A pulsed voltage study of the influence of an electric field on the optical permittivity of both smectic A and nematic phases of a liquid crystal

Abstract

Guided optical wave techniques have been used to measure the changes in the optical permittivity of a thin, well aligned layer of a liquid crystal, 4-n-octyl-4′-cyanobiphenyl, as a function of applied voltage. The behaviour of both TE and TM guided modes has been studied, in the nematic and smectic A phases, using a pulsed voltage technique to avoid heating effects. Small changes of both ε⊥ and ε∥ have been detected, of the order of 10⁻⁵, and these changes can be interpreted, for both smectic A and nematic phases, in terms of the suppression of director fluctuations. Close to the smectic A–nematic transition the observed change in (ε∥–ε⊥) in the smectic A phase reverses in sign. This unexpected behaviour is discussed in terms of dipole-dipole interactions.