The combination of integrated photonics and electron microscopy supports a method for highly efficient electron-beam modulation. The experimental work described by scientists at École polytechnique fédérale de Lausanne (EPFL), the Max Planck Institute for Biophysical Chemistry (MPIBPC), and the University of Göttingen is poised to spur development of advanced quantum metrology — particularly quantum measurement schemes — using electron microscopy. Using integrated photonics principles, the collaborators demonstrated coherent phase modulation of a continuous electron beam — an achievement that has remained out of reach for standard electron microscopes. Ultrafast electron microscopy enables scientists to observe free-electron quantum walks, attosecond electron pulses, and holographic electromagnetic imaging. Integrated photonics provides control of light-matter interactions in many quantum systems, including atoms, trapped ions, and quantum dots. The ability to combine electron microscopy with optical excitation is of increasing interest, but it is impeded by the weak interaction of propagating electrons with photons. A Swiss-German team headed by EPFL professor Tobias J. Kippenberg and professor Claus Ropers at the MPIBC and the University of Göttingen used silicon-nitride microresonators to enhance the optical field of a photonic circuit and demonstrate highly efficient electron-photon interactions in the continuous-wave regime. The researchers established full control over the optical input and output channels of a single, confined, integrated microresonator mode coupled to an electron beam. Scientists achieved efficient electron-beam modulation using integrated photonic circuits. The experiments could lead to new quantum measurement schemes in electron microscopy. A ring resonator (center) was optimized to match the speed of the electrons in the experimental work. Courtesy of Alex Mehler (Woogieworks). The researchers in Ropers’ group steered an electron beam through the optical near field of a photonic circuit to allow the electrons to interact with the enhanced light. They probed the electron-light interaction by measuring the energy of the electrons that had absorbed or emitted tens to hundreds of photons. Kippenberg’s research group built the photonic chips for the experiments so as to make the speed of light in the micro-ring resonators match the speed of the electrons. The photonic structures created by the researchers enabled single-optical-mode electron-light interaction with full control over the input and output light, resulting in a significant increase in electron-light interaction. The high finesse of the optical resonator and a waveguide designed for phase matching led to efficient electron-photon scattering at extremely low, continuous-wave optical powers. The technique enabled a strong modulation of the electron beam with only a few milliwatts (mW) from a continuous-wave laser — a power level that can be generated by a common laser pointer. The initial electron state was depleted at a cavity-coupled power of only 5.35 µW. More than 500 electron energy sidebands were generated for several mW. The approach simplifies the optical control of electron beams and makes optical control more efficient. Additionally, the technique can be seamlessly implemented in a standard transmission electron microscope. “Integrated photonic 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,” Kippenberg said. “Now [integrated photonics] prove to be a new ingredient for electron beam manipulation.” The work also introduces a platform for exploring free-electron quantum optics, and the research teams plan to extend their collaboration to explore quantum optics and attosecond metrology for free electrons. Future research will address strong coupling, local quantum probing, and electron-photon entanglement. The experimental setup used in the research, showing a transmission electron microscope and silicon-nitride microresonator used to demonstrate the electron-photon interaction. Courtesy of Murat Sivis. The approach, the scientists added, provides a versatile, efficient framework for enhanced electron beam control in the context of laser phase plates, beam modulators and continuous-wave attosecond pulse trains, resonantly enhanced spectroscopy, and dielectric laser acceleration. “Interfacing electron microscopy with photonics has the potential to uniquely bridge atomic scale imaging with coherent spectroscopy,” Ropers said. “For the future, we expect this to yield an unprecedented understanding and control of microscopic optical excitations.” The research was published in Nature (www.doi.org/10.1038/s41586-021-04197-5).
