Optical vector vortices generated with circularly planar and circularly hybrid nematic cells

Among the variety of optical elements used for the generation of vector and vortex light beams, liquid-crystal cells containing topological defects deserve a special attention. Nowadays, they represent one of the most popular techniques because of simplicity of their preparation. In this work we study optical singularities of the laser beams passing through a cell with a circular planar alignment of its director on both substrates, a so-called CC-cell. The other subject is a CH-cell with a circular planar alignment on one substrate and a homeotropic (i. e., perpendicular to the substrate plane) alignment on the opposite substrate. The CC- and CH-cells are characterized optically with both polarization optical microscopy and imaging polarimetry. The optical singularities of the object beams passing through these cells are visualized by means of their interference with quasi-spherical and quasi-planewave reference Gaussian beams. For the CC-cells, one expects formation of the defects with the topological strength q = + 1 on the surfaces. Due to an effect of escape into a third dimension, a singular director distribution does not propagate through the bulk. While reviewing the literature on the applications of liquid-crystal disclinations for generating the singular beams, we have found that the escape of the director into the third dimension is usually ignored for integer-strength disclinations. We also analyze in detail how the escape into the third dimension manifests itself when the light propagates through the CC-cells. For the case of CH-cells, we find that the optical patterns obtained with the polarization optical microscopy, the imaging polarimetry and the interference techniques indicate that the circular symmetry of a sample structure is essentially broken.