Quantum-Coherent Light-Electron Interaction in a Scanning Electron Microscope

The last two decades experimentally affirmed the quantum nature of free electron wave packets by the rapid development of transmission electron microscopes into ultrafast, quantum-coherent systems. So far, all experiments were restricted to the bounds of transmission electron microscopes enabling one or two photon-electron interaction sites. We show the quantum coherent coupling between electrons and light in a scanning electron microscope, at unprecedentedly low, subrelativistic energies down to 10.4 keV. These microscopes not only afford the yet-unexplored energies from $\sim 0.5$ to 30 keV providing the optimum electron-light coupling efficiency, but also offer spacious and easily configurable experimental chambers for extended, cascaded optical set ups, potentially boasting thousands of photon-electron interaction sites. Our results make possible experiments in electron wave packet shaping, quantum computing, and spectral imaging with low-energy electrons.

Figure 1

Quantum coherent electron-light coupling in an ultrafast SEM. Electrons photoemitted by ultraviolet laser pulses (purple) propagate through the column of a commercial SEM. The electron beam (green) is focused close to a tungsten needle tip (inset), where it interacts with the optical near-field excited by 1030-nm laser pulses, coupled into the SEM through a CF-100 window in the SEM sample chamber. The aspherical focusing lens (not shown) is 25 mm away from the tip, inside of the chamber. Electron spectra are recorded with a home-built compact double-stage magnetic sector electron spectrometer based on the Omega filter, placed inside the SEM. The dispersion plane of the spectrometer is imaged onto a microchannel plate detector, whose phosphor screen is optically recorded from outside of the vacuum chamber with a CMOS camera. An example image (bottom right inset), where individual electron counts (black dots) and photon orders (vertical dotted lines) can be easily seen by the eye. The PINEM spectrum is obtained by integrating the camera image vertically [38]. The incoherently averaged experimental spectrum (black), with the raw, binned data (blue), show 24 PINEM orders, 12 on each side, the maximum we observed. A magnified version is available in Ref. [38].

Figure 2

PINEM spectra as function of position along the needle tip. (b)–(g) Six electron energy spectra at a central beam energy of 17.4 keV, recorded and matched to positions indicated by the colored circles in (a). The color scale in (a) shows a simulation of the near-field coupling parameter $g$. Spectrum (b) does not show photon orders, as expected for a beam not passing the optical near field of the tip. Spectrum (c) shows 3 photon orders on either side of the zero-loss peak, spectrum (d) 4, and up to 7 photon orders in spectrum (f). Experimental data are displayed by the blue curve and gradient; numerical results are in red. Clearly, the numerical results match the experimental data very well. The zero-loss peak height depends on the laser and electron pulse lengths, where the electron pulse was assumed infinite in time and ignored in the simulation; instead, the first positive simulated energy peak’s amplitude was normalized to the experimental one. The circle’s positions in (a) are found such that the incoherent average of the simulated PINEM spectra match the experimental ones best. For details, see text and Ref. [38]. In each panel (b)–(g), the simulated value of the coupling parameter $g$ at the center of the electron beam spot is shown for maximum laser pulse strength, and an inset showing the cross section of the real part of the simulated normalized electric field in the $y$ direction is shown; with increase of diameter comes a mix of dipole, quadrupole, and higher multipoles of the field, whereas initially the dipole is almost exclusively excited. The aspect ratio of each inset is $1:1$. The inset in (a) was recorded with 10.4 keV electrons. Also at this low energy we observe 4 photon orders on each side.

Figure 3

Comparison of the full and approximated dependence of the critical angle required for conservation of the photon-electron interaction. The difference in the angle is quite small (as plotted on a semi-logarithmic scale), even for low (100 eV) electron energies, and requires state-of-the-art slow plasmons to observe a difference.