Arnout De Meyere

Viewing angle of liquid-crystal displays: representation on the Poincaré sphere

Using the extended Jones calculus for oblique incidence, we have succeeded in representing light propagation through birefringent media on the Poincaré sphere for arbitrary propagation directions. The situation is far more complicated than for the case of perpendicular transmission. To achieve a simple representation method we had to define appropriate conventions. One can use the method to evaluate the efficiency of existing viewing-angle compensation foils. The Poincaré sphere turns out to be a valid geometrical method for the design of new compensation foils that optimize the contrast ratio. The different parameters of a homogeneous compensation foil that is applied to a liquid-crystal display to improve the viewing-angle properties can be systematically determined.

Analytical Treatment of Grey Levels in Antiferroelectric Liquid Crystal Displays

Abstract Under certain simplifying assumptions, which are believed to be non-essential, it is possible to treat grey levels in antiferroelectric liquid crystal displays analytically. This shows more clearly than simulations which parameters play an essential role.

CALCULATION OF CHEVRON PROFILES IN FERROELECTRIC LIQUID-CRYSTAL CELLS.

Abstract The energetic aspects of layer deformation in ferroelectric liquid crystal cells are discussed and the actual chevron shape is calculated in some situations. We emphasize two, in our view, essential energy contributions. One term considers the layer curvature. The other one refers to the variations in the distance between layers and the consequent changes of the smectic cone angle. In some simple cases we can determine optimal shapes of the chevron layer structure by analytical solutions, based on these two energy terms. In more complicated situations other contributions have to be considered and the chevron profiles are simulated numerically. The influence of the applied voltage and the choice of parameter values are studied.

GRATING DIFFRACTION IN (ANTI-)FERROELECTRIC LIQUID CRYSTAL DISPLAYS

Abstract For many inhomogeneous structures in LCDs, such as antiferro- or ferrielectric LCDs and striped textures in classical or short-pitch ferroelectric LCDs, we present a strong, rigorous algorithm for the calculation of the optical transmission. We illustrate the technique in case of some simple alternating structures. The number of diffraction orders strongly depends on the grating period (Λ) - light wavelength (λ) ratio Λ/λ. For the alternating antiferroelectric state the transmission of visible light can be treated with only the zeroth order of diffraction. In this case, we find that the AFLC can be simulated as a birefringent layer with the principal axes parallel and perpendicular to the layers. In case Λ/λ > 1 higher orders of diffraction come into play. The dimension of the corresponding eigenvalue problem becomes too large for analytical treatment. We give some numerical results.

Modelling the molecular distribution in chevron FLCDs

Abstract Formulae are calculated for the main contributions to the energy in FLC-cells in the one-dimensional case. Special emphasis is given to the modelling of the smectic layers and to the influence of the electric field. For the study of structural questions in FLCDs, an efficient numeric simulation method is proposed. The use of it is illustrated with an example.

Homogeneous switching in antiferroelectric liquid crystal displays

Abstract We use a very simple bookshelf model to describe the behaviour of antiferroelectric liquid crystals in an electric field. Starting from the energy expression of the LC, which comprises antiferroelectric coupling, coupling with the electric field and surface anchoring, we can derive the dynamic equations. We carry out numerical simulations in order to solve these equations in different cases. We consider the three different switching modes: switching from the alternating state to one of the uniform states, switching from a uniform state to the alternating state, and switching between the two uniform states.

On higher order variational analysis in one and three dimensions for soft boundaries

Abstract For some problems in liquid crystal physics we need to use the Euler equation and the corresponding boundary equation in the three-dimensional case with soft boundaries. As a further complication the free energy expression, which should be minimized, might contain some second-order and third-order derivatives. These higher-order derivatives will cause the spatial derivatives of the boundary normal to appear in the boundary equation. Explicit formulae are given for the Euler equation and the corresponding surface equations for a general case. As an example, the theory is applied to nematic liquid crystals, where the general Euler equations and surface molecular fields are derived, including the effects of an imposed electric field.

Influence of ionic contaminations on SSFLCD addressing

Abstract In this paper a theoretical SSFLCD switching model is presented which focuses on the calculation of the internal electric field. Using this model we can investigate the joint dynamics of the FLC reorientation and charge carrier transport inside the LC-layer with or without applied voltage. We give some results on charge separation and addressing problems.

Light propagation and color variations in liquid-crystal displays

General theories for light propagation are applied to specific liquid crystal displays. For the case of a homogeneous molecular distribution as well as that of a uniform twist we elaborate the classical Poincare sphere method and a modified Jones calculus. The evolution of the polarization ellipse in the different configurations is studied and is depicted in an original way, which gives a better understanding of the color effects in liquid-crystal displays. Practical quantitative calorimetric conclusions are drawn in order to facilitate the experimental study of thickness variations in liquid crystal displays.

Geometrical averaging of AFLC dielectric tensors

The optical transmission of antiferroelectric liquid crystal layers is non-trivial. For optical wavelengths we average the dielectric tensors. The aim is to provide a geometrical way for this averaging. By a thorough mathematical analysis, we are able to construct the representation ellipsoid for the final e-tensor in most cases. In this way, we can easily predict the axes and indices of refraction of the structure. Especially the molecular states that occur during the hysteresis switching behaviour are studied.