Steve J. Elston

Hybrid method for modelling light leakage by a spherical object in a liquid crystal layer

A new hybrid method composed of the geometrical optics and the extended Jones matrix methods was developed, and applied to the modelling of the light leakage around a spherical object in a twisted nematic liquid crystal layer. Comparison with optical microscopy shows similar tendencies, and the new method may be useful for various practical applications such as structure optimisation for minimising light leakage in liquid crystal displays.

Light wave propagation in liquid crystal displays by the 2-D finite-difference time-domain method

Abstract Light wave propagation within liquid crystal (LC) devices is studied using a two-dimensional (2-D) finite-difference time-domain (FDTD) method. Computational space termination is provided by a combination of the perfectly matched layer absorbing boundary condition and periodic boundary conditions, to overcome the limitations imposed by previously proposed FDTD methods for LC optics. Both normal and oblique incidence cases are successfully implemented and a consistent method for magnitude and phase extraction is made available. This provides a rigorous numerical solution for the light wave propagating within LC devices, and a sample application for a small twisted nematic pixel is given.

Finite-difference time domain method for light wave propagation within liquid crystal devices

Light wave propagation within liquid crystal devices is determined by the application of an appropriate finite-difference time-domain (FDTD) method, accounting rigorously for electromagnetic wave propagation. The introduction of FDTD calculations is aimed to substitute the matrix-type solvers in cases where the stratified-medium approximation fails. Such cases are commonly encountered when a liquid crystal device exhibits variations of the director orientation along the transverse direction on the scale of the propagating optical wavelength, for instance, at pixel edges. The formulation and sample numerical application are focused on planar liquid crystal devices exhibiting bend/splay deformation.

Stretchable liquid-crystal blue-phase gels

Liquid-crystalline polymers are materials of considerable scientific interest and technological value. An important subset of these materials exhibit rubber-like elasticity, combining the optical properties of liquid crystals with the mechanical properties of rubber. Moreover, they exhibit behaviour not seen in either type of material independently, and many of their properties depend crucially on the particular mesophase employed. Such stretchable liquid-crystalline polymers have previously been demonstrated in the nematic, chiral-nematic, and smectic mesophases. Here, we report the fabrication of a stretchable gel of blue phase I, which forms a self-assembled, three-dimensional photonic crystal that remains electro-optically switchable under a moderate applied voltage, and whose optical properties can be manipulated by an applied strain. We also find that, unlike its undistorted counterpart, a mechanically deformed blue phase exhibits a Pockels electro-optic effect, which sets out new theoretical challenges and possibilities for low-voltage electro-optic devices.

Short pitch cholesteric electro-optical device based on periodic polymer structures

The helical flexoelectro-optic effect produces a submillisecond, temperature-independent in-plane rotation of the optical axis and is potentially interesting for the display industry. The main drawback is that it relies on a texture, the uniform lying helix (ULH), which is intrinsically unstable. We present a method based on the use of periodic polymeric microchannels to create highly ordered and stable ULH structures. Electro-optic measurements performed on a test device show a large contrast ratio between bright and dark states (better then 100:1), fast switching (200 μs), and large optical rotation (>30°).

Optical diffraction from a liquid crystal phase grating

The finite-difference time-domain method has been used in the numerical analysis of the optical diffraction properties of a liquid crystal phase grating. The grating is formed using a nematic material that is switched using striped electrodes with a unity mark-space ratio. Three different surface pretilts have been investigated: 0°, 30° parallel, and 30° antiparallel. The tilt geometry determines the degree of suppression of the zero order and the asymmetry of the diffraction. Highly efficient beam-steering devices are shown to be possible using the antiparallel tilt alignment.

Evaluation of optical anisotropy in the pretransitional regime in antiferroelectric liquid crystals

By measuring the transmission through an antiferroelectric liquid crystal device (placed between crossed polarizers) as a function of both orientation and applied field in the so-called electroclinic-like, pretransitional regime the behaviour of the effective optic axis tilt angle (psi) and effective optic anisotropy (Delta n) is determined. The relatively poor alignment obtained in antiferroelectric liquid crystal devices is allowed for in the data interpretation and high quality results are presented.

Flexoelectric switching in a zenithally bistable nematic device

A Q-tensor method is used to model a grating-aligned zenithally bistable nematic device in two dimensions. The flexoelectrically driven switching between the two ground states is shown to occur through the creation and annihilation of pairs of defects, and the sign and magnitude of the voltage thresholds demonstrated to depend on the relative dielectric permittivity of the grating material.

Wide-angle beam propagation method for liquid-crystal device calculations

A wide-angle beam propagation method suitable for analyzing anisotropic devices involving liquid crystals is presented. The mathematical formulation is based on a system of coupled differential equations involving an electric and a magnetic field component. The contribution of all dielectric tensor elements is included. A numerical implementation based on finite differences is used. Numerical examples are focused on light-wave propagation within twisted nematic pixels found in microdisplays, with all effects arising at pixel edges that are incorporated. A comparison between the results obtained and the prediction of finite-difference time-domain simulations is conducted, showing satisfactory agreement. The required computational effort is found to be minimal.

The Optics of Ferroelectric Liquid Crystals

Abstract Ferroelectric liquid crystals (FLCs) have been under investigation for use in electro-optic devices for over a decade. In this review the development of the studies of the optics of FLCs over this period is discussed. Starting from the first electro-optic switching studies undertaken in 1980 by Noel Clark and Sven Lagerwall, and the structure in a FLC cell proposed to explain the effects, the process of developing understanding is followed. The review concentrates on the structure revealed to be present in a FLC filled cell, as this is of central importance in the development of optical devices using this material. Problems encountered in device development, and the approaches taken to overcome them are discussed, including special surface alignments, AC stabilization, and treatment of cells with large electric fields. FLC continuum theory is also discussed as this is needed for calculation of the liquid crystal director profiles, and hence the determination of optical performance. More recent de...