Experimental measurement of surface plasmon resonance of pyramidal metal nanoparticle tips

We report on the fabrication, experimental characterization and modeling of atomic force microscope (AFM) probes with pyramidal optical antennas fabricated at the ends of the tips. These are being developed for tip-enhanced near-field scanning optical microscopy. We use focused ion beam milling to etch a gold-coated Si3N4 AFM tip, resulting in a pyramidal gold nanoparticle (188 - 240 nm long) at the end of the tip. Using finite-difference time-domain (FDTD) simulations, we estimate the electric field distribution around the nanoparticle as a function of incident wavelength for nanoparticles of various lengths. We experimentally measure the scattering spectra of fabricated probes and show enhanced scattering associated with the localized surface plasmon resonance of the tip. Both simulations and experiments show that an increase of the tip length results in a redshift of the tip resonance wavelength. These pyramidal metal nanoparticle tips could be used for either mapping the field distribution of nanophotonic devices or high spatial resolution spectroscopy.