Strong flexoelectric behavior in bimesogenic liquid crystals

In this paper, we demonstrate strong flexoelectric coupling in bimesogenic liquid crystals. This strong coupling is determined via the flexoelectro-optic effect in chiral nematic liquid crystals based on bimesogenic mixtures that are doped with low concentrations of high twisting power chiral additive. Two mixtures were examined: one had a pitch length of p similar to 300 nm, the other had a pitch length of p similar to 600 nm. These mixtures exhibit enantiotropic chiral nematic phases close to room temperature. We found that full-intensity modulation, that is, a rotation of the optic axis of 45 degrees between crossed polarizers, could be achieved at significantly lower applied electric fields (E < 5 V mu m(-1)) than previously reported. In fact, for the condition of full-intensity modulation, the lowest electric-field strength recorded was E=2 V mu m(-1). As a result of a combination of the strong flexoelectric coupling and a divergence in the pitch, tilt angles of the optic axis up to 87 degrees, i.e., a rotation of the optic axis through 174 degrees, were observed. Furthermore, the flexoelastic ratios, which may be considered as a figure-of-merit parameter, were calculated from the results and found to be large, ranging from 1.3 to 2 C/Nm for a temperature range of up to 40 degrees C. (c) 2006 American Institute of Physics.