One of the few ways to differentiate between molecular enantiomers are chiroptical spectroscopies like circular dichroism and circularly polarized luminescence. However, this type of spectroscopy is generally limited to the regime of high analyte concentrations due to the mismatch between the size of a molecule and the wavelength of a propagating electromagnetic wave. Previous studies have used plasmonic nanoparticles to compress the excitation field and dramatically increase ensemble-averaged chiroptical signals. This signal enhancement is attributed to an increase in the near-field optical chirality, however, the diffraction limit of light microscopy makes it difficult to measure how the signal enhancement depends on a molecule’s nanometer-scale position relative to a nanoparticle. We achieved super-resolution microscopy and spatially resolved on the nanometer scale the circular polarization of molecules near two-dimensional chiral nanoparticles using Points Accumulation for Imaging in Nanoscale Topography (PAINT) with a home-built imaging polarimeter. First, we demonstrated with full-field electromagnetic simulations that the optical chirality of the near-field near a plasmonic nanoparticle exceeds that of a propagating circularly polarized wave. Next, we experimentally found that the ensemble averaged dissymmetry factor for Cy5.5 dye molecules near a chiral nanorod dimer nanostructure has an absolute value of 0.25 when the excitation polarization matches the dominant circular polarization in the far-field differential scattering which is two orders of magnitude higher than typical ensemble averaged dissymmetry values. Additionally, we observe a change in the spatial distribution of the probability of molecule localizations in the vicinity of the nanostructure, and this pattern follows the calculated spatial distribution of optical chirality density. Our results indicate that the chiral near-field about a plasmonic substrate can be mapped out with super-resolution fluorescence microscopy and point to the potential of performing chiroptical spectroscopy at the single-molecule level.

Near-field optical chirality predicts the localization probability of molecules with high fluorescence dissymmetry near chiral gold nanorod dimers

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