Four in One: Parallel Determination of Optical Activity and Optical Anisotropy from Single Plasmonic Nanostructures

Chiral plasmonic nanostructures preferentially interact with left-handed or right-handed circularly polarized light, exhibiting unique optical properties that depend on the nanostructure's size, shape, and composition. Ensemble characterization techniques do not capture the heterogeneity inherently present in colloidally prepared chiral nanoparticle samples, making single-particle optical techniques necessary to relate specific structural features to the corresponding chiroptical response. Scattering-based approaches to measure single-particle chiroptical behaviors, such as circular differential scattering, mix circular dichroism with circular birefringence and linear dichroism. Here, we demonstrate the ability to individually measure and differentiate linear/circular dichroism and linear/circular birefringence observed in individual gold nanostructures using polarization-resolved calcite-assisted localization and kinetics microscopy. The circular and linear contributions of the chiroptical response can be isolated by monitoring how individual nanostructures alter the ellipticity and polarization azimuth of scattered light as the input azimuth angle of linearly polarized excitation is swept from 0° to 180°. Even for quasi-isotropic chiral plasmonic nanostructures, optical anisotropy in the form of linear dichroism and linear birefringence varies as a function of wavelength and must be properly accounted for when reporting chiral parameters. We demonstrate that substrates with large refractive indices and interfaces with a large refractive index difference significantly elevate the measured ellipticity values and must be carefully considered in polarization-based single-particle chiroptical measurements. Overall, the four wavelength-dependent parameters obtained, akin to linear/circular dichroism and linear/circular birefringence, provide a high-throughput, information-rich way to screen the chiroptical properties of individual nanostructures.