Near-field second-harmonic generation in single gold nanoparticles

Second-harmonic generation from single gold elliptical nanoparticles is experimentally investigated by a nonlinear scanning near-field optical microscope SNOM. The near-field nonlinear response is found to be directly related to local surface plasmon resonances and to particle morphology. The combined analysis of linear and second-harmonic SNOM images provides discrimination among different light extinction particle behaviors, not achievable just with linear techniques. The polarization state of the emitted second harmonic is also investigated, providing experimental evidence of second-harmonic particle emission modes peculiar to near-field excitation