BIREFRINGENCE AND OPTICAL-ROTATION IN CHIRAL FERROELECTRIC AND ANTIFERROELECTRIC LIQUID-CRYSTALS

The optical properties of birefringent and spatially modulated chiral liquid crystal phases can be conveniently described by the perturbativ e approach to the wave equation. Within the first order approximation, which is valid far from the points of degeneration, the optics of chi ral smectic phases is described by the space-averaged dielectric tense r. We have performed high resolution birefringence and optical rotatio n measurements in the Ferroelectric and antiferroelectric liquid cryst als and interpreted the results within the perturbative approach, Wher eas the fluctuation-induced changes of the birefringence are nearly ne gligible in the paraelectric phase of a ferroelectric liquid crystal 4 -(2'-methylbutyl)-phenyl 4'-n-octylbiphenyl-4-carboxylate (CE-8), we o bserve large, fluctuation-dominate birefringence changes in the parael ectric phase of an antiferroelectric liquid crystal 4-(1-methyl-heptyl oxycarbonylphenyl) 4'-octylbiphenyl-4-carboxylate (MHPOBC). The magnit ude of the order parameter, as deduced from the measured temperature d ependence of the birefringence, exhibits in both substances a crossove r of the order parameter exponent from beta = 0.5 close to the paraele ctric phase to the tricritical value beta = 0.25 far away from the tra nsition. This stresses the importance of the sixth order terms in the Landau free-energy expansion in ferroelectric and antiferroelectric li quid crystals.