Résumé
Hyper-Rayleigh scattering (HRS) has become the standard method for the determination of the first hyperpolarizability \beta (second-order non-linear optical polarizability, effectively cross-section for the frequency doubling process) of molecules and nanoparticles as the measurement can be simply performed directly from liquid suspensions. The measurement provides an average of the first hyperpolarizability over spatial orientation due to the macroscopic centrosymmetry of a liquid but microscopic instantaneous orientational fluctuations. For plasmonic spherical nanoparticles like gold or silver nanoparticles, it has been shown that this first hyperpolarizability finds its origin into the imperfection of the spherical shape and the nanoparticle surface. Nevertheless, this origin of the nonlinearity leads to several contributions to the nonlinear response, the relative weight of which is controlled by the shape and the size of the nanoparticles.