Electro-Optical Bistability in Ferroelectric Liquid Crystal Switching Devices for Use in Displays and Real-Time Holography

Abstract

Liquid crystalline (LC) materials which exhibit the ferroelectric Sc* phase showed early promise for device applications because of their fast electro-optic response and potential long-term electro-optical bistability. In combination these features should allow fast line at a time addressing and therefore potentially large passive matrix displays. Bistability should also facilitate DC balancing without the need for inverse frame or blanking procedures used in active matrix addressed liquid crystal over silicon devices. Such a feature is very valuable in the application of these materials to telecom devices where signal interruption for DC balancing purposes is very problematic. Long term bistability is also important in realising active tiling based displays. However the realisation of acceptable bistability has not been straightforward. Partly as a result, commercial passive matrix displays have been dominated by super twisted nematic (STN) devices, which have a limited voltage selection ratio that decreases with the number of pixel rows addressed. High quality flat panel LC displays for laptop computers, computer monitors and television has been delivered using active matrix thin film transistor twisted nematic (TFT TN) technology. The first generation of commercially important ferroelectric liquid crystal displays (for near-eye and projection applications) have in fact utilised active matrix liquid crystal over silicon (LCoS) device structures, where the speed of the FLC electro-optic response permits the use of frame sequential methods for generating colour and greyscales in images. Here we discuss the problems encountered in achieving useful bistability within FLC devices, the new applications that would greatly benefit from true FLC bistability and some interesting new results from some siloxane oligomers which suggest new possibilities in achieving true bistability. We hope to highlight the need for a deeper understanding of this problem.