BRAGG REFLECTION STUDIES OF BLUE PHASES IN CHOLESTERIC LIQUID CRYSTAL SYSTEMS

The cubic blue phases (BPI and BPII) in various liquid crystal systems have been studied using the spectroscopic method of optical Bragg reflection, with emphasis on the effects of chirality and electric field on the phase behavior.;In chiral-racemic mixture systems of a single compound, we have found that the temperature-chirality phase diagram is qualitatively consistent with theoretical expectation, but also has a significant deviation from the prediction of the model based on Landau theory. Comparison with several independent results of others available to date suggests that this deviation is a common phenomenon shared by different compounds. The responses of the BP's under the influence of an electric field have also been investigated. Field-induced phase diagrams have been obtained for a variety of different binary mixture systems, all having positive dielectric anisotropy. A monomorphic BPII at sufficiently high field strength has been identified for the first time, as well as the accompanying field-induced BPI-BPII transition. In some systems, the topology of the BPI-BPII phase boundary has been found to result in the occurrence of field-induced BPII-BPI-BPII transitions. Our results are discussed in relation to the Kirkwood-Helfrich theory, the possible existence of a field-induced hexagonal BP, and other experimental findings.