Near-field optical spectroscopy of individual surface-plasmon modes in colloid clusters

Local spectra of self-affine clusters of silver colloid particles recorded with subwavelength resolution by near-field spectroscopy are reported. Spec tra were also simulated computationally. The observed and calculated near-f ield spectra consist of several resonances with highly location-dependent f requencies. The most highly resolved of these resonances correspond to indi vidual surface plasmon (SP) normal modes. All of these features are only ob servable in the near field. Both theory and experiment also show that when excited by light in the SP region of the spectrum, the field-intensity dist ribution in the near field is very heterogeneous with most of the excitatio n concentrated in "hot spots" on the cluster surface that are strongly exci tation-wavelength dependent. This field-intensity localization provides a r ationale for recently reported surface enhanced Raman enhancements in exces s of 10(10). [S0163-1829(99)011 15-7].