Journal Article

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

Li, Yannian, Li, Quan, Bunning, Timothy J., Zheng, Zhi-gang, Bisoyi, Hari Krishna, Wang, Ling

2016

Overview

Chiral nematic liquid crystals are self-organized helical superstructures in which the helices can stand or lie, and lie in either a uniform or a random way; here, the helices are reversibly driven from a standing arrangement to a uniform lying arrangement and then rotated in-plane—solely by light. Manipulation of cholesteric liquid crystals This paper reports the manipulation of the helix axis accompanied by handedness inversion of an optically tunable, self-organized helical superstructure (a cholesteric liquid crystal) in three dimensions. Practical applications of chiral nematic liquid crystals (also known as cholesteric liquid crystals), rely on variation of the pitch length of these helices, or switching the helical axis between parallel or perpendicular to a substrate. This is typically achieved application electric or magnetic fields. Quan Li and colleagues now report such a manipulation using only light as a stimulus. Using this method, they achieve two-dimensional beam steering without the need for complex, multicomponent integrated systems, and they also construct a photoswitchable diffraction grating using a bilayer cell design. 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 1 , reflective displays 2 , tuneable colour filters 3 , 4 and mirrorless lasing 5 , 6 . Dynamic, remote and three-dimensional control over the helical axis of CLCs is desirable, but challenging 7 , 8 . 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 9 , by changing the anchoring conditions of the substrate, or by altering the topography of the substrate’s surface 10 , 11 , 12 , 13 , 14 , 15 , 16 ; separately, in-plane rotation of the helical axis parallel to the substrate can be driven by a direct-current field 17 , 18 , 19 . 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 20 and optical phased arrays 21 . 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.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/301/923/919 / 639/624/399/1022 / 639/638/898 / 639/925/357/341 / Analysis / Crystal structure / Crystals / Diffraction / Humanities and Social Sciences / letter / Light / Liquid crystals / multidisciplinary / Properties / Science / Substrates