Enhancing the efficiency of the topological phase transitions in spin–orbit photonics

A light beam reflected and refracted at a sharp interface can acquire a momentum-dependent Pancharatnam–Berry (PB) phase, which produces a topological phase transition from one kind of spin–orbit interaction (e.g., spin-controlled vortex generation) to another (e.g., photonic spin-Hall effect). However, this process is extremely inefficient and difficult to observe directly in experiments, which also hinders its applications. Here, we propose to enhance significantly the topological phase transitions by c-cut uniaxial crystals. We first give a full-wave theory to describe the spin–orbit interactions of a beam passing through a c-cut uniaxial crystal and experimentally observe the topological phase transition process of the transmitted beam when the angle between the beam propagation direction and the optical axis direction changes. It is found that the efficiency of the spin–orbit interactions caused by the momentum-dependent PB phase can be increased as high as 50%, which is much larger than that at isotropic sharp interfaces. Our findings provide an alternative approach for manipulating the spin and orbital angular momenta of light and exhibit potential applications in the future spin–orbit photonic components.