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The Flexoelectro-Optic Effect in Cholesteric Liquid Crystals

Doctoral thesis, 1997

The flexoelectrooptic effect in cholesteric liquid crystals is based on a linear coupling of the medium with an applied electric field. The electric field causes the optic axis, which is coinciding with the helix axis of the hardtwisted cholesteric (pitch < .lambda.), to rotate or tilt in a plane parallel to the cell glass plates. Hence, we have a uniaxial optical wave plate with a field-controlled direction of the optic axis, and essentially the same geometry as the electrooptic effects in the smectic C* phase (surface stabilized ferroelectric liquid crystals and deformed helielectrics) and the smectic A* (soft mode/electroclinic effect). This geometry, sometimes referred to as in-plane switching, guarantees a very wide viewing angle for displays and shutters. The field-induced tilt of the optic axis in the flexoelectrooptic effect is a linear function of the applied field which gives a well-controlled continuous grey-scale.
The effect has been examined in its static as well as dynamic aspects. A general expression, describing the field-dependence of the tilt, containing the individual splay and bend flexoelectric coefficients and the three Oseen elastic constants was derived for the case of .DELTA..epsilon. = 0. It predicts a striking range of linearity of the induced tilt and furthermore that this tilt is temperature independent as long as the helical wave vector of the material is constant. In certain mixtures with .DELTA..epsilon. = 0 we have in fact reached induced tilt angles of more than 30 degrees without deviation from linearity. Moreover, a tilt angle of 22.5 degrees is enough for providing 100% light modulation.
Several constant pitch cholesteric materials were investigated and our results confirm that the induced tilt of the optic axis is practically independent of temperature, which is a very attractive feature for applications. Moreover, the dynamics of the effect was studied. The electrooptic response is relatively fast, with response times of the order of 10.my.s to 100.my.s for the studied materials. We were also able to detect the immediate change in the flexoelectric coefficient due to a configurational change in the molecule, induced by light.
We discuss the influence on the flexoelectrooptic effect of material parameters such as the flexoelectric coefficients, the elastic constants, and the dielectric anisotropy. The dielectric effect gives a non-linear contribution to the electrooptic response and we conclude that the dielectric anisotropy must be very close to zero in order to get a large linear response. The fittings and simulations based on the analysis conform very well with our experimental findings.
So far, neither molecular materials nor alignment methods have been optimized for the flexoelectrooptic effect. The special uniform lying structure, of crucial importance for the effect, has a tendency to transform into Grandjean texture. We have solved this problem by creating a stabilizing polymeric network in the volume of the cell by means of photopolymerization of a small amount of reactive monomer added to the cholesteric.
At the same time as proposing a simple method for measuring sign and magnitude of flexocoefficients, we finally point out the inconsistency of data reported so far regarding these coefficients in general and propose a standard convention of sign which hopefully will find international acceptance.