  We report theoretical evidence that bulk nonlinear materials weakly
interacting with highly localized plasmonic modes in ultra-sub-wavelength
metallic nanostructures can lead to nonlinear effects at the single plasmon
level in the visible range. In particular, the two-plasmon interaction energy
in such systems is numerically estimated to be comparable with the typical
plasmon linewidths. Localized surface plasmons are thus predicted to exhibit a
purely nonclassical behavior, which can be clearly identified by a
sub-Poissonian second-order correlation in the signal scattered from the
quantized plasmonic field under coherent electromagnetic excitation. We
explicitly show that systems sensitive to single-plasmon scattering can be
experimentally realized by combining electromagnetic confinement in the
interstitial region of gold nanodimers with local infiltration or deposition of
ordinary nonlinear materials. We also propose configurations that could allow
to realistically detect such an effect with state-of-the-art technology,
overcoming the limitations imposed by the short plasmonic lifetime.
