Optical activity of cholesteric liquid crystals in the pretransitional regime and in the blue phase

Using the definition due to Landau, we present a closed-form expression for the optical-activity tensor for a cholesteric liquid crystal. The rotatory power is calculated in the pretransitional regime (above the isotropic-cholesteric transition), as well as in the ordered blue phase. The expression obtained for the first case reduces, in the low-chirality limit, to that of Dolganov et al. The result for the blue phase is new, and indicates the possibility of distinguishing between some of the ordered cubic structures that were recently proposed by various workers.