Topological Charge Conversion with Spiral-Slit Screens

We propose a method of converting the topological charge (TC) of an incident vortex light wave with spiral-slit screens. The phase in the incident vortex beam is redistributed via the optical path adjustment realized by changing the in-plane radial distance of the spiral slit. We numerically calculate the light field with Fresnel diffraction theory, and find the expected TC conversion on the observation plane. Our calculations also show that screens of pinholes distributed in a spiral curve can be used for the implementation instead of spiral slits. The method can also be used to distinguish the TCs of the incident vortices by reading the diffracted intensity patterns.