New type of high-field electro-optic response in nematics

Abstract

We have made concurrent measurements of ionic current and optical transmission between crossed polarizers on several nematics with positive dielectric anisotropy under the action of applied low frequency (< 1 KHz) square wave voltages. When the field E is low, the measured current is linear in E and there is no electrooptic response. Beyond some value of the field (E₀ approximately 100 esu), the current becomes independent of the field (phenomenon of limiting current). Further an electrooptic signal is measured at twice the frequency of the applied voltage, which exhibits a peak as a function of the field. The width of the peak is 3 to 4 times the value of E₀, and the signal level at the peak decreases as the frequency is increased. These measurements have been made on three highly polar compounds with cyano end groups. Careful observations do not show any evidence of electrohydrodynamic instabilities in the sample. It is argued that the observations can be understood if at the onset of the phenomenon of the limiting current, a strong electric field gradient is established near one of the electrodes due to the sweeping of an ionic species with high mobility. The field gradient produces a flexoelectric deformation of the director field, which in turn gives rise to the electrooptic effect. At higher fields, the stabilizing dielectric torque takes over to suppress this instability.