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Abstract
The optical properties of birefringent and spatially modulated chiral liquid crystal phases can be conveniently described by the perturbative approach to the wave equation. Within the first order approximation, which is valid far from the points of degeneration, the optics of chiral smectic phases is described by the space-averaged dielectric tensor. We have performed high resolution birefringence and optical rotation measurements in the ferroelectric and antiferroelectric liquid crystals and interpreted the results within the perturbative approach. Whereas the fluctuation-induced changes of the birefringence are nearly negligible in the paraelectric phase of a ferroelectric liquid crystal 4–(2'-methylbutyl)-phenyl 4'-n-octylbiphenyl-4-carboxylate (CE-8), we observe large, fluctuation-dominated birefringence changes in the paraelectric phase of an antiferroelectric liquid crystal 4–(1-methyl-heptyloxycarbonylphenyl) 4'-octylbiphenyl-4-carboxylate (MHPOBC). The magnitude of the order parameter, as deduced from the measured temperature dependence of the birefringence, exhibits in both substances a crossover of the order parameter exponent from β = 0·5 close to the paraelectric phase to the tricritical value β = 0·25 far away from the transition. This stresses the importance of the sixth order terms in the Landau free-energy expansion in ferroelectric and antiferroelectric liquid crystals.