OPTICALLY INDUCED REORIENTATION IN NEMATICS DOPED BY CHIRAL AGENTS

We have studied the optical Freedericks transition for normal incidenc e in chiral-nematic mixtures in cells coated for homeotropic alignment . In the case of linear polarization of the incident beam two optical thresholds were observed: the first one is second-order and the second one is first-order, accompanied by very large hysteresis loop. In the case of circular polarization, the double-threshold feature was notic ed only when the light elicity was the same as the sample chirality. W hen the elicity is opposite to the chirality, only one second order re orientational transition is produced. The experimental results were fo und only in partial agreement with the predictions of a simple plane-w ave model.