Nonlinear Dielectric Relaxation Spectroscopy of Ferroelectric and Antiferroelectric Liquid Crystals

研制了宽频带、高精度的非线性介电弛豫谱测量系统。该系统由波形发生器、电荷放大器、数字存储示波器和个人计算机组成。利用该系统测量了SMC^*相中部分铁电液晶的非线性光谱。所得的氟碳化合物的非线性介电常数比铁电聚合物的要大几十年。高阶非线性谱与线性谱对应的事实表明，非线性模起源于螺旋结构的涨落，称为Goldstone模。三阶非线性介电常数的负号意味着这种非线性的起源是外加电场对偶极子的取向饱和。三阶介电常数几乎与SMC^*相的温度无关，在SMA-SMC^*相变温度附近表现出临界行为。在非线性谱中，临界行为比在线性谱中更为显著。线性光谱和非线性光谱均可用唯象理论解释。我们还测量了反铁电液晶MHPOBC在不同近晶相中的非线性介电谱。例如，在铁电相中，我们发现电场的增加导致了从铁态到铁态的相变。从两种状态下的极化饱和值，我们估计了铁电相中的分子排列。通过目前的研究，这种光谱被发现是一个有用的工具，以获得关于电活性近晶液晶的结构和动力学信息。

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.