystal (PSFLC) samples under external applied electric field, using Snap-shot Mueller Matrix Polarimetry (SMMP) and Mueller Matrix (MM) formalism. Different polarimetric coefficients are simultaneously extracted from each channeled spectrum measured with this full-optical SMMP technique. The impact of the concentration of polymer present into the liquid crystal cell on this dynamic behaviour is studied, permitting a direct and quick characterisation of the material. The results obtained for PSFLC are compared with those already measured for pure Surface Stabilized Ferro-electric Liquid Crystal (SSFLC) samples, which correspond to a 0% concentration in polymer.

Characterization of the dynamics of surface stabilized ferroelectric liquid crystal under electric field by full optical snapshot matrix Mueller polarimeter

We investigated the layer dynamics of a conventional surface-stabilized ferroelectric liquid crystal (SSFLC) using a full-optical snapshot Mueller matrix polarimeter (SMMP) based on wavelength polarization coding. Time-resolved polarimetric measurements were performed with different SSFLC samples, and a strong correlation between the polarimetric parameters and the SSFLC under electric field at different exposure times was found. It has been shown that the SMMP polarimeter is able to determine the evolution of the trajectory of the liquid crystal director between the two addressed states, the reversible motion of the smectic layer while switching, as well as the irreversible transition from chevron to bookshelf texture.

Correlation between static and dynamic polarimetric properties and the texture of surface-stabilised ferroelectric liquid crystal cells

Snapshot Mueller matrix polarimetry was performed for static and dynamic analyses of surface-stabilised ferroelectric liquid crystal cells under an electric field. A strong correlation between the static (at fixed voltage) and dynamic (upon field reversal) polarimetric properties and the texture of ferroelectric liquid crystal cells was established. The birefringence properties were different between a rooftop/zigzag-textured cell and a stripe-textured cell. The trajectory of the optic axis, plotted over the transition between two addressed states, was analysed for each cell. The shape of the trajectories could be explained by a reversible motion of the smectic layers while switching.

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.

Fabrication of Defect-Free Ferroelectric Liquid Crystal Cells Using Alignment Films Rubbed in Antiparallel Direction

The fabrication of operable surface-stabilized ferroelectric liquid crystals (SSFLCs) is often hampered by the occurrence of zigzag defects. In this study, we try to fabricate a defect-free SSFLC with antiparallel-rubbed films that exhibit different properties between two substrates for the energy condition in two types of chevron to be different. As a result, the occurrence of zigzag defects can be suppressed as the differences in anchoring strength and pretilt angle increase between two alignment films, and then in the case of the large differences in physical properties, zigzag defect-free SSFLCs can be obtained.

Fabrication of Defect-Free Ferroelectric Liquid Crystal Cells Using Alignment Films Rubbed in Antiparallel Direction

The fabrication of operable surface-stabilized ferroelectric liquid crystals (SSFLCs) is often hampered by the occurrence of zigzag defects. In this study, we try to fabricate a defect-free SSFLC with antiparallel-rubbed films that exhibit different properties between two substrates for the energy condition in two types of chevron to be different. As a result, the occurrence of zigzag defects can be suppressed as the differences in anchoring strength and pretilt angle increase between two alignment films, and then in the case of the large differences in physical properties, zigzag defect-free SSFLCs can be obtained.

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.

Solitons induced by alternating electric fields in surface-stabilized ferroelectric liquid crystals

Propagation of solitary waves activated in thin ferroelectric liquid crystal cells under external, sinusoidally alternating electric fields is investigated using the electro-optic technique. It is shown that solitons give contributions only to the loss component of the response spectrum, within rather narrow ranges of frequencies and in sufficiently strong fields. The limit frequency, at which the amplitude of the velocity of the solitary waves is greatest, is found to be related to material constants of liquid crystals. Measuring this threshold frequency provides the capability to determine the elastic constant of surface stabilized liquid crystalline materials in the bookshelf or chevron layer geometries.

Rubbed Polyimide Thin Films for Zigzag Free Surface Stabilized Ferroelectric Liquid Crystal

We investigate the effects of rubbing on the surface free energies of polyimide thin films, and how the surface energetic properties influence the molecular alignment of ferroelectric liquid crystals and the formation of zigzag defects. Rubbing breaks the uniformity in free energy at the surface of polyimide thin film. By properly rubbing the polymer, a diminishment of the difference between the polar part of the surface energies along and against the rubbing direction, respectively, can be achieved, and will favour the formation of zigzag free ferroelectric liquid crystal layer.

Fabrication of Defect-Free Ferroelectric Liquid Crystal Cells Using Combination of Different Pretilt Alignment Films Rubbed in Anti-Parallel

Fabrication of operable surface-stabilized ferroelectric liquid crystals (SSFLCs) is often hampered by the occurrence of zigzag defects. This defect formation can, to some extent, be controlled or eliminated in SSFLCs in which the rubbing directions are set parallel, because two chevrons have different molecular alignment structures and different energies. In this study, we try to fabricate a defect-free SSFLC with anti-parallel rubbed films which possess different surface anchoring and/or pretilt angle between two cell substrates in order for the energy condition in two types of chevron to be different. In the case using a combination of the same alignment film material, it was confirmed that in all alignment film materials used in this research, the occurrence of zigzag defects can be reduced as the difference of rubbing strengths increases between two alignment films. It is found that the appearance of zigzag defects can be more strongly suppressed as the difference of pretilt angles increases between two alignment films, and then in the case of the large difference, zigzag defect-free SSFLCs can be obtained. Furthermore, it is found that the effect of the pretilt on the suppression of zigzag defect is much stronger than that of the anchoring strength.

Fast switching characteristics of a microlens array using the electroclinic effect of SmA^* liquid crystals

We present a microlens array characterized by the electroclinic effect of chiral smectic A (SmA(*)) liquid crystals, which show the very fast dynamic switching characteristics required in high-speed optical devices. In order to easily control the intensity at the focal length of the proposed dynamic microlens structure, we adopt a solid-type liquid crystal polymer with optical anisotropy, which can split the beam intensity into two directions, depending on the vectorial portion of the polarization state of the light. The proposed microlens shows a focal intensity tunable by controlling the polarization of light at the SmA(*) liquid crystal. The lens has a very fast switching time of about 24 micros, which is several times faster than conventional microlens arrays with surface-stabilized ferroelectric liquid crystals.

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.

66.4: Surface Polarity Controlled Horizontal Chevron Defect Free Surface Stabilized Ferroelectric Liquid Crystal Devices

AbstractThe asymmetrical surface polarity controlled hybrid alignment technique was proposed to suppress the alignment defects of half V‐shaped switching FLC materials. When the material was cooled from N* to SmC*, two kinds of layer arrangements were formed with opposite spontaneous polarization directions (Ps). Alignment layers with opposite surface polarity direction were utilized in asymmetrical hybrid cells and produced uni‐surface‐polarity direction. FLC Ps was only controlled by this surface polarity; therefore, double domains were suppressed into mono‐domain. A 2.4” prototype TFT‐LCD was presented with defect‐free alignment texture. This alignment technique provided promising FLC material with well alignment and fast switching for TFT‐LCD application.

Fast Visible-Near Infrared Switchable Liquid Crystal Filter

A conventional liquid crystal display consists of a liquid crystal cell between two crossed polarizers. By applying a voltage, the liquid crystal can be switched and the transmission of light can be manipulated. For thick devices, the transmission shows a strong wavelength dependency and this can be used to construct a tunable Lyot-Öhman filter. Such a filter consists of several liquid crystal cells with polarizers in between with a narrow transmission band that can be shifted over a certain wavelength range. Instead of aiming for a narrow transmission peak, in this work the aim is to achieve a wide transmission wavelength range. In this way the transmission band can be switched, for example, from the visible wavelength range to the near-infrared range. Because the device needs only two different states, bistable surface-stabilized ferroelectric liquid crystal devices can be used which offer short switching times in the order of 100 μs. Numerical optimization of this switch reveals that high contrast ratios can be achieved by using 2 fixed and 2 switchable retarders.

Probing the Electric-Field Modulated Ferroelectric Liquid Crystal Films with Phase-Sensitive Raman Imaging Technique

The electric-field-induced modulation of surface stabilized ferroelectric liquid crystal (SSFLC) films was in- vestigated with a phase sensitive Raman imaging. We found the technique not only effectively suppresses the nonresonant background of a Raman spectrum, but also yields an insight into the modulated amplitudes and phases of Raman peaks. By using the technique, the electro-optical switching SSFLC film doped with ZnO nanocrystals (nc-ZnO) was shown to switch in a more organized way and be less sensitive to the modulation induced disordering than an undoped SSFLC film. The method can be implemented in a conventional Raman spectroscopy to allow researchers probing into the spatial dis- tribution and modulation dynamics of electro-optic active species in a complex thin film.

Fabrication of Defect-Free Surface-Stabilized Ferroelectric Liquid Crystals Using Alignment Films Rubbed in Anti-Parallel

Fabrication of operable surface-stabilized ferroelectric liquid crystals (SSFLC) is often hampered by the occurance of zigzag defects. This defect formation can, to some extent, be controlled or eliminated in SSFLCs in which the rubbing directions are set parallel. In this study, we try to fabricate a defect-free SSFLC with anti-parallel rubbing films which possess different surface anchoring and pretilt angle between two cell substrates in order for the energy condition in two types of chevron to be different. It is confirmed that in all alignment films used in this research, the occurrence of zigzag defects can be suppressed as the difference of rubbing strengths increases between two alignment films. Furthermore, it is found that the appearance of zigzag defects can be more strongly suppressed as the difference of pretilt angles increases between two alignment films, and then in the case of the large difference, zigzag defect-free SSFLCs can be obtained.

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.

Probing Molecular Binding Effect from Zinc Oxide Nanocrystal Doping in Surface-Stabilized Ferroelectric Liquid Crystal with Two-Dimensional Infrared Correlation Technique

Doping liquid crystal (LC) with nanomaterials has been shown to yield some degrees of freedom for tailoring LC properties. This approach can be employed to produce new LC materials with high application potentials by blending instead of synthesizing new mesogenic molecules. In this paper, we show that doping ferroelectric liquid crystal (FLC) with ZnO nanocrystals improves the alignment order of a surface-stabilized FLC (SSFLC) in both steady-state and field-induced reorientation processes. We used the two-dimensional infrared (2D IR) correlation technique to reveal that the ZnO nanocrystals were uniformly dispersed into the FLC medium. The homogeneous dispersion of ZnO nanodots produces stronger correlations among the IR-active molecular normal modes of FLC molecules, which then leads to more concerted reorientation process at the submolecular level. A molecular binding effect originating from a dipolar interaction of the ZnO nanodot with surrounding CO groups of FLC molecules was proposed to illustrate our measured results. We estimated the total energy reduced by the doping to be about 1000 J/m3. The alignment stability gained is similar to that experienced by FLC molecules within 100-nm distance to an alignment surface with a strong anchoring strength of 1 × 10-4 J·m-2.

An improved equivalent circuit model and the dependence of hysteresis inversion frequency on typical parameters of V‐shaped switching SSFLC cells for both bookshelf and chevron structures

An improved equivalent circuit model able to be applied to conventional circuit simulators is presented for optical response prediction and drive circuit design of V‐shaped surface‐stabilised ferroelectric liquid crystal (SSFLC) cells. The model is derived from Moore and Travis' original model. Moreover, the impedance divider induced by the multilayer structure of the liquid‐crystal cells is taken into account, and both polar and non‐polar surface anchoring energy are considered to make the model more preferable. The model is then utilised to investigate the thresholdless switching characteristics for both bookshelf and chevron structures. With the circuit model: (1) the genuine V‐shaped switching occurs only at the hysteresis inversion frequency f i, and below (above) f i an anomalous (normal) hysteresis is observed; (2) the genuine V‐shaped switching is only observed when the transmission is plotted as a function of the total applied voltage; (3) f i increases with increasing conductivity of liquid‐crystal layer (G LC) and the amplitude of applied voltage, and the function of is almost linear; (4) For small spontaneous polarisation p s, f i grows as p s increases, however, on further increasing p s, f i reaches its maxima and then decreases (5) the V‐shaped switching can be observed in both bookshelf and chevron structures, however, f i is higher for the chevron structure. Results obtained from the circuit model compare favourably with the reported experimental and numerical results.

Comparison of dielectric and optical responses of chevron ferroelectric liquid crystals

AbstractAn analysis of consistency of dielectric and optical response methods is carried out for surface stabilized ferroelectric liquid crystals (SSFLC) with chevron geometry. The consistency is found both theoretically and experimentally for weak external electric fields of intermediate frequencies, for which the response of SSFLC is dominated by collective relaxation processes due to azimuthal reorientation of molecules arranging chevron layers. The methods are experimentally shown to lack consistency within very low, relatively low, and high field-frequency ranges. The disagreement appearing at relatively low frequencies is argued to be a consequence of different recording by dielectric and optical techniques the dynamics of zig-zag walls, forming defects in chevron structure.