Characterization of large tilt-angle flexoelectro-optic switching in chiral nematic liquid crystal devices

Abstract

The “flexoelastic ratio”, which is the ratio of the effective flexoelectric coefficient to the elasticcoefficients, is commonly used to characterize the electro-optic behavior of chiral nematic liquid crystal(LC) devices that exhibit flexoelectro-optic switching. In the uniform lying helix configuration, thiselectro-optic effect is manifested as a rapid (~100 s) rotation of the macroscopic optic axis when anelectric field is applied perpendicular to the helix axis of the chiral nematic LC and is attractive for bothintensity and phase modulation devices. There has been renewed interest in this electro-effect as newLC materials and mixtures have been developed that exhibit large tilt angles, , of the optic axis ( ≥45°) whilst maintaining a fast response time. In this Letter, we consider the relevance of the flexoelasticratio when characterizing the performance of the devices and find that an alternative ratio is required tocharacterize materials that can switch by = ±45° when the pitch is constrained. We show that for largetilt angles of the optic axis the values for the new and conventional flexoelastic ratios measurablydiverge. In addition, a simple way of determining this new characteristic ratio is presented that involvesdetermining the electric field amplitude at the point the transmission levels are the same for bothpositive and negative electric field polarities