An optical study of the structure of the helicoidal twist grain boundary (TGB(A)) phase

The optical properties of the liquid-crystal analogue of the Abrikosov phas e, the TGB(A) phase, have been examined for the first time using the Kossel technique. The Kossel diagrams observed between crossed polarizers each co nsist of a bright annulus on which a darker spiral pattern is superimposed. Identical patterns have been reported and explained previously for the chi ral nematic phase and the results presented here indicate that the optics o f the TGB(A) phase are analogous to those of the cholesteric phase. Both th e pitch and handedness of the TGB(A) phase have been measured from the Koss el diagrams. The pitch of the material was found to be approximately linear with temperature, ranging from around 550 nm at the TGB(A)-to-SmCalpha* ph ase boundary to 450 nm at the TGB(A)-to-isotropic phase boundary.