Second- and Third-Order Nonlinear Optical Properties of Noble Metal Clusters

Small metal nanoparticles, composed of several tens of metal atoms, show size-dependent properties. The nanoparticles, or clusters, bridge the gap between small molecules and the metallic state. While historically a domain of solid state physics, noble metal cluster chemistry has evolved into a widely studied field in chemistry. This is due to the development of wet-chemical methods to stabilize clusters in solution with ligands. For gold clusters, the ligands are mostly thiolates. Here, we propose to systematically investigate the nonlinear scattering and absorption properties of thiolate-stabilized gold clusters. Nonlinear optical effects arise when the intensity of incident light is very high and the responses are not only linear anymore. One prominent effect is second-order nonlinear scattering. Here, two photons of a given frequency are combined to one photon of doubled frequency by the scatterer (second-harmonic generation or frequency-doubling). Other effects are two-photon absorption or third-harmonic scattering. Frequency-doubling is the most intriguing effect, since it is symmetry-dependent. Thus, the study of frequency doubling (in solution by Hyper-Rayleigh Scattering) allows to gain insight into the electronic structure of the materials. We will investigate the size-dependent nonlinear optical properties of thiolate-protected gold clusters. Potential applications of NLO-active clusters are as contrast agents for biomedical imaging.