Enormous hyper-Rayleigh scattering from nanocrystalline gold particle suspensions

The recent emergence of advanced technological applications for colloidal gold suspensions and related particle assemblies and interfaces has created a demand for new chemical and physical techniques with which to characterize them. For macroscopic samples/interfaces, coherent second harmonic generation (SHG) has proven itself a useful characterization tool due, at least in part, to metal-based plasmon enhancement. In an effort to defeat or bypass the size restrictions inherent to SHG, we have utilized a related incoherent methodology, hyper-Rayleigh scattering (HRS), to interrogate aqueous colloidal suspensions of 13 nm diameter gold particles. The nanoscale particles have proven to be remarkably efficient scatterers; when evaluated in terms of the first hyperpolarizability (beta), HRS signals from the gold particles substantially surpass those observable from the best available molecular chromophores. Moreover, the present experiments indicate that beta is highly sensitive to colloid aggregation and imply that HRS is an effective tool for the characterization of symmetry-reducing perturbations of nanoscale interfaces.