Optical rotatory power in tilted smectic phases

Optical Rotatory Power measurements have been done on different chiral tilted smectics which lead to reproducible results. The high-temperature uniaxial SmA and SmC phases do not show any ORP due to the lack of in-plane anisotropy at the mesoscopic level (i.e. a few layers). At lower temperature one gets the synclinic C^* and the anticlinic C phases with a helical precession around the layer normal which is right-handed for C^* and left-handed for C . These phases are separated in a finite temperature range by a set of two so-called "ferrielectrics” SmC and SmC , which are not necessarily new phases, between which the handedness changes sign at a given temperature, while the change from synclinic to anticlinic tilt angle organization is spread out over the "ferrielectric” temperature range. In order to reconcile all experimental observations, otherwise apparently incompatible, it is proposed that the "ferrielectric” region is a diphasic one where the in-plane organization changes smoothly from the synclinic type to the anticlinic one. Received 7 January 2000 and Received in final form 7 April 2000