Imaging chiral optical excitations through inelastic electron-light scattering

Circular dichroism spectroscopy offers rich insight into biomolecular structure and nanoscale magnetic domain structure, but resolution is limited by the optical wavelength. We demonstrate a nanometer-resolution circular dichroism technique that employs electrons to probe optical near fields.

Electrons can exchange integer multiples of the photon energy with an optical field near a material [1,2]. The strength of the coupling between electron momentum states and photon number in the optical field depends on material structure and optical properties. Because sub-nanometer spotsize is routine in electron microscopes, this interaction can be employed to image optical fields with nanometer spatial resolution. By illuminating with left- and right-circularly polarized light and measuring the difference in coupling strength with electrons, we probe chiral optical near fields. This technique may enable the investigation of chiral optical and electronic states in plasmonic nanostructures, molecules and atoms with sub-nanometer spatial resolution.

[1] B. Barwick et al., Nature 462 (2009) 902.
[2] A. Feist et al., Nature 521 (2015) 200.