Spontaneous Polarization and Chirality in Ferroelectric and Antiferroelectric Liquid Crystals

本文用短脉冲激光热释电技术研究了近晶和非近晶液晶的宏观极化，如自发极化、表面极化和挠曲极化。已经提出了一种用于评估激光照射时液晶单元中的温度变化的新技术，该技术基于温度变化时的双折射变化。使用这种温度估计技术，表面和挠曲电极化的幅度和符号已被估计具有高精度。还测量了这些极化的温度依赖性。在垂直排列的5CB晶胞中，发现表面极化方向是从基体指向基底，即5CB分子的烷基取代基指向基底，这与二次谐波产生测量的结果相反。此外，在垂面界面的表面极化的大小是远远小于理论上从分子中的完全对齐的偶极矩评估。这些结果表明，四极子序参量的空间分布对表面极化起着重要的作用。对5CB、8OCB和MBBA的挠曲电系数的大小和符号进行了评价。5CB的符号为负，而8OCB的符号为正。这种差异可能源于分子中永久偶极矩的方向不同。

Macroscopic polarizations in nematic and smectic liquid crystals such as a spontaneous polarization, surface polarization and flexoelectric polarization have been investigated by means of pyroelectric technique using a short pulse laser. A novel technique for an evaluation of a temperature change in a liquid crystal cell upon the laser light irradiation has been proposed, which is based on a birefringence change upon the temperature change. Using this temperature estimation technique, the magnitude and the sign of the surface and flexoelectric polarizations have been estimated with a high precision. The temperature dependence of these polarization has also been measured. In a homeotropically aligned cell of 5CB, it is found that surface polarization directs from a balk to substrate and, namely, the alkyl substituents of 5CB molecules point toward substrate, which is opposite result to that estimated from second harmonic generation measurement. In addition, the magnitude of the surface polarization at the homeotropic interface is much smaller than that theoretically evaluated from the completely aligned dipole moments in the molecules. These results indicate that the spatial distribution of the order parameter of quadrupole play an important role in the surface polarization. The magnitude and sign of flexoelectric coefficients for 5CB, 8OCB and MBBA have been evaluated. The sign for 5CB is negative, while that for 8OCB is positive. This discrepancy might originate from the difference of the direction of the permanent dipole moment in the molecule.