Relative order parameters of cholesteric and blue phases

The wavelength and angle dependence of the Bragg reflection from the cholesteric "blue" phase is studied as a function of temperature. Dynamical "Darwin flat-top" behavior is found, and the width in energy and angle of the reflection are used to estimate the ratio of the order parameters in the cholesteric and both blue phases. For both blue phases, the order parameter is about $\frac{1}{3}$ that in the cholestric, and shows a distinctive temperature dependence, part of which may be associated with the presence of a blue-phase-isotropic two-phase region in the phase diagram of one of the mixtures studied. The resemblance of the reflection curve to its ideal form indicates that the blue phase's ordering is correlated over at least 50 lattice spacings, or 10 μm.