Optical properties of frustrated cholesteric liquid crystals

Abstract

In a previous article, we proposed a model to explain the unwinding transition in an electric field of a frustrated cholesteric liquid crystal sandwiched between two glass plates imposing a homeotropic anchoring. We found that three distinct solutions exist in materials of negative dielectric anisotropy: first, the homeotropic nematic at small thickness and small voltage, second, a translationally invariant configuration (TIC) at large voltage and, third, the cholesteric fingers. In this article, we study some optical properties of these solutions. We show first that the TIC rotates the polarization of light. Its ‘apparent’ rotatory power is calculated exactly and is compared with the experimental data when the TIC-nematic phase transition is second order. The agreement between theory and experiment is excellent. We show in particular that there exist discrete values of the voltage for which the TIC has a pure rotatory power. We then calculated the optical contrast of the fingers when they are observed between crossed polarizers. The agreement with experiment is still satisfactory, in spite of the approximate form of the director field chosen to describe the topology of the finger.