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

Wang, Xiuze; Fells, Julian A. J.; Welch, Chris; Tamba, Maria-Gabriela; Mehl, Georg H.; Morris, Stephen M.; Elston, Steve J.

Abstract

The “flexoelastic ratio”, which is the ratio of the effective flexoelectric coefficient to the elastic
coefficients, 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, this
electro-optic effect is manifested as a rapid (~100 s) rotation of the macroscopic optic axis when an
electric field is applied perpendicular to the helix axis of the chiral nematic LC and is attractive for both
intensity and phase modulation devices. There has been renewed interest in this electro-effect as new
LC 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 flexoelastic
ratio when characterizing the performance of the devices and find that an alternative ratio is required to
characterize materials that can switch by  = ±45° when the pitch is constrained. We show that for large
tilt angles of the optic axis the values for the new and conventional flexoelastic ratios measurably
diverge. In addition, a simple way of determining this new characteristic ratio is presented that involves
determining the electric field amplitude at the point the transmission levels are the same for both
positive and negative electric field polarities.

Citation

Wang, X., Fells, J. A. J., Welch, C., Tamba, M., Mehl, G. H., Morris, S. M., & Elston, S. J. (2019). Characterization of large tilt-angle flexoelectro-optic switching in chiral nematic liquid crystal devices. Liquid crystals, 46(3), 408-414. https://doi.org/10.1080/02678292.2018.1502822

• Journal Article Type: Article
• Acceptance Date: Jul 17, 2018
• Online Publication Date: Jul 31, 2018
• Publication Date: 2019
• Deposit Date: Sep 5, 2018
• Publicly Available Date: Nov 23, 2018
• Print ISSN: 0267-8292
• Electronic ISSN: 1366-5855
• Publisher: Taylor and Francis
• Peer Reviewed: Peer Reviewed
• Volume: 46
• Issue: 3
• Pages: 408-414
• DOI: https://doi.org/10.1080/02678292.2018.1502822
• Keywords: Flexo-electro-optic switching, chiral nematic large tilt angle mixtures
• Public URL: https://hull-repository.worktribe.com/output/961234
• Publisher URL: https://www.tandfonline.com/doi/full/10.1080/02678292.2018.1502822