Gap-Plasmon-Enhanced Nanofocusing Near-Field Microscopy

We report the observation of coherent light scattering from nanometer-sized gap regions in a nanofocusing scanning near-field optical microscope. When approaching a nanofocusing gold taper to the surface of a thin semitransparent gold film and detecting in transmission, we find a steep increase in scattering intensity over the last 5 nm in a near-field signal selected in k-space. This is confirmed as a signature of highly confined gap plasmons by detailed comparisons to finite element method simulations. The simulations reveal that the confinement is adjustable via the underlying probe sample distance control scheme even to levels well below the taper apex radius. This controlled experimental realization of gap plasmons and the extraction of their signature in a scanning probe microscope pave the way toward broadband spectroscopy at and below single-nanometer length scales, using parallel detection at multiple wavelengths, for instance, in transient absorption or two-dimensional spectroscopy.