Single Gold Bipyramid Nanoparticle Orientation Measured by Plasmon-Resonant Scattering Polarimetry

The 3D orientation of a single gold nanoparticle is probed experimentally by light scattering polarimetry. We choose high-quality gold bipyramids (AuBPs) that support around 700 nm a well-defined narrow longitudinal localized surface plasmonic resonance (LSPR) which can be considered as a linear radiating dipole. A specific spectroscopic dark-field technique was used to control the collection angles of the scattered light. The in-plane as well as the out-of-plane angles are determined by analyzing the polarization of the scattered radiation. The data are compared with a previously developed model where the environment and the angular collection both play crucial roles. We show that most of the single AuBPs present an out-of-plane orientation consistent with their geometry. Finally, the fundamental role of the collection angles on the determination of the orientation is investigated for the first time. Several features are then deduced: we validate the choice of the analytical 1D model, an accurate 3D orientation is obtained, and the critical contribution of the evanescent waves is highlighted.