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Polarization-resolved hyper Rayleigh scattering experiments were performed on gold metallic nanoparticles. These results unambiguously indicate that the second harmonic generation process of a nanoparticle is of pure electric dipole origin for the smallest particles as opposed to pure electric quadrupole, and that retardation effects come into play for larger sizes. It is furthermore demonstrated that this electric dipole emission at the harmonic frequency is due to a deviation of the particle shape from that of a perfect sphere and not from defects in the crystalline structure of the particle.
Nappa et al. (2005) studied this question.