Pretransitional optical rotation in the isotropic phase of cholesteric liquid crystals

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Abstract
We discuss the phenomenon of a pretransitional enhancement in the optical rotatory power of the isotropic phase of cholesteric liquid crystals. The effect is viewed theoretically as the modulation of dielectric properties of the isotropic liquid by chiral orientational correlations which destroy local inversion symmetry in a way that produces circular birefringence. De Gennes's theory of short-range order is applied to derive a quantitative result. This is experimentally tested with a system of "chiral-nematic" liquid crystals, achieving good rapport in the pretransition region. The existence of chiral orientational order in the isotropic phase is confirmed.