Liquid crystal VAN tilt bias by surface relief patterning

Liquid Crystal Displays require controlled alignment of the liquid crystal molecular director at its confining surfaces. These surfaces may be coated glass or in the case of 'Liquid Crystal On Silicon' (LCOS) technology, a silicon backplane. In the case of Vertically Aligned Nematic (VAN) cells an initially vertical orientation is used and from this the director may tilt in any direction. Some means is required to bias the tilt in a consistent direction to avoid the occurrence of differently oriented domains. For VAN cells one tilt bias method is oblique deposition of silicon oxide. An alternative method which eliminates concerns over consistency of deposition angle over a large substrate area is the use of surface relief structures to induce tilt bias. This is attractive for LCOS devices. Liquid crystal modelling tools [1] have been used to simulate the effects of rectangular and triangular shaped 'bumps' and 'dips' protruding from and extending into the LC's enclosing surfaces respectively. The director orientation and optical transmission of the LC pixels biased in this way are examined as a function of time during the switching cycle and spatially across the pixel to show that the combination provides controllable tilt bias

Finite-element modeling of liquid-crystal hydrodynamics with a variable degree of order

A finite-element model of liquid-crystal hydrodynamics based on the Qian and Sheng formulation has been developed. This formulation is a generalization of the Ericksen-Leslie theory to include variations in the order parameter, allowing for a proper description of disclinations. The present implementation is well suited to treat properly the various length scales necessary to model large regions yet resolve the rapid variations in the order parameter in proximity to disclinations

Modeling of weak anisotropic anchoring of nematic liquid crystals in the Landau-de Gennes theory

The anisotropic anchoring effect of a treated solid surface on a nematic liquid crystal is described in the Landau-de Gennes theory using a power expansion on the tensor-order parameter and two mutually orthogonal unit vectors. The expression has three degrees of freedom, allowing for independent assignment of polar and azimuthal anchoring strengths and a preferred value of the surface-order parameter. It is shown that in the limit for a uniaxial constant-order parameter, the expression simplifies to the anisotropic generalization of the Rapini-Papoular anchoring energy density proposed by Zhao et al. Experimentally measurable values with a physical meaning in the Oseen-Frank theory can be scaled and assigned to the scalar coefficients of the tensor-order-parameter expansion. Results of numerical experiments comparing the anchoring according to the study of Zhao et al. in the Oseen-Frank theory and the power expansion in the Landau-de Gennes theory are presented and shown to agree well

Modelling the optics of high resolution liquid crystal devices by the finite differences in the frequency domain method

A procedure combining accurate liquid crystal and electromagnetic modelling is developed for the analysis of wave propagation through liquid crystal devices. This is required to study the optics of high resolution liquid crystal cells or cells containing very small features, where diffraction effects occur. It is also necessary for the study of optical waveguiding devices using liquid crystal as variable permittivity substrates. An accurate finite element modelling program is used to find the permittivity tensor distribution, which is then used to find the response of the device to an excitation electromagnetic field by means of a finite difference in the frequency domain (FDFD) approach

3D Modelling of Twist Wall at the Electrode Edge of Liquid Crystal Cells

Q-tensor simulation of the liquid crystal structure at the edge of electrodes has been carried out. The modeling shows a twist wall, which reverses direction to form a zig-zag structure. The results are compared with experiment. Also a defect loop is found in micro-lenses formed using a hole electrode structure

Accurate modelling for the analysis and design of liquid-crystal-based microwave devices

Liquid crystal substrates have been shown to provide the means to develop low-cost, reconfigurable, adaptive and tuneable microwave and MM-wave devices for mobile and wireless communication systems. However, techniques for the characterisation of materials, device fabrication and design are necessary in order to take maximum advantage of the possibilities that these materials offer. This includes appropriate modelling methods to simulate accurately the switching behaviour of the liquid crystal and the characteristics of the wave propagation through the devices, taking full consideration of the point-by-point variation of the material tensor permittivity. We describe these techniques here and show their application in the design of a meander-line phase shifter