OAM mode conversion in a photonic chip

As an additional degree of freedom and emerging resource, orbital angular momentum (OAM) has been widely used in both classical and quantum information technologies. Flexible manipulation of OAM modes is fundamentally important to various OAM-based applications. In this Letter, we demonstrate OAM mode conversion in a photonic chip by properly engineering the three-dimensional laser direct writing. More specifically, we find that by increasing the direct-written power, the input OAM modes first remains unchanged, and then transforms into its superpositions. When mapping OAM into the waveguide, it gradually evolves from OAM and its superpositions to OAM. This mode conversion is essentially determined by the eigenmodes supported by the waveguide. Furthermore, the input OAM modes can be reversed by tuning the direct-written velocity. Our results demonstrate the more complete manipulation of twisted-light-inside a photonic chip and may further contribute to the integration of high-dimensional quantum information processing.