Photo-alignment control of topological defects in nematic liquid-crystal cells

Abstract

We report the experiments aimed at controlling the position of linear topological defect (disclination) in a so-called  -cell filled with a nematic liquid crystal sandwiched between circularly rubbed polyimide (Kapton) layer and photoaligning PVCN material oriented unidirectionally. To satisfy surface-alignment conditions, the director within this cell experiences a twist. The topological defect appears in a plane where the twist angle of the director becomes indeterminate (90º). In an achiral liquid crystal, orientation of the disclination coincides with the alignment occurring on the unidirectionally oriented substrate. The idea of this work is to control the alignment direction on the substrate by a polarized ultraviolet light. Photo-induced director reorientation on the substrate changes the twist angle in the bulk and so varies the azimuthal angle of the disclination. We monitor reorientation of the disclination as a function of light-polarization angle and irradiation dose.

Macroscopic Nematic Orientation Dictated by an Orientationally Frustrated Random-Field Surface: Equilibrium Structure and Kinetics

“…The standard approach involves treating the surface LC uniformly aligned by employing the rubbing or photo-alignment techniques, which in turn orients the macroscopic alignment of the bulk nematic (N) LCs, the so-called director n . On the other hand, generating spatial variation of the director with an azimuthal angle change by the photo-alignment technique is an effective pathway for providing pixelized electrooptic and mechanical functions. − These properties are enabled by the position-dependent refractive index ellipsoid, strain, and stress anisotropies, etc.…”