SPATIAL-DISPERSION AND ROTATORY POWER OF SHORT-PITCH PERIODIC DIELECTRIC MEDIA

By a Bloch-wave decomposition, we deduce a general expression for the effective optical dielectric tensor of a short-pitch periodic dielectr ic medium. The effective dielectric tensor describes an equivalent hom ogeneous medium presenting spatial dispersion. We consider in derail t he case of a cholesteric liquid crystal, deformed by a flexoelectric t orque induced by an electric field orthogonal to the helical axis. The distorted structure displays a true rotatory power that scales as the ratio between the pitch of the cholesteric and the vacuum light wavel ength. The approximate analytical results are compared with exact nume rical calculations, showing a good agreement even far from the limits of validity of the theory. Possible generalizations of these results a re discussed.