Nanotraffic Lights: Rayleigh Scattering Microspectroscopy of Optically Trapped Octahedral Gold Nanoparticles

We demonstrate a pronounced color change in the light scattered from thermally fluctuating optically trapped octahedral gold nanoparticles (OGPs) in water using a tightly focused near-infrared laser beam. Monodispersed OGPs with an average edge length of 67 ± 6.5 nm were synthesized using a polyol method. Using dark-field microscopy, we observed successive changes of color (i.e., red → green → yellow) scattered from the trapped OGPs. We analyzed this trapping behavior by means of Rayleigh scattering microspectroscopy and concluded that pairs of OGPs trapped in the potential well interact with each other to form dimers oriented in the direction of the electric field vector of the trapping laser light. We theoretically obtained the absorption and scattering cross sections and the enhancement factors of the electric field of an OGP dimer in three different configurations by means of finite-difference time-domain calculations. The calculations suggest that the dimers become preferentially oriented in a vertex-to-vertex configuration because of the high polarizability. These findings indicate that optical tweezers are a promising technique for creating highly ordered assemblies of plasmonic nanostructures whose coupled states can be monitored by means of microspectroscopy.