A. David, B. Gjonaj, and G. Bartal
Physical Review B 93, 121302 (2016)
Optical vortices possess unique topology which can benefit a broad variety of applications, ranging from particle trapping and microscopy to communications and light-matter interactions. Notwithstanding their proven applicability, being subjected to the diffraction limit typically inhibits the facilitation of vortex beams into nanoscale applications. Here, we present optical nanovortices of 60 nm size employing silicon-based waveguides that support guided modes with short wavelengths. We introduce nanovortices with on-demand topological charge and show azimuthal phase variations almost ten times faster than free-space oscillations of the illumination wavelength (671 nm) using high-resolution phase-resolved near-field measurements. Moreover, we apply superoscillations of two short-wavelength guided modes to further decrease the radial size of the vortex, achieving nanosized dimensions in both the azimuthal and radial directions.