Vortex generation in the spin-orbit interaction of a light beam propagating inside a uniaxial medium: origin and efficiency

It has been known that an optical vortex with a topological charge ±2 can be generated as a circularly polarized (CP) light beam propagates in a bulk uniaxial crystal, but its physical origin remains obscure which also hinders its practical applications. Here, through a rigorous full-wave analyses on the problem, we show that, as a CP beam possessing a particular spin (handedness) propagates inside a uniaxial crystal, two beams with opposite spins can be generated caused by the unique spin-sensitive light-matter interactions in the anisotropic medium. Flipping the spin can offer the light beam an vortex phase with a topological charge of ±2 owing to the Pancharatnam-Berry mechanism, with efficiency dictated by the material properties of the uniaxial medium and the topological structure of the beam itself. With its physical origin fully uncovered, we finally discuss how to improve the efficiency of such effect, and compare the mechanisms of vortex generations in different systems. Our findings not only provide deeper understandings on such an intriguing effect, but also shed light on other spin-orbit-interaction-induced effects.