Phase-sensitive spectroscopy of surface plasmons in individual metal nanostructures

We analyze both theoretically and experimentally the response of individual gold nanoparticles and nanoparticle dimers to a near-field excitation with broad-band radiation (a femtosecond white-light continuum) delivered through a subwavelength aperture. Because of the coherent superposition of the field emitted by the aperture and the secondary field reemitted by the nanostructure, the signals detected in the far zone exhibit a pronounced dependence on the phase of plasmon oscillations excited in the nanostructure. This phase sensitivity allows us to accurately determine positions of plasmon resonances not distorted by dielectric losses in a metal. In the near-field extinction spectra of individual nanoparticles, the plasmon resonance is observed as the ``zero-extinction'' point in which the transition from constructive (lower spectral energies) to destructive (higher spectral energies) interference occurs. By using spatially selective near-field excitation with a femtosecond white-light continuum, we are able to detect slight asymmetries in dimers composed of nominally identical nanoparticles.