Optical Properties of Cholesteric Materials used in Surface Stabilised Cholesteric Texture Devices

An in depth study has been made of the optical properties of a series of non-thermochromic chiral nematic materials to determine the influence of refractive index dispersion on pitch measurements. The materials used were mixtures of a chiral nematic material, BL131a, and a nematic material BL130 (from Merck Ltd.) in which the pitch is changed by varying the concentration of the nematic component. Detailed measurements were made of the ordinary and extraordinary refractive indices as a function of wavelength across the visible spectrum. Dispersion effects changed the birefringence by approximately 10% across the visible spectrum. Reflection spectra were obtained and used to deduce the helicoidal pitch. Failure to include dispersion in the calculation led to a significant error in calculations of both the bandwidth and pitch of the material. Finally, as SSCT devices are electro-optic in nature, the influence of the conducting surface of the device on the optical properties is examined.