Integrated photonics enables continuous-beam electron phase modulation

• Published in: Nature (London) Vol. 600; no. 7890; pp. 653 - 658
• Main Authors: Henke, Jan-Wilke, Raja, Arslan Sajid, Feist, Armin, Huang, Guanhao, Arend, Germaine, Yang, Yujia, Kappert, F. Jasmin, Wang, Rui Ning, Möller, Marcel, Pan, Jiahe, Liu, Junqiu, Kfir, Ofer, Ropers, Claus, Kippenberg, Tobias J.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 23.12.2021; Nature Publishing Group
• Subjects: 140/125; 142/126; 639/624/400/1103; 639/624/400/482; 639/766/483/1255; 639/766/483/3924; Analysis; Attosecond pulses; Continuous radiation; Electron beams; Electron energy; Electron microscopy; Electron states; Energy; Entanglement; Free electrons; Humanities and Social Sciences; Influence; Laser applications; Lasers; Light; Light scattering; Methods; Microscopy; Modulators; multidisciplinary; Phase matching; Phase modulation; Photonics; Propagation; Radiation; Science; Science (multidisciplinary); Sidebands; Silicon; Silicon nitride; Spectrum analysis; Waveguides; Switzerland; Germany
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/s41586-021-04197-5

Integrated photonics facilitates extensive control over fundamental light–matter interactions in manifold quantum systems including atoms 1 , trapped ions 2 , 3 , quantum dots 4 and defect centres 5 . Ultrafast electron microscopy has recently made free-electron beams the subject of laser-based quantum manipulation and characterization 6 – 11 , enabling the observation of free-electron quantum walks 12 – 14 , attosecond electron pulses 10 , 15 – 17 and holographic electromagnetic imaging 18 . Chip-based photonics 19 , 20 promises unique applications in nanoscale quantum control and sensing but remains to be realized in electron microscopy. Here we merge integrated photonics with electron microscopy, demonstrating coherent phase modulation of a continuous electron beam using a silicon nitride microresonator. The high-finesse ( Q 0  ≈ 10 6 ) cavity enhancement and a waveguide designed for phase matching lead to efficient electron–light scattering at extremely low, continuous-wave optical powers. Specifically, we fully deplete the initial electron state at a cavity-coupled power of only 5.35 microwatts and generate >500 electron energy sidebands for several milliwatts. Moreover, we probe unidirectional intracavity fields with microelectronvolt resolution in electron-energy-gain spectroscopy 21 . The fibre-coupled photonic structures feature single-optical-mode electron–light interaction with full control over the input and output light. This approach establishes a versatile and highly efficient framework for enhanced electron beam control in the context of laser phase plates 22 , beam modulators and continuous-wave attosecond pulse trains 23 , resonantly enhanced spectroscopy 24 – 26 and dielectric laser acceleration 19 , 20 , 27 . Our work introduces a universal platform for exploring free-electron quantum optics 28 – 31 , with potential future developments in strong coupling, local quantum probing and electron–photon entanglement. A silicon nitride microresonator is used for coherent phase modulation of a transmission electron microscope beam, with future applications in combining high-resolution microscopy with spectroscopy, holography and metrology.