  Optical field localization at plasmonic tip-sample nanojunctions has enabled
high spatial resolution chemical analysis through tip-enhanced linear optical
spectroscopies, including Raman scattering and photoluminescence. Here, we
illustrate that nonlinear optical processes, including parametric four-wave
mixing (4WM), second harmonic/sum-frequency generation (SHG and SFG), and
two-photon photoluminescence (TPPL), can be enhanced at plasmonic junctions and
spatio-spectrally resolved simultaneously with few-nm spatial resolution under
ambient conditions. More importantly, through a detailed analysis of our
spectral nano-images, we find that the efficiencies of the local nonlinear
signals are determined by sharp tip-sample junction resonances that vary over
the few-nanometer length scale because of the corrugated nature of the probe.
Namely, plasmon resonances centered at or around the different nonlinear
signals are tracked through TPPL, and they are found to selectively enhance
nonlinear signals with closely matched optical resonances.
