Dipole Determination by Polarimetric Spectroscopy Yielding the Orientation of Gold Nanorods.

Plasmonic nanorods and other colloidal nanoparticles are widely used probes for enhanced spectromicroscopies. Precise knowledge of the angular orientation of individual nanostructures, respectively of their resonant modes, is required for purposes such as chiroptical spectroscopy or metasurfaces. However, noninvasive measurements of structures below the diffraction limit prove to be challenging. In this article, dark-field spectroscopy requiring only four spectra in a simple microscope setup with a polarizer and an analyzer is employed in combination with an analytical dipole model to extract the orientation of the longitudinal dipolar mode of exemplary gold nanorods. This mode coincides with the azimuthal orientation of the colloid, and for irregularly shaped particles it can help to determine their dominant axis. The technique is demonstrated on multiple nanorods with varying aspect ratios. The spectroscopically determined orientations are compared to orientations extracted from electron microscopy images, resulting in standard deviations as low as ±2.4°. The method can be generalized to nanostructures with 3D orientations or multiple resonances.