Integrated photonics chip enhances electron microscopy

05 Jan 2022

EPFL platform could also assist research into quantum optics and entanglement.

Swiss research center EPFL has used an integrated photonics approach to efficiently modulate an electron beam, potentially offering a new platform for investigations into free-electron quantum optics.

As published in Nature, the approach could also lead to new quantum measurement schemes in electron microscopy by enhancing existing beam control schemes.

The project was a collaboration between EPFL and the Max Planck Institute for Biophysical Chemistry, joining the usually unconnected fields of electron microscopy and integrated photonics.

"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," said Tobias Kippenberg from EPFL. "They now prove to be a new ingredient for electron beam manipulation."

The team built upon existing work using photonic integrated circuits to guide light on a chip with ultra-low losses, an approach in which optical fields can be enhanced using micro-ring resonators.

Modulating electron beams in an analogous manner could help solve a challenge in near-field electron microscopy. Theoretical calculations suggest that under certain conditions optical coherence could be mediated by electrons, potentially integrating electron microscopy with coherent optical spectroscopy.

To date these theories have been limited to behavior in the ultra-fast regime, but the EPFL project achieved highly efficient electron-photon interactions in the continuous-wave regime instead, using an electron microscope and silicon nitride photonic integrated circuits.

Bridging atomic scale imaging with spectroscopy

The project's device employed a waveguide directing laser light into a ring microresonator of cross-section 2 microns × 650 nanometers. This photonic chip was positioned in a customized transmission electron microscopy rig, so that the electron beam was steered through the optical near field of the photonic circuit.

The photonic chips were engineered in such a way that the speed of the light moving in the micro-ring resonators exactly matched the speed of the electrons, drastically increasing the electron-photon interaction. These interactions were then probed by measuring the energy of electrons that had absorbed or emitted multiple photon energies.

In trials the platform provided coherent phase modulation of a continuous electron beam while employing only a few milliwatts of continuous wave laser power. This constitutes a dramatic simplification and efficiency increase in the optical control of electron beams, and the approach could be seamlessly implemented in a regular transmission electron microscope, according to EPFL.

The same technique could also make electron-photon interactions much more widely applicable than the previous theoretical restrictions have suggested, and the researchers plan to extend their research collaboration into new forms of quantum optics and attosecond metrology for free electrons.

"Interfacing electron microscopy with photonics has the potential to uniquely bridge atomic scale imaging with coherent spectroscopy," said Claus Ropers from the Max Planck Institute for Biophysical Chemistry. "For the future, we expect this to yield an unprecedented understanding and control of microscopic optical excitations."