Dielectric, Electro-Optical and Phase-Change Properties of Liquid Crystal Guest-Host Displays

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Abstract
We present novel measurements and correlations between the electric field-induced change of capacitance, optical appearance and material parameters of nematic as well as cholesteric guest-host layers. New guest-host mixtures comprising various dichroic dyes with transition moments parallel (L-dyes) and perpendicular (T-dyes) to the nematic director exhibiting large dichroic ratios and UV-stability were used. The material parameters determining the capacitive as well as the electro-optical characteristics, including all elastic constants, were measured. Good agreement was found between measureci and numerically calculated characteristics when using the experimentally determined material parameters in the formalism derived to calculate the transmission characteristics of homogeneous nematic guest-host displays. It is shown that the field-induced dielectric changes occurring in cholesteric guest-host layers with parallel and homeotropic wall alignment respectively differ considerably from their electro-optical characteristics in reflective displays. The phase-change calculations used so far are shown to be inadequate to explain the experimental findings.