Résumé
Hyper Rayleigh Scattering (HRS) is used to determine the absolute first hyperpolarizability of gold nanorods with an aspect ratio of 2.2 and 2.7. Two different long axis lengths are used, namely 25.5 nm and 64 nm. This allows for a discussion of the size effect for these centrosymmetric nanoparticles. A comparison of the first hyperpolarizabilities obtained with that of spherical nanoparticles, also centrosymmetric particles, is then made to discuss the role of the shape of these particles on the origin of the response. For the smallest nanorods, a strong hyperpolarizability normalized per atom is determined underlining the role of the interface and the shape in determining such a large absolute value. For the larger nanorods, the first hyperpolarizability per atom is smaller than that of the smaller nanorods but remains larger than the one obtained for gold nanospheres with a similar volume indicating that the shape and the surface response still continue to play a major role. These results are in agreement with the multipolar theory for the first hyperpolarizability of centrosymmetric nanoparticles but show that for nanorods, the surface regime pertains over a longer size range.