Pretransitional optical activity in a liquid-crystal system of high chirality

Correlations in the isotropic phases of chiral liquid crystals: the role of helicity modes

The phenomenological theory of chiral liquid crystals is further developed by generalizing the model of self-consistent correlations [J. Englert, L. Longa, H. Stark, and H.-R. Trebin, Phys. Rev. Lett. 81, 1457 (1998)]. In the present approach, not only a leading helicity mode of the tensor order parameter is retained but also the remaining four modes. By considering a full fluctuating spectrum of the order parameter, the role of correlations between helicity modes in the isotropic phases is studied. Additionally, an exact form of the two-point correlation function in real space is derived and its properties are thoroughly discussed. It is shown that for chiral isotropic liquids purely chiral modes could be identified that do not exist for an ordinary liquid. Detailed results of the numerical calculations are compared with those obtained from the earlier model and these show regions where the coupling between the modes becomes important, in agreement with the available experimental data. Though the analysis up to first-order cumulant expansion does not predict a direct phase transition between the blue phase III and the isotropic phase, it is fairly easy to identify two differently correlated regions in a temperature-chirality plane. Various structural quantities, such as optical activity and specific heat, also reveal a behavior characteristic of two isotropic phases with different correlation lengths.

Effective index of refraction, optical rotation, and circular dichroism in isotropic chiral liquid crystals

This paper concerns optical properties of the isotropic phase above the isotropic-cholesteric transition and of the blue phase BP III. We introduce an effective index, which describes spatial dispersion effects such as optical rotation, circular dichroism, and the modification of the average index due to the fluctuations. We derive the wavelength dependence of these spatial dispersion effects quite generally without relying on an expansion in powers of the chirality and without assuming that the pitch of the cholesteric P is much shorter than the wavelength of the light lambda, an approximation that has been made in previous studies of this problem. The theoretical predictions are supported by comparing them with experimental spectra of the optical activity in the BP III phase.

Pretransitional optical activity of short-pitched chiral nematic liquid crystals

We have developed, using the de Gennes theory of short-range orientational order in the isotropic phase, a closed-form expression for the temperature dependence of the pretransitional optical activity of chiral nematics. Detailed calculations are included and the results are expressed in a form that can be easily tested experimentally. The theoretical predictions are supported by experimental data.