Recent and Current Liquid Crystal Research Projects

Key personnel: Prof J Roy Sambles, Prof Fuzi Yang

FCPM imaging of liquid crystal structures

As the structures used to control the liquid crystal material moves from one-dimension to two-dimensions the characterisation techniques required to study them need to follow suit. One such three-dimensional imaging technique is fluorescent confocal polarising microscopy. This involves the liquid crystal structure under study being doped to low concentrations with a high quantum yield fluorescent dye composed of elongated molecules with both the excitation and fluorescence dipoles assumed to be along the long-axis of the molecule. The dye absorbs strongly at one wavelength and emits at one of a higher value. The molecules orientate parallel to the liquid crystal molecules, mimicking the director distortion through a cell. A beam of polarised light at the excitation wavelength is focussed into a small volume (<1mm³) in the sample.

For a linearly polarised excitation beam the efficiency of the light absorption of the dye and strength of the fluorescent emission is controlled by the angle between the polarisation axis of the incident light and the absorption axis of the dye molecule. This efficiency is recorded as a variation in the intensity of light detected, and from this the orientation of the director can be deciphered. By scanning in the x-y plane at a fixed depth z, an optical “slice” is recorded, and by repeating this at further distances through the cell, a 3D model of the director orientation can be produced. Recent examples of this work include:

• Imaging of defects around micro-particles embedded in nematic liquid crystal layers.
• Imaging of in-plane effects in cholesteric liquid crystal cells.
• Measurement of the director orientation and flow speed distribution in homeotropically aligned micro-channels under pressure-driven flow.
• Measurement of the alignment of liquid crystals on structured surfaces.
  

Conoscopic imaging of liquid crystals under flow (CJ Holmes)

Conoscopy is a powerful optical tool that allows the optical properties of a slab of birefringent medium to be measured over a range of angles simultaneously. A convergent monochromatic beam passing through a liquid crystal layer is focussed onto the back focal plane of an objective lens to form a real image. Each point in the focal plane receives the light from only one direction and when the sample is viewed between crossed polarisers the wealth of information contained in the resulting interference figures may be used to determine the alignment properties of the liquid crystal sample.

The use of conoscopy in association with a CCD imaging camera allows real-time imaging of the director orientation in a nematic under pressure-driven flow. Of particular interest is the influence of azimuthal orientation, relative to the direction of fluid flow as well as the influence of surface director tilt and applied electric fields.

Recent examples of this work include:
• Real time observation of conoscopic interference figure rotation as a function of liquid crystal (Plane Poiseuille) flow velocity for planar homogeneous alignment at 45° and ≈ 89° to the direction of flow.

"

Measured (left) and modelled (right) conoscopic interference figures for a static homogeneously aligned liquid crystal cell (5CB) at an azimuthal angle of 45° to the crossed polarisers

Ultra-thin metal-clad liquid crystal layers for metamaterial applications

The combined use of thin metal films and liquid crystals provides opportunity for novel types of optical device structures. For example confining a thin layer of liquid crystal between two metal layers allows an effective tunable Fabry Perot to be constructed to electrically control the transmission of light for optical communications. A rather more innovative structure combines the use of patterned metal films, with narrow slits and a very thin (less than 1 ?m) liquid crystal layer. Such a structure supports resonant optical modes whose wavelength is controlled by the spacing between the metal slits and not just the cell thickness. This type of structure is being explored for the first time to fully characterise its optical response and to establish if it has potential for switching use in optical telecommunications.

Fully-leaky guided mode (FLGM) studies of liquid crystal flow (F Yang)

As an anisotropic material in which the director is easily controlled by an externally applied electric field, liquid crystals play a key role in optical flat panel displays. However they are also a fascinating material in their own right, being an anisotropic fluid which is readily influenced by external conditions, for example the pressure gradient, surface anchoring and external electric/magnetic fields.. The original use of the FLGM technique to quantify the flow of these anisotropic fluids under pressure gradients allows the detailed testing of model theories and the quantification of many parameters of the liquid crystal.

Half-leaky guided mode (HLGM) studies of Biaxial Nematic Liquid Crystals (F Yang)

The recent discovery of novel biaxial thermotropic liquid crystals (LC) has produced great interest. Such materials are expected to have have potential for device applications in a new generation of faster biaxial LC displays. These expectations are based on the existence of a secondary director that can be controlled by external fields. However, to be put this into practice, requires a more detailed understanding of the behaviour of such a biaxial LC under applied electrical fields in a cell. This includes the characterisation of both director distributions the dynamic response etc. Although several measurement techniques have been employed in the partial characterisation of different biaxial LC materials, determining the degree of optical biaxiality by optical techniques is still a major challenge. Thus a special design of the HLGM geometry has been set up at Exeter to allow the determination of the biaxial refractive index and both director distributions in a LC cell to pave the way for device applications with these new LC materials.