Ferroelectric volume effect and surface induced fast instabilities for rapid electrooptical displays in liquid crystals

2014 One system exist which is capable of microsecond electrooptical switching in liquid crystals, on polarized light. It is the well-known SC* valve, which uses a volume smectic ferroelectric material and a weak surface anchoring. A new effect is presented which gives rise to localized polar surface instabilities. Some of these surface induced instabilities persist for high frequency (1 MHz) AC excitation. Their mechanism is not yet understood. Their nematic-like response time is of an order of magnitude shorter, at equivalent field, than the one of the SC*. A control of the surface excitation mechanism would possibly lead to new electrooptical applications. Revue Phys. Appl. 21 (1986) 669-672 NOVEMBRE 1986, Classification Physics Abstracts 61.30 73.90 68.45 Nematic (N) liquid crystals are now widely used in electrooptical displays [1]. This is due to three properties : 1) their strong coupling with light waves, through their large birefringence, 2) their large sensitivity to external electric fields, because of their large dielectric anisotropy and 3) their weak curvature elasticity which allows large distortions with small fields. The nematic materials for electrooptical applications are now well optimized, to improve the contrast threshold and multiplexability of displays. They are used in well defined texture, oriented by a king of epithaxy from carefully treated boundary surfaces. The corresponding anchoring or the nematic molecules at the surfaces is « strong », i.e. it will not change under the action of the command electric field, during the change of texture which gives rise to the display. One of the major drawback of these nematic display is their long intrinsic response time due to their weak elasticity curvature. Ferroelectric smectic C* liquid crystals have been proposed to make fast submicrosecond electrooptical switches [2]. Recently, we have demonstrated in Orsay [3] the existence of surface polar instabilities, under the action of an electric field. A more or less distributed surface distortion is observed, with a mechanism related to the ferroelectric character of nematic-solid interface, and to the flexoelectric property of nematics. As SC* this system necessitates weak anchoring conditions to work. Superimposed to the surface flexo-ferroelectric instabilities we have observed other localized surface induced instabilities which can present a fast response time, in the microsecond range. The mechanism of these localized instabilities is not yet fully understood, but their response time seems nematic-like, controlled by the large dielectric anisotropy of the liquid crystal. If well controlled, it could lead to fast electrooptical effects. In this paper, we try to describe the physical problems involved in these fast electrooptic switchings, for both the materials themselves and the associated weak anchoring interfaces. We discuss in particular the threshold effects, related to the multiplexability of these displays. We compare the response time of the bulk (SC*) and .fast surface induced (N) displays, which turns out to be an order of magnitude more rapid for the same field, at room temperature. The interest of the SC* ferroelectric system is its linear coupling Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:019860021011066900