Propagation of Vortex Beams Diffracted with Circular and Triangular Apertures

Laguerre-Gaussian (LG) laser beams are called vortex beams because of optical vortices centered on the beam axis: the topological charge (TC) or number of vortices equals the index l in their e^ilϕ azimuthal phase term, and each photon carries a quantity lħ of orbital angular momentum (OAM). Vortex beams have immense potential for application in nuclear physics. JLab is considering generating vortex gamma ray sources, focusing vortex beams on photocathodes for polarized electron sources, and creating high power vortex beams with Fabry-Pérot interferometers to probe nuclear structure: these new tools may shed light on fundamental problems like the proton spin crisis. Here we investigate the propagation of vortex beams diffracted by circular and triangular apertures theoretically and experimentally. The properties of these diffracted beams are of interest as the diffraction patterns depend on their TC’s sign and magnitude, and can thus be used to measure their OAM directly. Simulations indicate that the diffraction patterns reach a stable state in the far field, where they become symmetrical and vortices propagate without change, and we find good agreement in our experimental results.