Optical activity in the isotropic and blue phases of two cholesterics

Optical activity measurements were performed in the isotropic phase and blue phases of two short-pitch cholesteric liquid crystals (cholesteryl oleate and cholesteryl oleyl carbonate). The data in the isotropic phase are interpreted in terms of five structural modes of the order parameter. Coupling between these modes gives rise to an extremum and a change of sign in the pretransitional optical rotation. The intensities of the polarized and depolarized component of the transmitted beam were measured as an extral tool to locate the phase transitions.