Electrically induced bandwidth broadening in polymer stabilized cholesteric liquid crystals

Abstract

The reflection notch bandwidth of a cholesteric liquid crystal (CLC), equal to the product of the liquid crystal (LC) birefringence (Δn), and the pitch length (po), is typically on the order of 50-100 nm in the visible portion of the electromagnetic spectrum. Static bandwidths greater than 100 nm can be observed in CLCs that possess a pitch gradient throughout the thickness of the cell. In this work, we report on polymer stabilized CLC (PSCLC) systems that exhibit electrically controllable, dynamic bandwidths governed by the strength of a direct current (DC) electric field applied across the sample. Symmetric notch broadening which increases linearly with field and reaches a maximum value at 4 V/μm is observed. Removal of the field returns the PSCLC cell to its original optical properties. A seven fold increase in bandwidth was observed for 22 μm thick cells which contained LCs with a small birefringence (∼0.04). A variety of CLC mixtures with small positive or negative dielectric anisotropies are shown to exhibit this reversible dynamic bandwidth broadening. The magnitude of the effect was dependent on the amount of polymer stabilization controlled by initial monomer content. The underlying mechanism is partially elucidated by examining cells simultaneously in transmission and reflection and observing differences when modulating the DC polarity across the cell. Different mechanisms for the observed effects are discussed in terms of consistency with our experimental results.