Optical Tensor Configuration Research Articles

A Study of Elastic Deformation During Voltage Switching of a Ferroelectric Liquid Crystal Cell

Ferroelectric liquid crystal devices are primarily of interest owing to their truly bi-stable operation and fast switching times. At present our understanding of the switching processes in ferroelectric liquid crystal cells is incomplete. In order to develop a more complete understanding it is necessary to have a detailed knowledge of the optical tensor configuration adopted by liquid crystalline materials in cells. In this paper we present results of a half-leaky guided mode study of voltage induced elastic deformations of the director profile in a cell containing a homogeneously aligned 40% chiral mixture of the ferroelectric liquid crystal SCE8. The experimentally deduced changes in the director structure are highly accurate and may be used to test theoretical models of surface stabilised ferroelectric liquid crystal devices.

Combined X-Ray and fully leaky guided mode studies of the smectic layer and optic tensor configuration in a ferroelectric liquid-crystal cell.

X-ray scattering together with optical characterization using fully leaky guided modes have been used for the first time to study the same ferroelectric liquid-crystal cell. This enables direct calculation of an accurate cone and chevron description of the liquid-crystal director profile since the layer structure and optic tensor configuration are both well known.

Experimental investigation of the compressible continuum theory of a homeotropically aligned ferroelectric liquid crystal

The optical tensor configuration in a homeotropically aligned ferroelectric liquid crystal (FLC), SCE13 cell, is investigated by means of optical excitation of half leaky guided modes. A thin slab of FLC is confined between a high index pyramid and a low index substrate whose indices bound those of the liquid crystal. In this geometry there exists a small angle range over which a series of sharp resonant modes may propagate in the liquid crystal. Detecting the angular dependent reflectivity for plane polarized radiation and subsequently fitting the data by iteratively modelling from multilayer Fresnel theory, a full characterization of the tilt and twist profile throughout the cell is achieved. The temperature dependence of the tilt of the principal director, which is related to the smectic cone angle, and of the optical permittivity, as well as the pitch have been obtained. The tilt director profile across the cell is interpreted using a compressible continuum theory for SmC* liquid crystals which includes the possibility of variable cone angle and layer spacing.

Guided wave study of the optical tensor configuration in FLC cells

Abstract In the absence of a complete continuum theory to describe the director configuration within ferroelectric liquid crystal cells detailed optical tensor characterisation is vital. There are available a range of optical techniques to study this tensor arrangement, the most powerful being generally some form of resonance guided wave excitation. This study reviews the geometries used with guided wave excitation and shows how the recently utilised half-leaky guided wave technique is extremely powerful in unravelling in detail the optical tensor configuration in ferroelectric liquid crystal layers.

Optical guided-wave study of a homeotropically aligned ferroelectric liquid crystal

Abstract The excitation of optical modes is used to study the optical tensor configuration in a thin ferroelectric liquid crystal layer, cooled from the initially homeotropically aligned nematic phase. By monitoring the angular dependent reflectivity for plane polarized radiation coupled into the guided modes in the smectic C* layer and subsequently fitting the recorded data to predictions from multilayer optics theory, the optical tensor configuration in the layer is fully evaluated. Iteratively modelling the full tilt/twist profile in the cell, progressively converging the predicted reflectivity to experimental data, gives a complete and very well specified picture of the optical tensor throughout the cell. By studying the cell at various temperatures, the temperature dependence of the tilt of the major axis of the optic tensor (which may be related to the cone angle if the smectic layers are parallel to the cell surface) has been established. The temperature dependent optical dielectric constants have ...

The optical tensor configuration in a surface stabilized ferroelectric liquid crystal determined by using half leaky guided modes

Abstract The optic tensor configuration in a surface stabilized ferroelectric liquid crystal cell is investigated using optical excitation of half leaky guided modes. A thin ferroelectric liquid crystal layer is confined between a high index pyramid, with an index greater than the maximum of the liquid crystal, and a glass substrate having an index less than the minimum of the liquid crystal. Using standard attenuated total reflection experimental procedures, over a small angle range a series of sharp resonant peaks are recorded in the s-polarized reflectivity using p-polarized incident light. These peaks are extraordinarily sensitive to details of the optical tensor configuration within the cell. Fitting theoretically modelled reflectivities from multilayer Fresnel theory to the data allows determination of near surface alignment, bending of the chevron, surface tilt angle and biaxiality. To give a clear physical explanation for the great sensitivity of the technique, the electromagnetic field component ...

Optic mode determination of the relaxed state configuration in a ferroelectric liquid crystal cell

Abstract Here the method of propagation of optic modes to examine the alignment in a ferroelectric liquid crystal layer is fully discussed. A sample geometry which allows the excitation of optic modes in a ferroelectric liquid crystal cell is presented and the data obtained in terms of reflectivity. The data are analysed by modelling the expected reflectivity from such a cell for a selection of proposed models for the optic tensor configuration. Models where the smectic layering lies perpendicular to the cell surfaces, or bends slowly across a cell are seen to be incorrect. It is found that a discontinuity in the smectic layering in the middle of the cell must be included to model the data well. This is consistent with the chevron structure seen in X-ray scattering work, and leads to an optic tensor alignment which is largely uniform across the ferroelectric layer. The relation between this and observation of conventional ferroelectric cells is discussed.

The Configuration in a Ferroelectric Liquid Crystal Cell in Terms of a Rigid Chevron Structure

Abstract Reorientation of the optic tensor configuration in a ferroelectric liquid crystal (FLC) cell is investigated under the application of a DC field. This is performed by the excitation of guided optic modes in a thin FLC layer. A careful analysis of the structure is performed by comparison of reflectivity data (showing the mode excitation) to theory for a variety of proposed models of the optic tensor profile. It is seen that the only way in which the data can be explained is by the retention of the chevron structure in the smectic layering under the application of a field. The FLC is seen to reorient within this structure for moderate fields, leaving a pinned point in the middle of the sample at the chevron cusp. This shows that simple models which allow the smectic layer structure to distort under field application are inadequate, and provides direct optical observation of the chevron structure.

Examination of the optic tensor structure in the electroclinic effect in liquid crystals

The optic tensor configuration of a thin ferroelectric liquid crystal layer exhibiting the electroclinic effect is examined by the excitation of optic modes in the layer. It is seen that the restructuring involves no tilt of the optic axis out of the plane of the cell surfaces. This is interpreted in terms of a quasi-bookshelf structure in the smectic layering of the cell.