Flexoelectric polarization changes induced by light in a nematic liquid crystal

In a nematic liquid crystal where the director field has a splay-bend deformation mediated by the boundary conditions there is a certain intrinsic polarization density distribution characteristic of the flexoelectric properties of the medium. By application of an electric field perpendicular to the director, a twist is induced which is a measure of the flexoelectric anisotropy [I. Dozov, Ph. Martinot-Lagarde, and G. Durand, J. Phys; (Paris) Lett. 43, L-365 (1982)]. We show that in molecules susceptible to a photoinduced configurational change we may change the volume polarization by UV illumination and relate the change in flexoelectric coefficients to the configurational change in the molecule. In the experiment we use the British Drug House (Merck) nematic mixture E7 doped with 5 wt % of the dye 4-hexyloxy-(4'-hexyl)azobenzene. The photoinduced trans-cis configurational change leads to an increase by 40% in the flexoelectric anisotropy (e(s)-e(b))/K.