Hyper-Rayleigh scattering from gold nanoparticles: Effect of size and shape

We report hyper-Rayleigh scattering (HRS) properties of gold nanoparticles (GNPs) of five different shapes, quasi-spherical (similar to 10 and similar to 20 nm diameter), rod (aspect ratio similar to 2), and branched shapes, tetrapod, flower and star with 800 nm, 150 fs laser excitation. Using similar to 10 nm spherical GNPs as reference, the first hyperpolarizability (beta) values were calculated for all other shapes. Star and flower shaped GNPs have the highest hyperpolarizability (similar to 130 and similar to 52 times higher, respectively), while rod and tetrapod shaped GNPs only have modest enhancement (similar to 7 times), which is similar to similar to 20 nm size quasi-spherical particles. These enhancements are attributed to reduced symmetry as well as the presence of sharp tips on GNP surface. When the beta values are normalized with respect to the number of atoms per particle, the flower and star shaped GNPs still have the highest hyperpolarizability values. The polar plots of vertically polarized HRS signal as a function of the angle of polarization of the incoming incident light shows two lobes, indicating that excitation is predominantly dipolar in nature although the size of some GNPs are big enough to show a quadrupolar response. It is believed that the presence of sharp tips at the surface of these large sized GNPs is responsible for the observed dipolar response. This study shows that GNPs having sharp tips might be a better candidate when their nonlinear properties are used for sensing applications. (C) 2014 Elsevier B.V. All rights reserved.