Three-dimensional control of the helical axis of a chiral nematic liquid crystal by light.

Chiral nematic liquid crystals--otherwise referred to as cholesteric liquid crystals (CLCs)--are self-organized helical superstructures that find practical application in, for example, thermography, reflective displays, tuneable colour filters and mirrorless lasing. Dynamic, remote and three-dimensional control over the helical axis of CLCs is desirable, but challenging. For example, the orientation of the helical axis relative to the substrate can be changed from perpendicular to parallel by applying an alternating-current electric field, by changing the anchoring conditions of the substrate, or by altering the topography of the substrate's surface; separately, in-plane rotation of the helical axis parallel to the substrate can be driven by a direct-current field. Here we report three-dimensional manipulation of the helical axis of a CLC, together with inversion of its handedness, achieved solely with a light stimulus. We use this technique to carry out light-activated, wide-area, reversible two-dimensional beam steering--previously accomplished using complex integrated systems and optical phased arrays. During the three-dimensional manipulation by light, the helical axis undergoes, in sequence, a reversible transition from perpendicular to parallel, followed by in-plane rotation on the substrate surface. Such reversible manipulation depends on experimental parameters such as cell thickness, surface anchoring condition, and pitch length. Because there is no thermal relaxation, the system can be driven either forwards or backwards from any light-activated intermediate state. We also describe reversible photocontrol between a two-dimensional diffraction state, a one-dimensional diffraction state and a diffraction 'off' state in a bilayer cell.