Tailorable spin-to-orbital angular momentum degree-of-freedom complete conversion with cascaded metasurfaces

Spin-to-orbital angular momentum (OAM) conversion combined with the polarization shaping expands capabilities in optical communications and quantum optics. Whereas a single metasurface can create different OAMs depending on the input polarization, it cannot arbitrarily tailor output polarizations. We reveal how to overcome this limitation with cascaded metasurfaces, achieving complete control over the polarization and optical phase of OAMs combined with a near-unity efficiency, which can mediate the creation of quantum-entangled states. Our analytical framework of multi-channel OAM conversion predicts the optimal metasurface separation distance, and diffraction theory calculations confirm that the designed performance is preserved within nanofabrication tolerances.