Near-field imaging with a localized nonlinear light source.

We demonstrate high-resolution near-field imaging and spectroscopy using the nonlinear optical response of a gold nanoparticle pair as an excitation photon source. Femtosecond pulses of frequencies omega(1) and omega(2) are used to induce a nonlinear polarization at the four wave mixing (4WM) frequency 2omega(1) - omega(2) in the junction of the nanoparticle dimer. The nonlinear response leads to localized photon emission, which is employed as an excitation source for fluorescence and extinction imaging. The principle of this imaging technique is demonstrated for samples of fluorescent nanospheres and tubular J-aggregates.