Near-field observation of modes in subwavelength plasmonic slot waveguides

Metal-insulator-metal (MIM) plasmonic slot waveguides may guide light with subwavelength confinement, and as such have a potential use for on-chip light guiding and processing. So far only input-output measurements have been performed. We have fabricated such slots in an 80 nm thick layer of gold deposited on a glass substrate. We have experimentally visualized the modes of these waveguides. We quantitatively determine the wavevector as a function of slot width. The results are in excellent agreement with numerical calculations, which also reveal the subwavelength extent of these modes. The symmetry of the modes is investigated to explore their nature.