The transmission electron micro­scope (TEM) can image molecular structures at the atomic scale by using electrons instead of light, and has revolutionized materials science and structural biology. The past decade has seen a lot of interest in combining electron micro­scopy with optical excitations, trying, for example, to control and mani­pulate the electron beam by light. But a major challenge has been the rather weak inter­action of propa­gating electrons with photons. In a new study, researchers have success­fully demonstrated extremely efficient electron beam modulation using integrated photonic micro­resonators. The study was led by Tobias J. Kippen­berg at EPFL and by Claus Ropers at the Max Planck Institute for Biophysical Chemistry and the University of Göttingen.

The two laboratories formed an uncon­ventional colla­boration, joining the usually unconnected fields of electron microscopy and inte­grated photonics. Photonic integrated circuits can guide light on a chip with ultra-low low losses, and enhance optical fields using micro­ring reso­nators. In the experiments conducted by Ropers’ group, an electron beam was steered through the optical near field of a photonic circuit, to allow the electrons to interact with the enhanced light. The researchers then probed the interaction by measuring the energy of electrons that had absorbed or emitted tens to hundreds of photon energies.

The photonic chips were engineered by Kippen­berg’s group, built in such a way that the speed of light in the micro­ring resonators exactly matched the speed of the electrons, dras­tically increasing the electron-photon inter­action. The technique enables a strong modu­lation of the electron beam, with only a few milli-Watts from a continuous wave laser – a power level generated by a common laser pointer. The approach constitutes a dramatic simplification and effi­ciency increase in the optical control of electron beams, which can be seamlessly imple­mented in a regular trans­mission electron micro­scope, and could make the scheme much more widely applicable.

“Integrated photonics circuits based on low-loss silicon nitride have made tremendous progress and are intensively driving the progress of many emerging technologies and fundamental science such as Lidar, tele­communi­cation, and quantum computing, and now prove to be a new ingredient for electron beam mani­pulation,” says Kippenberg.

“Interfacing electron micro­scopy with photonics has the potential to uniquely bridge atomic scale imaging with coherent spectro­scopy,” adds Ropers. “For the future, we expect this to yield an unprece­dented under­standing and control of micro­scopic optical excitations.” The researchers plan to further extend their colla­boration in the direction of new forms of quantum optics and atto­second metro­logy for free electrons. (Source: EPFL)

Reference: J.-W. Henke et al.: Integrated photonics enables continuous-beam electron phase modulation, Nature 600, 653 (2021); DOI: 10.1038/s41586-021-04197-5

Link: Center for Quantum Science and Engineering, EPFL, Lausanne, Switzerland • Nano-Optics and Ultrafast Dynamics, Georg-August-University Göttingen, Göttingen, Germany