Optical activity measurements at long wavelengths in the blue phases of highly chiral liquid crystals

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Abstract
A recent calculation of the optical activity in the blue phases using Landau-de Gennes theory predicts the wavelength dependence where φ/d is the optical activity, λ is the wavelength (assumed to be much greater than the pitch), and A 2 and A 4 are coefficients which depend on the structure of the blue phases. Measurements of the optical activity between 0.78 and 1.1 μm in all three blue phases of the CE2/7S5 system confirm this relationship. For certain values of A 2 and A 4 the optical activity passes through zero within the range of wavelength investigated. The values of A 2 and A 4, however, vary in a way which cannot be explained by the theory. The data suggest that fluctuations in the order parameters, which are not considered in the theory, are large in the blue phases of these highly chiral systems.