Enhancement of Freedericksz's threshold voltage of nematic liquid crystals in thin cells

We report the measurement of Freedericksz's threshold voltage (Vth) in two pure calamatic nematic as well as a mixture of a calamatic nematic and a bent core (banana) liquid crystal at different thicknesses, in the ranges 1-3 μm and 7 to 15 μm cells. Rubbed polyimide-coated glass plates are used to get planar alignment of the nematic director. We show that the transmitted intensity varies gradually across the transition region with the applied voltage in the thin cells compared to the thick cells. We find a significant enhancement of Vth in thin cells in both the compounds and in the mixture and the enhancement is larger at as the temperature is lowered in the nematic phase. Part of it is contributed by the enhancement of the orientational order parameter (Vth2 ~ S) in thin cells. Finally we discuss the role of ionic contribution to increase the space charge polarization in thin cells in further enhancing the threshold voltage.