Per Rudquist

Solution of the dark state problem in antiferroelectric liquid crystal displays

So far, it has proven impossible to achieve an acceptable dark state between crossed polarizers for antiferroelectric liquid crystals (AFLCs), which otherwise would have an enormous potential for electro-optic applications, in particular for high-resolution full color displays. The reason lies in static and dynamic spatial fluctuations of the optic axis. As both have intrinsic causes it is not likely that the problem is ever going to be solved by improvement in alignment and addressing methods. We show that if the directors in alternating layers are orthogonal to each other, the AFLC acquires new optical properties such that the problem is eliminated, and a dark state extinction is achieved which is only limited by the quality of the polarizers. After having synthesized such a material, we have been able to demonstrate the predicted unique electro-optical properties of this new class of AFLC materials.

Antiferroelectric liquid crystals with 45° tilt - a new class of promising electro-optic materials

Abstract Antiferroelectric liquid crystals with a tilt angle of 45 degrees have very interesting optical properties, which seem to have been overlooked so far - perhaps because such materials have hardly been available. We have prepared a four-component mixture of partially fluorinated compounds with a SmCa* phase in the interval between 27.4°C and 121.6°C, in which the tilt angle θ saturates at 45 degrees for T≤80°C, and we investigate the optical properties, theoretically and experimentally. One of the surprising features of 45 degree materials is that they permit a remarkably high contrast by virtue of an excellent dark-state, in spite of the fact that AFLC materials are notoriously difficult to align. This is because a 45° AFLC turns out to be (negatively) uniaxial instead of biaxial. We describe these properties and propose a number of potentially interesting new applications, including a polarizer-free display mode and a three-level “phase-only” modulator.

Electro-optic characteristics of de Vries tilted smectic liquid crystals: Analog behavior in the smectic A* and smectic C* phases

Chiral smectic A liquid crystal materials of the de Vries type (with molecules tilted relative to the layer normal) exhibit analog field-induced (electroclinic) optic axis rotation accompanied by an increase in birefringence. We identify two such de Vries smectic A* materials and use them to develop and test models for these characteristic electro-optic effects. These materials also exhibit colossal analog field-induced optic axis rotation in the lower temperature smectic C* phase, a consequence of polarization charge stabilization, and of polarization screening of the applied field in the liquid crystal.

Linear electro‐optic effect based on flexoelectricity in a cholesteric with sign change of dielectric anisotropy

The linear electro‐optic effect in short‐pitch cholesterics is based on the linear coupling of the medium with an applied electric field. It has a number of remarkable properties. The electric field causes the optic axis to tilt in a plane parallel to the surfaces of the cell glass plates, giving the same symmetry as the electro‐optic effects in the smectic C* phase (surface stabilized ferroelectric liquid crystals and deformed helix mode) and the smectic A* phase (soft mode/electroclinic effect). For shutters and displays this guarantees a very wide viewing angle. The induced tilt is a linear function of the applied field, at least for small fields, which gives a well‐controlled continuous grey scale. Furthermore, it is practically independent of temperature. Response times of the order of 100 μs are easily achievable. The most interesting development in this effect would be to extend the linear regime to much larger tilt angles, in particular to 22.5°, where light could be modulated from 100% to zero tr...

The case of thresholdless antiferroelectricity: polarization-stabilized twisted SmC* liquid crystals give V-shaped electro-optic response

We have studied the three-component liquid crystal mixture reported to exhibit ‘thresholdless antiferroelectricity’ [Inui et al., J. Mater. Chem., 1996, 6, 671]. We find that the thresholdless or V-shaped switching is obtained in the absence of antiferroelectricity. This analog electro-optic response is due to the field-induced switching of a twisted smectic C* structure stabilized by polar surface interactions and by electrostatic bulk polarization charge interactions. The latter confine the director twist to thin surface regions leaving the bulk of the cell uniform, which gives good extinction at zero field. In sufficiently thin cells, such thresholdless switching can in fact be followed down to much lower temperatures, where the bulk would be antiferroelectric, but is maintained in the cells in the ferroelectric state by hysteresis from surface action.

Volume-stabilized ULH structure for the flexoelectro-optic effect and the phase-shift effect in cholesterics

Flexoelectric coupling gives rise to a linear electro-optic response in cholesterics (flexoelectrooptic effect) with a uniformly lying helix (ULH) structure and this electro-optic effect is strongly reliant on the homogeneity and quality of the texture. The ULH structure, unfortunately, is complicated in itself and may be perturbed by factors such as dielectric coupling, surface/liquid crystal interactions and phase transitions, and often there is a tendency for relaxation into the Grandjean texture (standing helix structure) with time. Hence, in order to exploit the flexoelectro-optic effect in cholesterics any instability of the ULH structure must be ruled out. We have overcome these problems by incorporating a polymer network by means of photopolymerization of a reactive monomer added to the cholesteric. The volume stabilized ULH structure still exhibits the flexoelectro-optic effect, it is stable and it is also retained after heating to the isotropic phase and going back to the cholesteric phase. In a...

On the phase sequence of antiferroelectric liquid crystals and its relation to orientational and translational order

The substance MHPOBC is the oldest and still most important reference antiferroelectric liquid crystal (AFLC). There is still considerable controversy concerning the correct phase designations for this material and, in particular, about the presence or absence of SmC* in its phase sequence. By means of dielectric spectroscopy and polarizing microscopy, we show that whereas the pure compound lacks the SmC* phase, this phase rapidly replaces the SmC*β subphase through the reduced purity resulting from temperature-induced chemical degradation which is hard to avoid under standard experimental conditions. X-ray investigations furthermore show that this change in phase sequence is coupled to a decrease in translational order. This explains the large variations in the reported phase sequence and electro-optic behaviour of MHPOBC, in particular concerning the SmC*β phase which has been said to exhibit ferro-, ferri- as well as antiferroelectric properties. It is likely that the sensitivity of the AFLC phase sequence to sample purity is a general property of AFLC materials. We discuss the importance of optical and chemical purity as well as tilt and spontaneous polarization for the observed phase sequence and propose that one of the key features determining the existence of the different tilted structures is the antagonism between orientational (nematic) and translational (smectic) order. The decreased smectic order (increased layer interdigitation) imposed by chemical impurities promotes the synclinic SmC* phase at the cost of the AFLC phases SmC*α, SmC*β, SmC*γ and SmC*a. We also propose that the SmA* phase in FLC and AFLC materials may actually have a somewhat different character and, depending on its microstructure, some of the tilted phases can be expected to appear or not to appear in the phase sequence. AFLC materials exhibiting a direct SmA* -SmC*a transition are found to be typical ‘de Vries smectics’, with very high orientational disorder in the SmA* phase. Finally, we discuss the fact that SmC*β and SmC*γ have two superposed helical superstructures and explain the observation that the handedness of the large scale helix may very well change sign, while the handedness on the unit cell level is preserved.

The Orientational order in So-Called de Vries Materials

There is a great interest in smectic materials that show no layer shrinkage (NLS) in the transition from smectic A to smectic C. Such materials are often discussed in terms of “de Vries materials” or “de Vries behavior” after A. de Vries, who proposed different mechanisms for this NLS behavior, involving a significant tilt β of the individual molecules in the smectic A phase. According to the original proposition of de Vries, the molecules are already tilted in this kind of smectic A phase, with a large constant tilt and the same tilt direction in each layer but random tilt direction between different layers. Despite the individual molecular tilt the smectic A phase remains uniaxial and the transition to the biaxial smectic C state is seen as a global ordering in the tilt directions. The model thus ad hoc predicts that there is zero layer shrinkage at the A – C transition. We refer to this model as the “hollow cone distribution”. As later pointed out by de Vries there are, however, other possible models for describing a tilt disorder (thermodynamically unavoidable) in the A phase. Nevertheless, the hollow cone has been a widely accepted model in the literature and is repeatedly referred to in discussing de Vries behavior or even taken as a basis for theories describing it. We discuss different smectic A orientational distribution functions that could be related to de Vries behavior or de Vries transitions. We find that two opposite models have comparable predictive power but only one gives a consistent picture together with existing data. Our conclusion is that the smectic A – smectic C transition can have a continuously changing character from a “pure tilt” to a “pure de Vries”, and we illustrate the orientational distribution functions in the A and C phases for these two limiting cases. We find that de Vries behavior is not related to any exotic distribution of hollow cone or similar kind in the A phase, but instead to an unusual combination of low nematic order and high smectic order in the de Vries smectic A. As the technical interest in such materials is considerable, a directed effort toward the synthesis of new optimized materials would be important.

The flexoelectro-optic effect in cholesterics

The flexoelectro-optic effect in short-pitch cholesterics [1] is analysed in terms of applied electric field strengths and material parameters such as the two flexoelectric coefficients es and eb and the three elastic constants. Starting from the free energy density of the uniformly lying-helix (ULH) configuration, including the flexoelectric polarization term, the equation describing the field-induced tilt angle of the bulk optic axis is derived. It is convenient to introduce the flexoelectric “anisotropy” Deltae as the difference between the splay and bend flexoelectric coefficients, hence defined by Deltae es eb. Our results then show that Deltae is the essential material parameter controlling the sign and magnitude of the electrically induced tilt. In the region of linear approximation, the tilt is proportional to Deltae and to the electric field E, and inversely proportional to the helical wave vector k, as well as the effective elastic constant (K11 K33). The individual values of the elastic constan...

Orthoconic antiferroelectric liquid crystals

The ubiquitous liquid crystal display (LCD) is based on nematic liquid crystals (LCs) and has during 40 years developed from simple few digit displays into high-resolution flat-panel displays. A last and very important step towards the present TVs was the combination with thin film transistors that also led to the introduction of several new switching modes. Despite the enormous success of this technology, there is presently a renewed interest in LCs with faster electrooptic response, especially for future 3D vision display systems and possibly for field-sequential-colour (FSC) generation displays. Here, I focus on the so-called orthoconic antiferroelectric LCs (OAFLCs), which can provide the fast switching of conventional antiferroelectric LCs, but combined with a potentially much higher optical contrast. The reason is that the dark state of orthconic materials has a homeotropic optic axis, which makes the extinction ideally complete between crossed polarisers and independent of in-plane alignment fluctuations characteristic of all antiferroelectric LCs. The basic features and device physics issues of orthoconic LCs are reviewed and the most important remaining challenges to be met in terms of materials development are discussed in this article. Furthermore, a few examples of new applications made possible with the use of present and future orthoconic materials are given.