Optimization of second harmonic generation of gold nanospheres and nanorods in aqueous solution: the dominant role of surface area

Size and shape of gold nanoparticles (AuNPs) have a strong influence on their second order nonlinear optical properties. In this work, we propose a systematic investigation of surface and shape effects in the case of small gold nanoparticles. Colloidal solutions on AuNPs with different sizes and shapes have been synthesized, i.e. nanospheres (diameters 3.0; 11.6; 15.8; 17.4; 20.0 and 43 nm) and nanorods (aspect ratios 1.47; 1.63 and 2.30). The first hyperpolarizability beta values of these AuNPs have been measured by harmonic light scattering (HLS) at 1064 nm. For nanospheres and nanorods, we found that their beta values are governed by a purely local, dipolar contribution, as confirmed by their surface area dependence. As an important consequence of these surface effects, we have revisited the previously reported aspect ratio dependence of beta values for gold nanorods, and evidenced the predominant influence of nanoparticle area over aspect ratio considerations.