Multipolar Contributions of the Second Harmonic Generation from Silver and Gold Nanoparticles

Hyper Rayleigh scattering is used to investigate the second harmonic light collected from a liquid suspension of silver metallic particles, the diameter of which ranges from 20 nm up to 80 nm. From the dependence of the quadratic hyperpolarizability as a function of the particle size, it is inferred that retardation effects of the electromagnetic fields play a major role in the frequency conversion process. The hyper Rayleigh scattering intensity was also recorded as a function of the angle of polarization of the incident fundamental wave. For the particles with a diameter of 20 nm, the harmonic response is dominated by the dipolar contribution arising from the deviation of the particle shape from that of a perfect sphere. It is therefore concluded that the origin of the nonlinear polarization for the smaller silver particle sizes arises from the surface similarly to the case of the gold metallic particles. For larger diameter particles, retardation effects in the interaction of the electromagnetic fields with the particles cannot be neglected any longer, and the response deviates from the pure dipolar response, exhibiting a strong quadrupolar contribution. The weighting parameter Vis used, equaling unity for a pure quadrupolar contribution and vanishing for a pure dipolar response, in order to quantify the relative magnitude of these two dipolar and quadrupolar contributions. The size dependence of this parameter obtained for the silver particles is compared to the dependence previously reported for gold metallic particles.