Applications of field-enhanced near-field optical microscopy.

Metal nanostructures such as sharp tips can enhance emission yields through shape-induced local field enhancement. The enhancement originates from two mechanisms: surface plasmons and electrostatic lightening rod effects. We present fluorescence imaging using the strong local field created at the apex of a gold tip and demonstrate optical resolution of 25 nm. The enhancement effect gives also rise to photoemission from the tip itself. Measured spectra of the tip emission show a broad band continuum together with a second-harmonic peak. Both continuum and second-harmonic are confined at the apex of the tip. We find that, depending on the spectral position, the photoluminescence originates either from intraband or from interband transitions. The nonlinear response can be described by a single dipole oscillating at the second-harmonic frequency and oriented along the tip axis. These unique properties can be used to map focal fields distributions.