Improving vector vortex waveplates for high-contrast coronagraphy

Vector vortex waveplates (VVWs) open the door to new techniques in stellar coronagraphy and optical communications, but the performance of currently available liquid-crystal-polymer-based VVWs tends to be limited by defects in the axial region of the vortex pattern. As described here, several steps allow for a reduction in the size of such axial defects, including the use of photoalignment materials with high photosensitivity and reversible response, and a reduction in exposure energy. Moreover, redistributing the writing beam's intensity from the axial region to its periphery (using a VVW) allows the production of large area VVWs with a small defect area. Finally, using VVWs as linear to axial polarization converters allows producing VVWs of higher topological charge, while also reducing the photoalignment time to a few minutes. These steps have allowed the fabrication of VVWs with topological charges of 1 and 2 with central defect sizes below 3 μm.