Real-space spin-orbit interaction from quarter-wave plate nonuniformity

Abstract Spin–orbit interactions couple light polarization to its transverse structure. Although widely studied at interfaces, in tilted anisotropic plates, and under tight focusing, their role in paraxial confocal systems remains unclear. We show that a commercial quarter-wave plate between crossed polarizers at nominal normal incidence converts the rejected Gaussian field into a Hermite–Gaussian-like two-lobe mode. Its reduced overlap with the collecting fiber mode enhances the polarization extinction ratio by over two orders of magnitude. For an ideal plate at normal incidence, the leading momentum-space contribution vanishes by symmetry. An effective angular model reproduces the symmetry but not the absolute intensity or the beam-size dependence. We trace this discrepancy to first-order real-space nonuniformities of the plate, amplified by the confocal geometry and isolated through beam-size scaling. Cross-polarization confocal detection thus renders weak spatially varying polarization responses observable as transverse-mode transformations.