LINE-SHAPES OF FIELD-INDUCED BLUE-PHASE-III SELECTIVE REFLECTIONS

We have measured the electric-field dependence of the reflectivity of the blue phase III (BP III) of cholesteric liquid crystals when the dielectric anisotropy epsilon-a < 0. We propose a model which assumes that the boundary aligns the sample to a depth L and in which L is increased by the applied field. The free energy is proportional to E(n), where n depends on the structure of BP III. The data are fitted well by a Lorentzian line shape, and the peak intensity and linewidth agree well with the theory for n = 2.1. This value is consistent with the double-twist model of BP III, but not with cubic and quasicrystal models, since the energies of these latter structures depend on higher powers of E.