The generation of vortices in the focusing of circularly polarized light beams results from the spin-orbit interaction (SOI), which is very similar to the generation of vortices when a light beam normally passes through an optically thin slab. However, its physical origin and the intrinsic relationship between both systems are still not well elucidated. Here, we establish a unified full-wave theory to revisit this problem. It is revealed that the origin of vortex generation in the transverse fields of the focusing system is the momentum-depende...
