Alignment of Liquid Crystals by Surface Gratings

Abstract

Abstract : Square-wave grating structures with periodicities ranging from 3200 A to 12 micrometers were etched into fused quartz substrates, and the effect of such gratings on liquid crystal alignment was studied. Gratings with periodicities below 4 micrometers appear to be required to align typical room- temperature nematic liquid crystals. At larger periodicities a pronounced defect texture forms. The defect texture is created during nucleation and growth of the nematic phase as it cools from the isotropic phase. The defect texture is stabilized by adsorption of an oriented molecular layer on the substrate surfaces. This adsorbed layer exerts an orienting torque on the bulk liquid crystal. Experiments were performed to demonstrate the existence of such an adsorbed layer. The tilt angle of the nematic director from the plane of quartz substrates was measured for liquid crystals used in the alignment experiments. M24 and the 'heptyl/butyl mixture' align with the director in the substrate plane. MBBA aligns with a tils angle of about 23 deg on fused quartz, whether or not a grating structure is present. Surface gratings were also formed by patterning a 'monolayer' of DMOAP. Such patterned organic monolayers, which have no appreciable surface relief, are effective at aligning liquid crystals. This represents a new approach to liquid crystal alignment. High-quality alignment of the smectic A phase of M24 was induced by a 3200-A-period square-wave surface-relief grating. A novel twisted-nematic liquid crystal display which uses metal gratings for polarization of light as well as for liquid crystal alignment was fabricated.