Optical vortices are light beams characterized by a phase change of an integer multiple of 2π along a closed path around the center of the beam, where the phase of the beam is undetermined (phase singularity) and the field amplitude vanishes. Optical vortices could find a series of appealing applications to optical tweezers, optical spectroscopy, digital imaging, and quantum information processing.

We demonstrate that plasmonic helical gratings consisting of metallic nanowires imprinted with helical grooves or ridges can be used efficiently to generate plasmonic vortices with radius much smaller than the operating wavelength. In our proposed approach, these helical surface gratings are designed so that plasmon modes with different azimuthal quantum numbers (topological charge) are phase-matched, thus allowing one to generate optical plasmonic vortices with arbitrary topological charge. The general principles for designing plasmonic helical gratings that facilitate efficient generation of such plasmonic vortices are derived and their applicability to the conversion of plasmonic vortices with zero angular momentum into plasmonic vortices with arbitrary angular momentum is illustrated in several particular cases. Our analysis, based both on the exact solutions for the electromagnetic field propagating in the helical plasmonic grating and a coupled-mode theory, suggests that even in the presence of metal losses the fundamental mode with topological charge m=0 can be converted to plasmon vortex modes with topological charge m=1 and m=2 with a conversion efficiency as large as 60%. The plasmonic nanovortices introduced in this study open new avenues for exciting applications of orbital angular momentum in the nanoworld.

This work was published on Scientific Reports [Sci. Rep. 5, 13089; doi: 10.1038/srep13089 (2015)].