Natural optical activity and liquid crystals

Optical activity of matter is related to the chirality of its constitutive molecules. In liquid crystals, chiral molecules can give rise to superstruc tures in which the local dielectric tensor rotates in space describing a he lix, a fact which greatly enhances the optical activity of the medium. The structures and the optical properties of some helical phases are well-known , as for instance the cholesteric and some chiral smectic phases. For short enough helix pitches, the periodic medium can be considered optically as h omogeneous and described by the same constitutive equations used to define the optical properties of solid crystals. Such liquid crystal phases repres ent an ideal tool to apply the methods, used since a long time in optics, t o define homogeneous models for non homogeneous media and to discuss their limits of validity. A brief account is given of the main results recently f ound in this research area.