Role of the Cholesteric Phase In The Formation Of Twisted Smectic Structures

ABSTRACTThe smectic structure obtained by cooling the material from the cholesteric phase often produces different structures in the twisted ferroelectric configuration, which depends not only on the nature of the material, but also on the rate of cooling. In this paper, we have investigated the origin of this behavior and have concluded that the twist in the cholesteric phase is responsible for the apparent unpredictability. Furthermore, we show that for a given chirality of material and the same rubbing condition at the surface, two possible smectic domains are produced. While the electro-optic properties of both these domains show gray-scale capabilities, these domains have different optical properties. This comparison was made possible by using a specially prepared sample in which we rubbed two glass plates, each with two areas in which the rubbing direction was both horizontal and vertical. Using this sample, we have been able to show that the presence of pitch inversion close to the cholesteric to smectic phase transition is the apparent reason the results can not be reproduced. Furthermore, our experiments clarify the importance of sample cooling rate. If rapidly cooled during the fabrication from the cholesteric to the smectic phase, the sample retains a memory of the non-equilibrium pitch, providing us with a means to capture the desired pitch. This important discovery will aid us in modifying the material as well as carefully tailoring its pitch in order to insure that cooling of the cholesteric material leads naturally to the twisted smectic structure.