Optical confirmation of the extended mean-field theory for a smectic-C*–smectic-Atransition

Half-leaky guided modes are used to determine accurately the cone angle in a smectic-${\mathit{C}}^{\mathrm{*}}$ liquid crystal as a function of temperature. Homeotropic alignment is obtained using only an in-plane electric field and electroclinic and voltage-induced effects are made negligible. The data obtained is fitted fully to an extended mean-field theory using the cone angle as a measure of the order parameter. The results show unequivocally for this material, the ferroelectric liquid crystal SCE13, that the free energy is fully specified through an expression containing terms to third order in the cone angle squared.