FORM BIREFRINGENCE IN HELICAL LIQUID-CRYSTALS

The optical properties of cholesteric and chiral smectic liquid crysta ls are studied, in the limit of pitch smaller than the wavelength of v isible light, by means of a formalism borrowing techniques developed t o explain form birefringence effects in particulate media. The behavio ur of actual stratified anisotropic substances is shown to be accurate ly approximated, in this limit, by an effective homogeneous uniaxial c rystal, whose dielectric tensor elements are easily obtained from the exact values corresponding to the stratified medium. In two notable ca ses, i.e., for normal incidence of light and in smectic-C liquid cryst als with a critical value of the tilt angle, the optical properties of the stratified material may only be accounted for by introducing an e ffective medium suitably characterized by optical activity. In all the examined cases, the periodic media appear to be completely equivalent to homogeneous crystals displaying optical activity, up to pitch valu es corresponding to one half of the first Bragg diffraction band. The model's results having higher relevance both in fundamental studies an d applications are described and discussed.