Switching behavior of antiferroelectric liquid-crystalline mixtures

Abstract

Goc, W. Kuczynski, R. Dbrowski, B. Stryla and H. GierszalInstitute of Molecular Physics, Polish Academy of Sciences,ul. Smoluchowskiego 17, 60-179 Poznan, Poland, e-mail: wkucz@ifmpan.poznan.plMilitary Academy of Technology, Warszawa, PolandABSTRACTResults of investigations of the switching phenomenon in antiferroelectric liquid crystals are presented. Mixtures possessingthe smectic C A phase in very broad temperature range, including the room temperature, were investigated. The influence ofseveral experimental conditions (temperature, electric field strength, frequency and shape of electric pulses) on switchingphenomenon was studied. The switching process was detected using two independent methods. In the electrooptic method,the intensity of light passing the sample placed between crossed polarizers was registered. In the second method the currentflowing across the cell during the change of the polarization direction was measured. Both methods have given consistentresults. The critical electric field strength needed for changing the polarization direction by 1800 has been determined asfunction of temperature, pulse shape and frequency. Two kinds of switching were observed. At low frequency of triangle-shaped voltage (below 10 Hz) the switching between three stable states (one antiferroelectric state and two field inducedferroelectric states) took place. At higher frequencies, or when the voltage has rectangular shape, we observed the switchingbetween two ferroelectric states. A simple and accurate method of measurement of the threshold field for bistable switchingis proposed.Keywords: liquid crystal, smectic, chiral, antiferroelecrtic, switching1. INTRODUCTIONChiral smectic liquid crystals are currently very extensively investigated. The reason for the large interest in this class ofmaterials is the occurrence of many interesting physical properties on the one hand, and the large application potential on theother.When the average direction of long axes of chiral molecules does not coincide with the smectic layers normal, then anordering of dipoles (producing a spontaneous polarization) in each layer occurs. The direction of the polarization vector ingiven layer is perpendicular to the tilt plane of molecules [1]. The correlation of polarization vectors in successive smecticlayers can be realized in different ways. When these vectors are parallel (or nearly parallel) then the liquid crystal exhibitsferroelectric (or helielectric) properties. On the other hand, if the polarization vectors in neighboring layers are antiparallel,the liquid crystal is antiferroelectric. The ferroelectric liquid crystals were discovered about 20 years ago and have already