Consider the simplest electrically-controlled nematic optical device. A thin (say, 10 micron) nematic slab is confined between two glass plates. Their inner surfaces are covered with a transparent electroconductive layer of indium tin oxide (ITO). The initial "horizontal" orientation of molecules is fixed by specially treated (rubbed) polymer alignment layers deposited on the top of ITO. The electric field is now applied across the cell by connecting the two ITO layers to a voltage generator. Because of the dielectric anisotropy of the nematic, the molecules tend to reorient along the vertical direction of the field (provided the dielectric anisotropy is of the positive type. This reorientation, however, cannot be uniform across the cell. Close to the bounding plates, surface interactions are strong enough to keep horizontal orientation of the adjacent molecules; in the middle of the cell, these surface interactions are of little importance and n is close to the direction of the field. In general, the director orientation is the function of both the applied voltage and the vertical coordinate Z. The effect, first discovered by V. Frederiks, is at the heart of modern electro-optical applications of LCs.
How to access the director dependence on the vertical Z-coordinate in such a thin nematic layer? Most techniques would provide information that is essentially an integral over the director configuration along the Z axis. The technique that does allow to reconstruct the Z-dependence of orientation and the whole 3D director pattern is that of fluorescence confocal polarizing microscopy (FCPM).
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