Nonlinear optical properties of metal nanoparticles have been investigated at different excitation wavelengths as a function of particle size. The second order nonlinear response has been determined in two different nanometric
systems, a) Gallium nanoparticle monolayers embedded in dielectric matrix producing collinear second harmonic generation, and b) Au/water colloids giving rise to diffuse hyper-Rayleigh scattering. Modeling of the mechanisms involved provides a key to the interpretation of the dependence of the signal from temperature and size
