Spin-orbit coupling induced by ascorbic acid crystals

Some anisotropic materials form semicristalline structures, called spherulites, which observed in a polarisation microscope, exhibit a characteristic ``maltese-cross"-like pattern. While this observation has been hitherto considered as a tool to characterize these materials, we show that these patterns are associated with a strong light's spin-orbit coupling induced by the spherulite structures. We experimentally demonstrate these effects using samples of crystallized ascorbic acid and observing the creation of optical vortices in transmitted laser beams, as well as the formation of inhomogeneous polarisation patterns. Our findings suggest the use of spherulites in frequency ranges, e.g. in the THz domain, where polarisation and spatial shaping of electromagnetic radiation is still a challenging task.