Gray optical vortices in a hollow waveguide

Summary form only given. Optical vortices (OV) have attracted a lot of attention for their fascinating dynamics as well as potential applications in areas like all light induced waveguiding. A helical wavefront and a dark circular core whose intensity becomes zero at the central point owing to the phase singularity is the characteristic of an OV. This "darkspot" or phase singularity is often used to judge the existence or the exact location of an OV in a bright beam background. Usually an OV is diffracted and expands along with the bright background beam that carries it. Although self-defocusing nonlinearity can counteract the diffraction of an OV and results in the formation of an optical vortex soliton, it is unable to maintain the bright background. Thus we contemplate using a hollow metallic waveguide to confine the bright beam. Here, we report the first experimental observation of optical vortices with non-zero core intensity, i.e. gray core, in a hollow waveguide. We subsequently verify this remarkable phenomenon with computer simulations and identify various parameters that lead to the creation of gray OV.