Vortex Light at the Nanoscale: Twists, Spins, and Surprises -- A Review

For over three decades, the study of optical vortex beams carrying orbital angular momentum (OAM) has been at the forefront of optics, driven by fundamental questions about optical momentum as well as diverse applications in quantum information, communications, and optical manipulation. Most work has focused on paraxial beams, whose transverse fields are accurately described by conventional wave optics and the Stokes formalism. By contrast, when light is confined to the nanoscale and tightly focused beyond the paraxial regime, vortex beams exhibit complex electromagnetic structures that transcend these conventional models. In this deeply non-paraxial regime, the resulting fields display rich and often counterintuitive behaviour, opening new perspectives on light-matter interactions. This review unifies the emerging physics of nanoscale optical vortices by developing a coherent theoretical framework and offering a critical synthesis of recent advances, guiding readers toward a deeper understanding and stimulating future work in this rapidly evolving field.