Reflective cholesteric liquid crystal gels

In this paper, we present electro-chromatic switching of a cholesteric gel, which exhibits a shift in the reflection band with the application of an electric field, but preserves the intensity of the reflected light. The reflecting color of the cholesteric gel was controlled by the strength of an electric field. When optimized to reflect visible light the reflecting color of the gel reversibly shifts to shorter wavelengths with increasing voltage. The reflection intensity remains the same over a range of voltage and begins to decrease with further application of the electric field, eventually leading to homeotropic alignment. We have systematically investigated the mechanism of the switching by studying the optical spectra and measuring the dielectric constant of a cholesteric gel as a function of voltage. Using a simple model that includes tilting and untwisting of a helical structure, we show that uniform electro-chromatic switching is mostly initiated by a helix tilting mechanism, whereas decrease in the intensity of reflection is mainly caused by helical untwisting.