  We report on measurements of visible extinction spectra of semicontinuous
silver nanoshells grown on colloidal silica spheres. We find that thin, fractal
shells below the percolation threshold exhibit geometrically tunable plasmon
resonances. A modified scaling theory approach is used to model the dielectric
response of such shells, which is then utilized to obtain the extinction cross
section in a retarded Mie scattering formalism. We show that such spherical
resonators support unique plasmon dynamics: in the visible there is a new
regime of coherently driven cluster-localized plasmons, while crossover to
homogeneous response in the infrared predicts a delocalized shell plasmon.
