Optical polarization analogue in free electrons beams

TL;DR

This paper introduces an optical polarization analogue for free electron beams using singular wave states, enabling EELS to measure local optical spin density and polarization-related phenomena at the nanoscale.

Contribution

It defines a new polarization analogue for electrons and demonstrates how EELS can directly measure polarized electromagnetic local density of states.

Findings

EELS can measure local optical spin density.

Electron waves can access polarization-related quantities.

The method advances nano-optics research.

Abstract

Fast electrons spectromicroscopies enable to measure quantitatively the optical response of excitations with unrivaled spatial resolution. However, due to their inherently scalar nature, electron waves cannot access to polarization-related quantities. In spite of promising attempts based on the conversion of concepts originating from singular optics (such as vortex beams), the definition of an optical polarization analogue for fast electrons has remained a dead letter. Here, we establish such an analogue as the dipole transition vector of the electron between two well-chosen singular wave states. We show that electron energy-loss spectroscopy (EELS) allows a direct measurement of the \textit{polarized} electromagnetic local density of states. In particular, in the case of circular polarization, it measures directly the local optical spin density. This work establishes EELS as a quantitative technique to tackle fundamental issues in nano-optics, such as super-chirality, the local polarization of dark excitations or polarization singularities at the nanoscale.