  Circular dichroism spectroscopy is an essential technique for understanding
molecular structure and magnetic materials, but spatial resolution is limited
by the wavelength of light, and sensitivity sufficient for single-molecule
spectroscopy is challenging. We demonstrate that electrons can efficiently
measure the interaction between circularly polarized light and chiral materials
with deeply sub-wavelength resolution. By scanning a nanometer-sized focused
electron beam across an optically-excited chiral nanostructure and measuring
the electron energy spectrum at each probe position, we produce a
high-spatial-resolution map of near-field dichroism. This technique offers a
nanoscale view of a fundamental symmetry and could be employed as "photon
staining" to increase biomolecular material contrast in electron microscopy.
