A pupil engineered with a polarization vortex can have a profound impact on certain classes of optical systems.
This paper describes the recent history, ongoing activity, and applications of polarization vortices in optical
system design, with special attention to the impact of a vortex filter placed in the pupil of an illumination
system. Two systems of particular interest for these types of fields are confocal microscopy, in which a dark field
imaging mode is accessible, and immersion lithography, in which azimuthal illumination is favored.
Thomas G. Brown, Thomas G. Brown,
"Polarization vortices in optical design", Proc. SPIE 6667, Current Developments in Lens Design and Optical Engineering VIII, 66670H (18 September 2007); doi: 10.1117/12.731963; https://doi.org/10.1117/12.731963