Artifact-Free Dark-Field Scattering Microspectroscopy for Single-Particle Chiral Measurements at the Nanoscale.

Abstract

Characterizing the chirality of plasmonic nanostructures at the single-particle level is essential to reveal structure-optochirality relationships. Single-particle dark-field scattering microspectroscopy, commonly used to measure circular differential scattering, suffers from substantial artifacts arising from linear dichroism, limiting measurement accuracy. Here, we develop a dark-field scattering microspectrometer with hyperspectral imaging to address these challenges. This method employs unpolarized excitation to maintain circular polarization at the sample plane with random phases and detection optics comprising a broadband quarter waveplate and a Wollaston prism. We attribute the artifact of linear dichroism in the circular differential scattering of achiral gold nanorods to the small deviation in the waveplate's retardance. Through dual measurements with the fast axis of the waveplate aligned at ± 45° relative to the optical axis of the Wollaston prism, the artifact of linear dichroism for achiral nanoparticles is eliminated while circular differential scattering of chiral nanostructures is preserved. Finite element simulations reveal that the circular differential scattering of the dimer originates from the near-field coupling of nanorods. Our approach offers a robust solution for accurate chiral measurements without the need to modify existing instrumentation, making it particularly well-suited for the rapidly evolving field of chiral nanooptics and nanoplasmonics.