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

## TL;DR

This paper introduces a new method using a polarisation-insensitive label to track liquid crystal orientation changes in display cells.

## Contribution

A resonance Raman active internal reference is introduced for real-time tracking of mesogen alignment in liquid crystal cells.

## Key findings

- A resonance Raman active compound enables real-time depth probing in liquid crystal cells.
- The method allows tracking of mesogen alignment changes at the solid/liquid interface.
- Optical transmission changes can be sensed during electric field-induced alignment switching.

## 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.

A polarisation insensitive label in the alignment layer of a liquid crystal (LC) cell enables sensing of the changes in optical transmission to be tracked as the alignment of a mesogen is switched with electric fields.