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The rich structure of bright and dark surface-plasmon modes localized in individual and coupled gold nanoparticles is unveiled by electron- energyloss spectroscopy performed in a scanning transmission electron microscope. Spatially resolved maps of surface-plasmon modes in the ̃1.5-2.5 eV range (wavelengths ̃500-800 nm), collected for individual nanorods, coupled nanorod dimers, and touching nanosphere dimers, are in excellent agreement with theory. Surface-plasmon maps constructed from the spatially and spectrally resolved energy-loss signais are shown to mimic rather well the near fields calculated for extemal illumination in the case of bright surface-plasmon modes (i.e., those coupling to external light). Dark surface-plasmon modes that cannot be excited by optical means are also found, and our electron probing technique provides further insight into their corresponding spatial distribution and symmetry, which are not accessible to any other existing techniques. Our resutts initiate the study of a whole set of new dark surface-plasmon modes that should become a source of new applications in sensing and microscopy but have escaped experimental scrutiny so far.

All surface-excitation studies of Au in the past focused on the well-known 2.4 eV surface plasmon polariton in the visible spectral regime. The existence of surface exciton polaritons is believed to be pristine to the spectral regimes, showing strong excitonic absorptions. The presence of surface exciton polaritons in far-UV in Au (≥10 eV), where the optical and electronic properties of Au are dominated by broad interband transitions that display characters of rather weak and diffused excitonic oscillator strengths, is not expected and has never been discussed. Re-examining the reports of Yang and using electron energy-loss spectroscopy with a 2Å electron probe in aloof (optical near-field) setup and real-space energy-filtered imaging, we firmly establish the existence of surface exciton polaritons in individual Au nanoparticles in the far-UV spectral regime. These results indicate that surface exciton polaritons indeed can be excited in weak excitonic onsets in addition to their general believing for the sharp excitonic oscillations. Our experimental observations are further confirmed by the theoretical calculations of electron energy-loss spectra. The unmatched spatial resolution (2Å) of the electron spectroscopy technique enables an investigation of individual nanomaterials and their surface excitations in aloof setup. The surface exciton polaritons in individual Au nanoparticles thus represent an example of surface excitations of this type beyond the visible spectral regime and could stimulate further interests in surface exciton polaritons in various materials and applications in novel plasmonics and nanophotonics at high energies via manipulations of the associated surface near fields. © 2008 The American Physical Society.