Non-linear electrooptic effect in antiferroelectric liquid crystal

Electrooptic phenomena caused by weak electric fields, much lower than those needed for the helix unwinding, in helical smectic liquid crystals were studied in thin planar samples. The investigations were performed in chiral liquid crystal 4-(1-methyl-heptyloxycarbonyl) phenyl 4′-(3-butanoyloxy propyl-1-oxy) biphenyl-4-carboxylate which exhibits antiferro-electric properties. We have found that electric field applied to a helical smectic liquid crystal caused two effects. First, the helix was deformed and the position of effective optic axis changed by an angle proportional to the field strength. The second effect, quadratic in field, causes the change in the shape of the indicatrix. As a consequence, the relative changes in the light intensity caused by external electric field consist of two components. The first component represents the modulation with the fundamental frequency and the second one with the doubled frequency (second harmonic of the electrooptic effect). The ab- solute values of the first- and second-order electrooptic coefficients have been determined and their temperature dependence discussed.