Reflection from imperfect cholesteric liquid crystals: Basic properties and applications

There is a great deal of interest in the Bragg reflective properties of imperfect (multi-domain) cholesteric liquid crystals (CLC) from both a fundamental viewpoint and recently for their use in reflective displays. By the introduction of a polymer network the domain structure can be controlled, enabling quantitative experiments to be conducted and making a practical reflective display feasible. This research involves the development of an approach to calculate and measure the reflection spectra from an imperfect CLC in the planar state. Reflected luminance, contrast ratio and chromaticity are measured as a function of incident light and detector angle. Models are developed to explain the observed results and good agreement is found. From the models and measurements, optimal illumination conditions for display applications are determined. The effect of dichroic dyes on the reflection spectra is investigated and shown to improve color quality. Several approaches are developed to produce a multi-color display.;Domain structure is characterized with optical microscope and SEM analysis as a function of polymer concentration, UV irradiation time and type of polymer. Domains are determined to be single layer with planar boundaries that decrease in size with increasing polymer concentration. The distribution in helical axis orientation is established by angular reflectance measurements to be gaussian with standard deviation increasing as the polymer concentration increases. Fitting the observed reflected spectra shapes with those calculated using the 4 x 4 Berreman method shows that the polymer also causes a distribution in the pitch length.;Reflected luminance from the planar state is a function of incident light angle and peaks at the specular angle. The contrast ratio is relatively less dependent on the angle of incidence when viewed from the front. Light lost due to scattering by defects at the higher polymer concentrations is around 50 percent. Illumination directly from the front is the optimal condition for the material studied in this research and there is found to be little loss in display properties for illumination angles out to 30 degrees. Two-dimensional polar plots of luminance and contrast ratio are presented.;The inherent broadening of the reflected spectrum as the reflected wavelength increases results in a loss of color quality. The use of dichroic dyes to narrow the reflection spectrum and thus improve color quality is investigated. The reflection spectra from CLC material with different dyes at various concentrations are measured and agreement found with calculated reflection spectra using the dynamic theory of reflection. Measured chromaticity coordinates show an improvement in reflected color quality and the incorporation of a dichroic dye is currently being used in the manufacture of commercial reflective cholesteric displays.;In addition, lithographic/etch techniques are utilized to form a rib/channel arrangement to spatially separate CLC material of different pitch formed with photo-cleavable chiral material to produce a multi-color display.