Abstrakt
Electron vortex beams for chirality probing at the nanoscale
In the recent years there have been new methods which enable optical manipulation of free electrons and generation of tailored quantum states. Electron beam modulation becomes a powerful tool for ultrafast nanoscale imaging. In this contribution we propose a method for near-field chirality probing with nanometer resolution utilizing electron vortex beams (EVB).
Upon interaction with localized light modes, the swift electrons may emit and absorb photons leading to modification of their energy, momentum and in certain cases also angular momentum. The interaction between free electrons and nanostructure near-fields serves as the basis for Photon-Induced Near-field Electron Microscopy (PINEM) [1]. If we take this concept further, we can show that when free electrons shaped into EVBs interact with the near-fields of chiral nanostructures the EVBs carry a footprint of the near-field chirality in the electron spectra.
Here we present a case study for the interaction of EVBs with the near-field of a golden nanosphere illuminated by spherically polarized light in both idealized and more realistic experimental alignment, demonstrating the robustness of the proposed method. The near-field is calculated analytically using Mie theory. The chirality of the scattered sphere field stems from spin-orbit coupling of light [2]. However, the interaction of intrinsically chiral structures is also discussed. We propose that high-resolution chirality scanning of near-fields is achievable thanks to the nanometer size of the electron vortex in the interaction site.
EVBs can be generated fully-optically via ponderomotive scattering from an optical travelling wave which carries orbital angular momentum (for instance if one of the optical beams that generates the travelling wave is an optical vortex beam) [3]. The advantages of such interaction are that there is no requirement for special phase-plates or holographic masks and it can be achieved entirely in vacuum.
[1] Feist, A. et al. (2015) Nature. 521. 200-203.
[2] Bliokh, K., et al. (2015) Nature Photonics. 9. 796-808
[3] Kozák, M., (2021) ACS Photonics. 8 (2), 431 435.