Nonlinear plasmonics with gold nanoparticle antennas

We investigate the nonlinear optical properties of gold nanoparticle pairs. Two excitation beams of frequencies ω1 and ω2 are used to induce nonlinear polarizations at the junction of a particle dimer. Nonlinearities of the second and third order can be controllably induced as a function of the dimer geometry, leading predominantly to second-harmonic generation (SHG), sum frequency generation (SFG) and four-wave mixing (4WM). Due to their center symmetry, dimers with identical particle diameters give rise to a very weak second-order response, without affecting the third-order response. Therefore, a sharp probe functionalized with a symmetric metal dimer acts as a nanoscale photon source emitting narrow-band photons of frequency 2ω1 − ω2. We demonstrate that this source can be employed as a near-field optical probe for high-resolution fluorescence imaging.