Topology-Induced Phase Transitions in Spin-Orbit Photonics

Spin-controlled vortex generation and spin-Hall effect, two distinct effects discovered in optics, have been extensively studied recently. However, while physical origins of two effects are both due to spin-orbit interactions, their inherent connections remain obscure which also hinders further explorations on the manipulations of them. Here, in studying the scattering of a spin-polarized light beam at sharp interfaces, an intriguing phase transition between vortex generation and spin-Hall shift trigged by varying the incidence angle is revealed. After reflection/refraction, the beam contains two components: normal and abnormal modes acquiring spin-redirection-Berry phases and Pancharatnam–Berry phases, respectively. Inside the abnormal beam, two classes of wave components gain Pancharatnam–Berry phases with distinct topological natures, generating intrinsic and extrinsic orbital angular momenta (OAM), respectively. Enlarging incidence angle changes the relative portions of these two contributions, making the abnormal beam undergo a phase transition from vortex generation to spin-Hall shift. Such intriguing effect is experimentally observed at a purposely designed metamaterial slab, exhibiting efficiency enhanced by several-thousand times compared to that at a conventional slab. These findings unify two previously discovered effects in a single framework, reinterpret previous results with clearer pictures, and shed light on understanding other physical effects involving the competition between intrinsic and extrinsic OAM.