Nonlinear Dielectric Relaxation Spectroscopy of Ferroelectric and Antiferroelectric Liquid Crystals

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We developed the measurement system for the nonlinear dielectric relaxation spectrum in a wide frequency range and with high accuracy. This system consists of the wave generator, the charge amplifier, the digital storage oscilloscope and the personal computer. We measured the nonlinear spectra of some Ferroelectric Liquid Crystals (FLCs) in the SmC^* phase by use of this system. The obtained nonlinear dielectric constants of FLCs are much larger in magnitude by several decades than those obtained for ferroelectric polymers. The fact that the higher-order nonlinear spectrum has a counterpart in the linear one shows that the nonlinear mode is originated from the fluctuation of the helical structure, called the Goldstone mode. The negative sign of the third-order nonlinear dielectric constant implies that the origin of this nonlinearity is the orientational saturation of the dipolemoments by the applied electric field. The third-order dielectric constant is almost independent of the temperature in the SmC^* phase and shows the critical behavior in the vicinity of the SmA-SmC^* phase transition temperature. The critical behavior appears more remarkably in the nonlinear spectrum than in the linear one. The linear and nonlinear spectra are explainable by the phenomenological theory. We also measured the nonlinear dielectric spectra for an antiferroelectric liquid crystal MHPOBC in various smectic phases. In the ferrielectric phase, for example, we found a phase transition from a ferri-state to a ferro-state induced by the increase of electric field. From the saturated value of polarization in both the states, we estimated the molecular arrangement in the ferrielectric phase. Through the present study, this spectroscopy is found to be a useful tool to obtain the information on the structure and dynamics of the electro-active smectic liquid crystals.