Bright-Field Polarimetry of a Single Plasmonic Nanostructure Combining Polarization and Position Modulation Techniques

Being able to measure the chiroptical properties or more generally the specific response of individual nanoparticles with respect to the polarization of light is a major challenge in the domain of nanophotonics, whether for a fundamental purpose or for understanding and shaping the properties of metamaterials built from these entities. The only few published experiments in this field are essentially dedicated to identifying the signature of circular dichroism. On the basis of conventional methods employed in the polarimetric study of macroscopic samples, we propose an alternative experimental technique which allows a complete determination of the relevant optical anisotropy parameters of a single nano-object (circular and linear dichroisms and birefringences). The retrieval of weak signals from bright-field extinction measurements is made possible by the original combination of polarization and spatial modulation spectroscopies. The framework and applicability of the method and data processing are discussed in detail. They are illustrated through the study of two different types of nanoparticles: achiral homodimers of gold nanospheres and chiral gold nanostructures lithographied on a transparent substrate.