Human psychophysical discrimination of spatially dependant Pancharatnam-Berry phases in optical spin-orbit states

We tested the ability of human observers to discriminate distinct profiles of spatially dependant geometric phases when directly viewing stationary structured light beams. Participants viewed polarization coupled orbital angular momentum (OAM) states, or ``spin-orbit'' states, in which the OAM was induced through Pancharatnam-Berry phases. The coupling between polarization and OAM in these beams manifests as spatially dependant polarization. Regions of uniform polarization are perceived as specifically oriented Haidinger's brushes, and study participants discriminated between two spin-orbit states based on the rotational symmetry in the spatial orientations of these brushes. Participants used self-generated eye movements to prevent adaptation to the visual stimuli. After initial training, the participants were able to correctly discriminate between two spin-orbit states, differentiated by OAM $=\pm1$, with an average success probability of $69\%$ ($S.D. = 22\%$, $p = 0.013$). These results support our previous observation that human observers can directly perceive spin-orbit states, and extend this finding to non-rotating beams, OAM modes induced via Pancharatnam-Berry phases, and the discrimination of states that are differentiated by OAM.