Optical oscillation modes of plasmon particles observed in direct space byphase-contrast near-field microscopy

Light distributions near resonant metal nanoparticles are recorded by a sca ttering-type scanning near-field optical microscope (s-SNOM), for the first time with a sub-particle-size resolution (< 10 nm) and with simultaneous a mplitude and phase contrast. The images depict the optical oscillation patt erns of single plasmon particles. Examples are presented of particles excit ed in dominantly dipolar and quadrupolar modes, and also of closely spaced particles sustaining a gap mode. The gap mode can provide enhanced optical fields in nanometric spots for nonlinear and single-molecule spectroscopy a pplications.