Optical Properties of Chiral Nematic Liquid Crystals

Abstract The selective reflection of light from thermochromic liquid crystals is characterized by the peak wavelength (λp) and width (Δλ) of the reflection spectrum and may be related theoretically to the average refractive index ([nbar]), the birefringence (Δn) and the pitch (p) of the material. We have examined the influence of each of these parameters, varied and measured independently, for two series of new chiral nematic mixtures. Materials are described with birefringences in the range 0.04 ≤ Δn ≤ 0.16 and the equivalence of λp/Δλ and λ/Δn has been studied. We have shown that the relationship is valid for large values of An, but holds less well as An becomes small (Δn ≤ 0.1). In the second series of materials, we have considered the influence of twisting power alone. In the absence of other variables (i.e., birefringence, transition temperatures, etc.), a decrease in the twisting power gave only an associated increase in the selective reflection wavelength. We conclude that changes in birefringence ...