Frequency-modulated textural formation and optical properties of a binary rod-like/bent-core cholesteric liquid crystal

Abstract

We explore the mechanisms of voltage-induced textural switching and the corresponding electro-optical responses of a binary cholesteric liquid crystal (CLC) composed of the rod-like nematic LC E7 with positive dielectric anisotropy and the bent-core LC dimer CB7CB with large flexoelectric coefficients. Our results indicate that the minimal voltage (V-H) required for retaining the CLC in the homeotropic state and the optical transparency of the CLC after treating with decreasing voltage (from V-H to zero) are dependent on the voltage frequency. The observed frequency modulated electro-optical properties are characterized by the frequency regimes separated by the critical frequencies of flexoelectric polarization and dielectric relaxation in dielectric dispersion. These unusual features, specific to the CB7CB-doped CLC, are explained by the dielectric and frequency-dependent flexoelectric responses of LC molecules to an external AC voltage. Accordingly, in addition to the known Grandjean planar and focal conic states, the uniform lying helix as a third stable state can be feasibly generated in the binary CLC with 45 wt% CB7CB by two pathways. The first one is to treat the cell directly with a sufficient voltage lower than V-H ; the other is to decrease the voltage gradually from V-H to zero in the low-frequency regime, where the flexoelectric strength is significant. Manifested by its optical tristability, frequency-controllable optical transparency, and fast flexoelectro-optical response, the proposed binary rod-like/bent-core CLC system is promising for developing a variety of memory- and dynamic-mode photonic and optoelectronic devices. (C) 2019 Chinese Laser Press