Nonuniformity of Phase Vortices and Singularity in Plasmonic Field Produced by Silver Spiral Slit Under Linearly Polarized Illumination

We experimentally investigate the phase vortices and singularity in the surface plasmon polariton (SPP) field produced by oil-immersed silver spiral slit under illumination of linear polarized plane wave. Experimentally, the Mach-Zehnder interferometer system is constructed to detect the scattered SPP field with the phase retrieved from the interferogram. We find six nonuniform vortices and a singularity line segment in the central part of the slit. We propose the experiment method of assistive circular ring-slit to obtain the expression of the excited secondary SPP source and use the Huygens-Fresnel principle for the SPP wave propagations to explore the formation of vortices. From the experimental and the theoretically computed results, it is demonstrated that the topological charge of each vortex is nonuniform despite of its unity mean value. Based on the maps of phases and intensities, we find that the orbital angular momentum of a photon is azimuthally nonuniform and keeps almost unchanged in radial direction, and its density is approximately proportional to distance squared from the core.