The enhancement of optical second-harmonic generation at individual defects on metal surfaces was studied. Second-harmonic generation was excited locally at chosen defects by using a near-field optical microscope with an uncoated fibre tip. Second-harmonic intensity-enhancements of up to ten times were observed at the apex of micron-sized defects on a Au surface, while the average enhancement was of the order of 20%. The observed enhancement was attributed to a lightning-rod effect.

Second-Harmonic Generation from Individual Surface Defects under Local Excitation. A.V.Zayats, T.Kalkbrenner, V.Sandoghdar, J.Mlynek: Physical Review B, 2000, 61[7], 4545-8