Surface-Plasmon Resonances in Single Metallic Nanoparticles

Using a near-field optical antenna effect, we measure the homogeneous line shape of the surface-plasmon resonance in single gold nanoparticles. The surface-plasmon dephasing times extracted from the near-field spectra of the individual particles vary around 8 fs. This mean value agrees with calculations based on Mie theory, which neglect surface effects. Deviations of individual particles from this value are interpreted as being due to variations in the local nanoenvironment. We also observe double-peaked line shapes caused by electromagnetic coupling between close-lying particles.