M. Matuszczyk

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.

Dielectric anisotropy and dielectric torque in ferroelectric liquid crystals and their importance for electro‐optic device performance

Measurements of the relevant anisotropic part of the dielectric constant for ferroelectric liquid crystals allows a calculation of the dielectric torque acting upon the director during addressing conditions in a matrix‐multiplexed device. The frequency dependence of the dielectric anisotropy is presented for different commercially available ferroelectric liquid‐crystal materials in the range 103–106 Hz. The field dependence of the switching time has been measured for one of the materials in order to determine the field at the minimum value. The ratio between this field and the field at which switching ceases has been determined and found to be in good agreement with the theoretically calculated value. The results enable us to consider the torque balance as a function of field and frequency, and thereby identify appropriate driving conditions.

Fast-Switching Low-Temperature Liquid Crystal Mixtures

Abstract The metastable chiral smectic C phases of the compounds have been made technically useful by alloying to different two- and three-component systems with enantiotropic behaviour. These systems can serve as a base for multi-component systems of higher order for optimizing various properties. In a similar way, the inconveniently high-lying ferroelectric phases of the compounds have been taken down to a useful temperature range. The mixtures have an excellent UV stability and are well adopted for use in high speed bistable SSFLC devices.

Smectic A* materials with 11.25 degrees induced tilt angle for full gray scale generation

Abstract In the SmC* phase, 22.5° tilt materials are optimum for cells operating in the birefringent mode (under idealized bookshelf conditions), while for SmA 11.25° at present is of special interest, since it makes it possible to induce a full 90° rotation of incoming plane-polarized light using a double-cell, or even a single cell in reflective mode. We present one of the first practical A mixtures yielding 11.25° tilt at room temperature and at moderate field strengths, at the same time being easily aligned and stable. The mixtures presented are ideally suited for use in continuous grey-scale double cells, in reflective single cells and, eventually in combination with semiconductive elements, in applications such as SLM:s (Spatial Light Modulators).

Electrically controlled optical attenuators and switches with ferroelectric liquid crystals

Abstract Electro-optic modulators can be used as choppers, function generators or optical power controllers. The materials dealt with are PLZT ceramics and nematic LC compared with smectic A and C LCs. PLZT is fast in the scatter mode, but its contrast is poor. The NCAP nematic LC structure has a high contrast, but is slow. Both PLZT ceramics and smectic phases can achieve microsecond switching times in the polarization mode. The achieved contrast is high with PLZT, while with LCs it depends on the switching speed. A modulator based on polarization has a complicated structure when non-polarized light is used.

Atomic Force Microscopy of Rubbed Polyimide Aligning Films for Liquid Crystal Displays

Abstract The surface of polyimide (PI) films for aligning liquid crystals were studied by means of atomic force microscopy (AFM). The surface of the unrubbed PI films consisted of polymer clusters of different sizes which are randomly distributed over the film area. After rubbing, however, these polymer clusters formed long chains along the rubbing direction. The cluster chains were separated with periodicity about 100 nm for weak rubbing strength. Deeper grooves are also present, separated with periodicity about 2 μm that could be related to the microstructure of the fibres from the rubbing cloth. On increasing the rubbing strength further on, the cluster chains coalesce into wider ones. The rubbed PI films show optical retardation, which was increased rapidly with increased the rubbing strength and reached a constant value of about 1.4nm.

A 1″ × 0.5″ FLC bar-graph display with short-term memory

Abstract The development of an SSFLC cell to be used as an optical switching element is presented. The bar-graph type cell is composed of 96 stripe-electrodes of 200 micron width. Technical aspects of cell construction, such as electrode and thickness control, are discussed together with the electro-optic peiformance and contrast of cells with different surface coatings. Finally, the driving technique for the cell is presented.

On the seemingly antiferroelectric behaviour of certain ferroelectric liquid crystals

Abstract Double current peak, double hysteresis loop and even in some sense three electro-optic states, the main characteristics of the antiferroelectric liquid crystal phase, are found also in the ferroelectric smectic C* phase, observed so far on several liquid crystal materials possessing a C*—N* phase sequence. In the observed cases, the confining surfaces bring about a pronounced monostable surface anchoring of the liquid crystal molecules forcing the layers to be broken in two alternating directions, corresponding to the equivalent surface directions of positive and negative molecular tilt relative to the layer normal. The resulting striped texture is very reminiscent of certain domain patterns in solid ferroelectrics. The seemingly anomalous electric switching behaviour of this C* texture mimics an antiferroelectric and may thus easily lead to an incorrect designation of the phase if not supplemented by other identification methods. Hysteresis shape and switching current peaks are thus by themselve...

Comparative studies of the multiplexability of SSFLC devices with respect to alignment characteristics

Abstract Several series of Surface-Stabilized Ferroelectric Liquid Crystal (SSFLC) cells were prepared using different aligning techniques (obliquely evaporated SiOx, buffed polyimides and nylon). The surface structure of aligning layers were studied by Atomic Force Microscopy (AFM). The samples were filled with various commercial FLC materials from Merck, BDH and Chisso. Our investigations were focused on the correlation between the aligning layer and the electro-optic characteristics of the SSFLC samples. The strength of the elastic torque is related to the surface conditions and evaluated using a specially designed test waveform. The switching properties and image sticking phenomenon have been studied under real addressing conditions.

Biaxial dielectric properties in the chiral smectic C phase of single compounds and their relation to molecular structure

Abstract In an attempt towards relating the dielectric biaxiality to the molecular structure, the dielectric anisotropy and biaxiality and their temperature dependence in the chiral smectic C phase have been determined for seven single compounds. It is found that, on a molecular level, compounds with a strong polar group connected with a cyclohexane ring exhibit a relatively large dielectric biaxiality. Among the substances with large dielectric biaxiality, it is noticed that attaching the polar group to the linking point of the cyclohexane ring and the alkyl chain containing the stereocenter results in the largest biaxiality. The studied compounds possess a smectic C phase preceded either by a smectic A or a nematic phase. As a general rule (with exceptions) we found that a large biaxiality is also coupled to a high value of the spontaneous polarisation. While the dielectric biaxiality for all studied materials is positive, the dielectric anisotropy (at 100 kHz) was found to be negative, especially at lo...