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 <i>k</i>-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

Excitation of Mesoscopic Plasmonic Tapers by Relativistic Electrons: Phase Matching <i>versus</i> Eigenmode Resonances

We investigate the optical modes in three-dimensional single-crystalline gold tapers by means of electron energy-loss spectroscopy. At the very proximity to the apex, a broad-band excitation at all photon energies from 0.75 to 2 eV, which is the onset for interband transitions, is detected. At large distances from the apex, though, we observe distinct resonances with energy dispersions roughly proportional to the inverse local radius. The nature of these phenomena is unraveled by finite difference time-domain simulations of the taper and an analytical treatment of the energy loss in fibers. Our calculations and the perfect agreement with our experimental results demonstrate the importance of phase-matching between electron field and radiative taper modes in mesoscopic structures. The local taper radius at the electron impact location determines the selective excitation of radiative modes with discrete angular momenta

Excitation of Mesoscopic Plasmonic Tapers by Relativistic Electrons: Phase Matching <i>versus</i> Eigenmode Resonances

We investigate the optical modes in three-dimensional single-crystalline gold tapers by means of electron energy-loss spectroscopy. At the very proximity to the apex, a broad-band excitation at all photon energies from 0.75 to 2 eV, which is the onset for interband transitions, is detected. At large distances from the apex, though, we observe distinct resonances with energy dispersions roughly proportional to the inverse local radius. The nature of these phenomena is unraveled by finite difference time-domain simulations of the taper and an analytical treatment of the energy loss in fibers. Our calculations and the perfect agreement with our experimental results demonstrate the importance of phase-matching between electron field and radiative taper modes in mesoscopic structures. The local taper radius at the electron impact location determines the selective excitation of radiative modes with discrete angular momenta