Mapping Optical Chirality with Single Fluorescent Molecules

Single fluorescent molecules, acting as ideal point dipoles, offer a unique means to probe light-matter interactions at the nanoscale. Here, we exploit this property to map the chiral and vectorial structure of tightly focused optical fields using individual, immobilized terrylene diimide molecules. By scanning the excitation focus under linear and circular polarization, we obtain three-dimensional fluorescence excitation maps that directly visualize the handedness and symmetry breaking inherent to circularly polarized light. The measured patterns show excellent quantitative agreement with a full vectorial diffraction model, enabling the accurate determination of both molecular orientations and the local field structure. This approach establishes single molecules as quantitative nanoprobes of optical chirality, offering new strategies for characterizing complex light fields and polarization effects in nanophotonic, plasmonic, and anisotropic materials.