Mapping complex optical light field distributions with single fluorescent molecules

Single fluorescent molecules, behaving as ideal electric dipole emitters, are powerful nanoscopic probes of complex optical fields. Here, this property is exploited to precisely map the polarization and vectorial structure of tightly focused laser beams, utilizing both linear and circular polarization states. The resulting three-dimensional fluorescence excitation maps strikingly reveal the intrinsic chiral and non-chiral structure of the light field, in perfect quantitative agreement with a full vectorial wave-optical model. This precise correspondence not only enables the reliable determination of absolute molecular orientations but also allows for the accurate characterization of the field's properties. These results fundamentally advance our understanding of light-matter interaction at the single-molecule level and open new avenues for characterizing complex light fields, with broad applications in super-resolution microscopy and nanophotonics.