Second-harmonic generation from nanoscopic metal tips: Symmetry selection rules for single asymmetric nanostructures

Second-harmonic generation (SHG) from individual nanoscopic metal tips has been investigated. Compared to both planar interfaces, as well as spherical or ellipsoidal nanoparticles, very different polarization selection rules and SH-emission directions result. As a partially asymmetric nanostructure the tip allows for the distinction of otherwise inseparable local surface and nonlocal bulk second-harmonic polarizations. This provides opportunities for second-harmonic investigations of nanoparticles and in scattering-type near field microscopy.