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 ζ V is 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.

The second-harmonic generation of 150 nm spherical gold nanoparticles is investigated both experimentally and theoretically. We demonstrate that the interference effects between dipolar and octupolar plasmons can be used as a fingerprint to discriminate the local surface and non-local bulk contributions to the second-harmonic generation. By fitting the experimental data with the electric fields computed with finite-element method (FEM) simulations, the Rudnick and Stern parameters weighting the relative nonlinear sources efficiencies are evaluated and the validity of the hydrodynamic model and the local density approximation approaches are discussed.

We report the optical second harmonic generation from individual 150 nm diameter gold nanoparticles dispersed in gelatin. The quadratic hyperpolarizability of the particles is determined and the input polarization dependence of the second harmonic intensity obtained. These results are found in excellent agreement with ensemble measurements and finite element simulations. These results open up new perspectives for the investigation of the nonlinear optical properties of noble metal nanoparticles.

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The near-field coupling between a gold and a silver spherical nanoparticle is investigated theoretically. Fano profiles are observed in the absorption cross section of the gold nanoparticle due to the coupling between the spectrally localized surface plasmon resonance of the silver nanoparticle and the continuum of interband transitions of the gold one. The effect of dimer internal characteristics (particle sizes and distance), surrounding medium (through the refractive index), and external excitation (polarization and propagation directions) are addressed. In particular, it is shown that the near-field coupling can be tuned from the weak to the strong regime by rotating the polarization direction, and that the Fano profiles are sensitive to the shadowing effect even for small particle sizes.

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By developing a new method for synthesizing atomically monodisperse Au15 nanoclusters stabilized with glutathione molecules and using the current state-of-the-art methods for synthesizing monodisperse protected Au25 nanoclusters, we investigated their nonlinear optical (NLO) properties after two-photon absorption. The two-photon emission spectra and the first hyperpolarizabilities of these particles were obtained using, in particular, a hyper-Rayleigh scattering technique. The influence on NLO of the excitation wavelength, the size as well as the nature of the ligands is also explored and discussed. Au15, the smallest stable thiolated gold nanocluster, presents remarkable nonlinear properties with respect to two-photon processes. The two-photon absorption cross-section at 780 nm for Au15 is ∼65,700 GM. This experimental cross-section value points to a quantum yield for two-photon emission of about 3 × 10(-7) at 475 nm for Au15. The first hyperpolarizability β for Au15 clusters (509 × 10(-30) esu), as compared to Au25 clusters (128 × 10(-30) esu), is larger considering the difference in the number of gold atoms. Also, 10(30) β per atom values reported for Au15 and Au25 clusters are more than two orders of magnitude larger than the values reported for Au NPs in the size range 10-50 nm, outlining the quantum cluster regime.