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

The optical properties of the liquid-crystal analogue of the Abrikosov phase, the TGB(A) phase, have been examined for the first time using the Kossel technique. The Kossel diagrams observed between crossed polarizers each consist of a bright annulus on which a darker spiral pattern is superimposed. Identical patterns have been reported and explained previously for the chiral nematic phase and the results presented here indicate that the optics of the TGB(A) phase are analogous to those of the cholesteric phase. Both the pitch and handedness of the TGB(A) phase have been measured from the Kossel 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* phase boundary to 450 nm at the TGB(A)-to-isotropic phase boundary.