Probing thick uniaxial birefringent medium in confined geometry: A polarised confocal micro-Raman approach

In order to have a simple and non-destructive tool to analyse the optical axis distribution in a liquid crystal cell induced either by the boundary conditions or by a quasi-static electric field or an optical field, we have calculated the Raman signal backscattered by a uniaxial birefringent and twist free material confined in a cell. We account for both the deformation of the focal point due to the crossing of extra layer by the focused pump beam and the depolarisation of both that pump beam and the backscattered signal, due to the propagation through the bireffingent material. It is shown that, whatever the deformation of the focal point and the associated reduction of depth resolution, it is possible to check whether a sample is homogeneous or not. This is done by comparing, for a given polarised line (herein a stretching mode), the Raman intensities recorded as a function of the sample height position with respect to the microscope (z-scan) and for two perpendicular directions of the polarisation of the pump beam. As a sample is found to be heterogeneous, it is possible to extract the optical axis distribution from those curves by a conventional data processing, on the condition that the focal point is not distorted too much and remains smaller than the sample thickness. The shape of the polar diagram obtained by plotting the Raman intensity versus the angular position of the sample with respect to the polarisation direction of the pump beam gives also information on the optical axis distribution across the sample. These results have been tested experimentally using nematic liquid crystal cells, specially prepared with different optical structures. (C) 2006 Elsevier B.V. All rights reserved.