Linear electro-optic effect in a cholesteric liquid crystal

Abstract

J. Patel and R. B. Meyer [Phys. Rev. Lett. 58, 1538 (1987)] found an electro-optic effect in a cholesteric layer with a unidirectionally aligned helix axis, when an electric field was applied normal to it. The effect is flexoelectric in origin and exhibits a linear behavior in the electric field, at least for low fields, where the dielectric coupling between the liquid crystal molecules and the applied field can be neglected. At high fields, however, the electro-optic response might considerably deviate from the linear character due to the field-induced helix unwinding. We have extended the study, both experimentally and theoretically, to take the dielectric phenomena into account. We do this by investigating the electro-optic response in the case of a field-dependent helix profile, making not only the optic axis direction but also the basic optical parameters strongly field dependent. Computer simulations of the electro-optic response, performed on the basis of this model, show an excellent agreement with the experimental results. This means that the combination of flexoelectric and dielectric coupling is sufficient to describe satisfactorily the physics of this electro-optic effect up to reasonably high fields. We finally address the question: How may the region of linear response be extended further by the design of new materials?