Electro-optical effects in liquid crystals with dielectric dispersion

Abstract

ABSTRACT The dielectric dispersion in uniaxial nematic liquid crystals creates a dielectric memoryŽ effect whereby the polarization induced by the electric field decays exponentially with time rather than instantaneously, as in materials without dispersion. The induce d polarization couples linearly with the electric field. This linear coupling allows one to accelerate the director relaxation towards the offŽ state by a specially designed electric pulse of a proper polarity and duration We show theoretically and experimentally the possibility of electrically driving the director towards the off state, thereby decreasing the switching time. Keywords: liquid crystal, dielectric anisotropy, dielectric memory, dielectric dispersion, switching speed, response time. 1. INTRODUCTIONNematic liquid crystal molecules, reoriented via the coupling of the dielectric anisotropy ( u0 ) to an applied electric field E are used in many electrooptical applications, including liquid crystal displays (LCDs)[1]. A great deal of effort has gone into the improvement of LCDs, especially towards increasing the switching speed. The switching dynamics can be divided into two distinct phases, the switch onŽ phase and the switch offŽ phase. Here the terms onŽ and offŽ refer simply to the cases where the display is electrically driven to a desired orientation (on) and then when it relaxes back to its initial (off) state. The on state is driven by an applied electric field and, for Umuch greater than the th reshold voltage, the characteristic time is given by