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

Abstract

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.

Stimuli‐Directed Dynamic Reconfiguration in Self‐Organized Helical Superstructures Enabled by Chemical Kinetics of Chiral Molecular Motors

“…Light‐driven dynamic self‐organized helical superstructures, i.e., cholesteric liquid crystals (CLCs), could be obtained upon doping the chiral molecular motors into achiral LC hosts, where the orientation of helical axes is typically expressed as three configurations: (1) parallel to the substrate with a fingerprint texture (lying helixes) resulting from the periodical helical pitch ( p ) defined as the distance as the molecular rotation by 360° twist along the helical axis; (2) perpendicular to the substrate with a Grandjean texture (standing helixes) performing unique Bragg reflection of circularly polarized light at wavelength λ = np , where n is the average refractive index of LC; (3) spatial disorder alignment with a focal conic (FC) texture, which can strongly scatter incident light. Accordingly, light‐directed dynamic control over the helical axes as well as the helical pitch in one, two and three dimensions has been a burgeoning area for promising applications, including photodynamic photonic crystals, tunable mirrorless lasers, holographic microlenses, and light‐directed diffraction gratings 15, 16, 17, 18, 19, 20, 21…”

Switching between Phosphorescence and Fluorescence Controlled by Chiral Self‐Assembly

“…To date, a variety of helical assemblies, constructed from chiral self‐assembly, have been demonstrated to display highly potential applications in catalysis,2 sensing and recognition,3 chiroptical switches,4 chiral templates,5 biology,6 and circularly polarized luminescence (CPL) 7. On the other hand, helical self‐assembly may play a unique role in regulating localized excitations of π functional systems, which can bring highly perfect multi‐scale orders and show a special effect to tune the energy of electronics, vibration, and rotation of molecules.…”

“…Crystallization12 and mechanical stimulation13 induced fluorescence‐phosphorescence dual emissions have been obtained, respectively. Furthermore, simultaneous occurrence of emission from isolated molecules and excited oligomers in a single luminescent emitter, simultaneously boosted fluorescence and phosphorescence from a single molecule ensemble, and single‐emitting components involving two fluorescence domains corresponding to an excited monomer and an excimer have also been reported 4, 5, 6, 14…”

Dynamic Tuning and Memory Switching of Defect Modes in a Hybrid Photonic Structure

“…Since LC phases with periodically helical structures can intrinsically be regarded as PCs, various LC-based PC devices with tunable optical features by light irradiations [8,9] or by electric-field treatments [10,11] have previously been demonstrated. Alternatively, recent investigations concerning the tunability of defect modes as well as the design of electrically tunable photonic devices have extensively been proposed in the literature by incorporating various types of LCs individually in one-dimensional (1D) multilayer PCs [12].…”