Probing Chirality with Inelastic Electron-Light Scattering

Abstract

Circular dichroism spectroscopy is an essential technique for understanding molecular structure and magnetic materials; however, 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 subwavelength 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.

Spontaneous and stimulated electron–photon interactions in nanoscale plasmonic near fields

“…The derivation of the electron-near-field coupling constant from the PINEM spectra was performed following a similar procedure as described in the supplementary information to ref. 30 . Here, the initial electron energy distribution (i.e., prior to the near-field interaction) was modeled by a pseudo-Voigt profile with a Lorentzian-like contribution of 25% and an FWHM of 0.9 eV.…”

“…As a consequence, the initial electron energy spectrum is expanded with sidebands, evenly spaced by the photon energy hω L . The population of these sidebands varies with the near-field integral along the electron trajectory and the statistics of the incident light 25 , enabling spatially resolved near-field measurements [26][27][28][29][30][31][32] with fs-and astemporal 31,33 and meV-spectral resolutions 34 . The key to the PINEM mechanism is the fact that the evanescent near field provides spatial Fourier components with sufficiently large momenta to bridge the phase mismatch between the electron field and the optical pump field in free space.…”

Optical polarization analogue in free electrons beams

“…Spatially resolved pEELS map polarized EMLDOSs, and the dichroic circular pEELS probabilities are directly related to the density of electromagnetic spins. In particular, this permits a comprehensive description of the local polarization of both bright and dark optical excitations, while the otherwise highly sucessfull cathodoluminescence [53,54] or PINEM [41,55] polarized experiments are restricted to bright ones. Remarkably, through the mapping between Bloch and Poincaré spheres, our work establishes a Jones formalism for electrons.…”

“…The only difference is the rotation speed of the linear polarization, being related to the light wavelength c/ω and the wavelength of the electromagnetic field following the electrons v/ω. Since the electron speed can be changed at will, this makes EELS a quite tunable tool for the investigation of chiral structures, as already suggested for photon induced near-field microscopy (PINEM) [41].…”

Control of atom-photon interactions with shaped quantum electron wavepackets

“…These effects have fueled the development of light sources [83][84][85][86][87][88], particle accelerators [89][90][91][92][93] and detectors [94][95][96][97][98][99], and medical devices. Electron-photon interactions form the basis of innovative diagnostic tools including electron energy-loss spectroscopy (EELS) [100][101][102][103][104][105][106][107][108][109] and its variants [101,109,110], cathodoluminescence (CL) [101,108,109,111] and photon-induced near-field electron microscopy (PINEM) [107][108][109][112][113][114][115][116][117][118][119][120]. These tools have enabled the study of the most fleeting and minute excitations in both light and matter.…”