Polarimetric dark-field spectroscopy of gold bipyramids: Measuring single particle 3D orientation

We determine the 3D orientation of single gold nano-bipyramids (AuBPs) deposited on a glass substrate. These nanoparticles possess a well-defined localized surface plasmon resonance along their main axis which can be modeled as a single dipole. Orientation is deduced by polarization resolved dark-field scattering. For that purpose, a specific experiment has been built to select, analyze in polarization and resolve the scattering angles at the single nanoparticle level. Usually, the out-of-plane angle is deduced by measuring the degree of polarization (DOP) and doing numerical simulations taking into account the nature of the dipole (1D, 2D) and all experimental parameters. We compare this method with a complete measurement of the DOP as function of the collected numerical aperture (controlled by Fourier filtering). The results provide the accuracy of the polarimetric measurements. The complimentary method proposed in this article can easily be extended to more complex system.