Sample quality and second-harmonic generation from gold nanoparticles

The second-order nonlinear optical response of metal nanostructures is very sensitive to the symmetry of the structures and the symmetry of the local-field distribution in the structure. Thus, even small defects in the particles, which break the ideal symmetry and can attract strong local fields, can lead to significant second-harmonic signals forbidden by the ideal symmetry. Designing nonlinear metamaterials with desired properties is not possible if the fabrication defects significantly affect the optical response of the structure. We show that recent improvements in sample quality allow the selection rules for second-harmonic generation to be much better fulfilled than ever before. We are thus approaching the limit where such structures can be used to design nonlinear metamaterials for future applications.