Spin orbit coupling of light

Light is described by a vector potential and thus has internal degrees of freedom, which represents its intrinsic spin carrying an internal momentum. Now, if this beam of light travels along a curved trajectory, it shall also posses an orbital momentum, which can be manifested in the way it transforms under coordinate rotation. These two momenta may interact with each other. Differing from the situation with electrons, the spin-orbit coupling of light is geometric by origin. Here is a description of the expriments observing this coupling.