Spin-Orbit Photonics with Potato Starch Lenses

Advanced flat optics based on metasurfaces or liquid crystals control the dynamic and geometric (Pancharatnam-Berry) phases of light through in-plane variations of refractive index and birefringence. This study introduces spherulites, structures inherent to starch and other polycrystalline materials, as unique volume optical elements that modulate the dynamic phase through changes in the geometric path of rays, while simultaneously controlling the geometric phase via 3D variations in anisotropy. The shape-dependent dynamic phase focuses light, while the geometric phase, resulting from the structural radial anisotropy, generates optical vortices, converting light's spin into orbital angular momentum. This phase interplay establishes spherulite-based spin-orbit photonics. After the challenging holographic verification of both phases, starch spherulites extracted from potato tubers are demonstrated as standard and vortex microlenses, with their operation controlled by light polarization. The suitability of spherulites for light sensing is demonstrated by measuring vortex topological charges and fully reconstructing any Poincaré sphere polarization state of incident light from the spherulite's focal intensity spot. A vectorial Shack-Hartmann experiment with starch spherulites showcases this novel polarimetric sensing alongside wavefront measurement. By transferring the discovered properties of spherulites to artificial metamaterials, new polarization lenses, on-chip vortex detectors, and polarization-sensitive wavefront sensors can be developed.