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

The phase adopted by liquid crystals, in applications such as displays, is controlled by alignment layers. Typically this phase originates from a layer of unidirectionally rubbed polymer on a solid support. The extent to which the alignment layer orders the liquid crystal can be determined by the transmission of polarised light, however this quantifies the net effect of the alignment over the entire depth of the LC layer. The polarisation dependence of the Raman scattering of liquid crystal molecules, e.g., 5CB, can allow for molecular anisotropic ordering (alignment) at the interface to be characterised directly by polarised Raman microspectroscopy. However, the optical properties of the oriented LC phases and ITO on glass electrodes impacts the depth resolution achievable. Here we introduce a resonance Raman active component, [Fe(bpy)3](BArF)2, into a PMMA alignment layer as an isotropic reference for Raman scattering. The Raman scattering from this complex is insensitive to the directing of polarisation of the excitation laser and enables estimation of the confocal depth probed in complete liquid crystal cells. The Raman scattering of [Fe(bpy)3](BArF)2 in the PMMA allows for changes in LC orientation near the interface to be followed and compared with changes to the orientation of molecules in the bulk of the liquid crystal. This approach provides spatial resolution (depth) when studying the realignment of LC molecules by electric fields and enables the balance in orientational control by the alignment layer and the electric field to be assessed.