Internal reference for determining liquid crystal orientation at alignment layers in liquid crystal cells by confocal polarised Raman microscopy

Abstract

The transmission of light through liquid crystal (LC) displays is controlled by reversible switching of the alignment of a mesogen using electric fields. In the absence of an electric field, the orientation of the mesogens is controlled by the layer of polymer, rubbed unidirectionally, on an ITO (indium titanium oxide) electrode on glass. The realignment induced by an applied electric field, to switch a pixel, is inefficient close to the solid liquid interface where the alignment layer has greatest interaction with the LC molecules and thereby reduces the darkness that can be achieved with LC display pixels. Characterising changes in orientation of liquid crystal molecules, e.g., 5CB, at the alignment layer/LC interface is potentially possible by making use of the polarisation dependence and spatial resolution of confocal Raman microspectroscopy (CFRM). However, the optical properties, e.g., refractive index, of the LC phases are dependent on LC orientation also, which limits control over spatial (depth) resolution in CFRM. Here, we introduce a resonance Raman active component, [Fe(bipy)₃](BArF)₂, into a PMMA alignment layer as an isotropic internal reference for CFRM. The Raman scattering from this compound is insensitive to the direction of polarisation of the excitation laser and enables estimation of the confocal depth probed in complete liquid crystal cells under operation. This layer enables changes in the depth of focus, due to change in refractive index, to be determined in real time when a potential is applied across the LC cell. This reference approach enables following the alignment of mesogens at the solid/LC interface in real time.