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. Spectra were also simulated computationally. The observed and calculated near-field spectra consist of several resonances with highly location-dependent frequencies. The most highly resolved of these resonances correspond to individual surface plasmon (SP) normal modes. All of these features are only observable 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 distribution in the near field is very heterogeneous with most of the excitation concentrated in “hot spots” on the cluster surface that are strongly excitation-wavelength dependent. This field-intensity localization provides a rationale for recently reported surface-enhanced Raman enhancements in excess of ${10}^{10}.$