Spontaneous Polarization and Chirality in Ferroelectric and Antiferroelectric Liquid Crystals

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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.