Switching topological charge of optical vortex by two-dimensional structures

Optical vortex beams have found wide applications in telecommunications, biophotonics, medicine and other fields due to their unique properties. Here, we propose a method to generate a vortex beam with post fabrication variable topological charge using inherently two-dimensional structures based on phase change materials. We fabricate Ge-Sb-Te thin films with a spatial pattern of amorphous and crystalline states written by using the direct laser writing technique. In our report the rewritable pattern represents a pitch-fork grating. We verify that the mechanical rotation of the films leads to a change in the topological charge of the output beam. We demonstrate an erase-and-rewrite modification of the pattern, that results in the change of the topological charge of the generated beam, when all optical elements are fixed, i.e. no mechanical displacement takes place.