CMMPE - University of Cambridge

In 1987 J.S. Patel and R.B. Meyer presented a new linear electro-optic effect which was based upon the flexoelectric effect in liquid crystals. The so-called flexoelectro-optic effect in chiral nematic liquid crystals is a fast switching (typically 10–100 microseconds) in-plane rotation of the optic axis which is linear-in-the-field. Furthermore, the direction of rotation is determined by the polarity of the electric field. By applying an electric field perpendicular to the helix axis, flexoelectric coupling to the applied field results in a space-filling splay-bend deformation and the macroscopic effect is an in-plane rotation of the optic axis. The basic principle of this electro-optic effect is illustrated in Figure 1, whereas the cell configuration and alignment of the helix is shown in Figure 2.

Figure 2. Cell configuration: Since the electric field has to be applied perpendicular to the helix axis, a uniform lying helix texture (ULH) has to be used instead of the standard grandjean texture.

Recently, we have demonstrated that mixtures based upon these compounds can show very high switching angles (>80deg). Furthermore, by using polymer stabilisation, we have shown that the uniform lying helix texture can be recovered both on heating from the crystalline phase and cooling from the isotropic phase, and that the response times can be minimised whilst at the same time maintaining large switching angles.

Figure 4. The standing helix mode.

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Figure 1. Rotation of the optic axis (OA) with the application of an electric field. The tilt angle of the optic axis is linear with electric field.

The main drawback with the flexoelectro-optic effect is that materials hitherto have shown very small flexoelectro-optic switching. This is due, in the most part, to the fact that dielectric coupling dominates the response of the liquid crystal to the applied electric field. Our research in recent years has been focussed on developing new liquid crystal compounds so as to maximise the flexoelectric response in chiral nematic liquid crystals and thus improve the performance characteristics of the flexoelectro-optic device. In short, theory informs us that strong dipole moments are good for flexoelectric coupling but are bad in the respect that they increase the dielectric coupling to the applied electric field. As a solution to this conundrum we have developed bimesogenic compounds which incorporate strong dipole moments, and thus increase flexoelectric coupling, whilst at the same time reducing the dielectric anisotropy so as to minimise dielectric coupling. The generic molecular structure is shown in Figure 3.

Figure 3. Generic chemical structure of new bimesogenic compounds.

In addition to the materials development program, we have also demonstrated that the flexoelectro-optic effect could be used as a phase device in the standing helix mode (Figure 4). In this case the helix axis is perpendicular to the plane of the surfaces and the in-plane electrodes satisfy the condition of an electric field perpendicular to the helix axis. At present, research is focussed on demonstrating that this standing helix mode is a viable candidate for high quality moving image displays because the response is sufficiently fast to enable frame sequential colour and that a perfect black state is obtained when no electric field is applied.

Strong Flexoelectric Behaviour in Bimesogenic Liquid Crystals
Coles H.J., Clarke M.J., Morris, S.M., Broughton B.J., Blatch A.E.
J.Appl.Phys., 99(3), 034104

Effect of Polymer Concentration on Stabilised Large-Tilt-Angle Flexoelectro-Optic Switching
Broughton B.J., Clarke M.J., Morris S.M., Blatch A.E., Coles H.J.
J.Appl.Phys., 99(2), 02351

Structure-flexoelastic properties of bimesogenic liquid crystals
Morris, S.M., Clarke M.J. , Blatch A.E. and Coles H.J.
Physical Review E, 75, 041701

Optimised Flexoelectric Response in a Chiral Liquid Crystal Phase Device
Broughton B.J., Clarke M.J., Blatch A.E., and Coles H.J.
J.Appl.Phys. 98(3), 134109

Research at CMMPE — Electrical/optical phenomena — Flexoelectro-optic effect

Further reading