Reflective Spin–Orbit Geometric Phase from Planar Isotropic Interface

Manipulating angular momentum of light relies on the change of geometric phase in anisotropic nanostructures or optical interface phenomena. Spin–orbit interactions demonstrated in normal incidence at a planar isotropic interface are conditioned by Brewster reflection, limited to the light beam with a single radial frequency in the k‐space. This study demonstrates a geometric (Pancharatnam–Berry) phase that overcomes Brewster angle limitations providing high variability in the modulation of light reflected from a planar isotropic interface. The examined geometric phase stems from the reflection anisotropy imitating the structural anisotropy of photoaligned liquid crystals and photonic metasurfaces. Using a geometric‐phase modulation of tightly focused light reflected from a glass slab, three modes of spin to orbital angular momentum conversion are demonstrated. The discovered reflective spin–orbit interaction allows for design of a double‐helix focusing sensor to be used in microscopy imaging for precise depth measurements (accuracy 109 nm and precision 7 nm in the range of 8 µm). Experiments with a focused astigmatic beam prove that a glass plate can perform very complex modulation of the geometric phase using illumination structuring. The results thus provide the foundation for the design of low‐cost geometric‐phase systems relying on the space‐variant reflection anisotropy. Spin–orbit interactions are extensively investigated in nanomaterials and at interfaces. Analogous to nanostructures, space‐variant reflection anisotropy is examined, allowing spatial modulation of the geometric phase and variable spin–orbit interactions at the planar interface. Double‐helix focusing sensors are demonstrated with discovered effects, and their use for geometric‐phase reflection optics is proposed.