Abstract
Integrated photonics facilitates extensive control over fundamental light–matter interactions in manifold quantum systems including atoms1, trapped ions2,3, quantum dots4 and defect centres5. Ultrafast electron microscopy has recently made free-electron beams the subject of laser-based quantum manipulation and characterization6–11, enabling the observation of free-electron quantum walks12–14, attosecond electron pulses10,15–17 and holographic electromagnetic imaging18. Chip-based photonics19,20 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 (Q0 ≈ 106) 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 spectroscopy21. 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 plates22, beam modulators and continuous-wave attosecond pulse trains23, resonantly enhanced spectroscopy24–26 and dielectric laser acceleration19,20,27. Our work introduces a universal platform for exploring free-electron quantum optics28–31, with potential future developments in strong coupling, local quantum probing and electron–photon entanglement.
Highlights
Our set-up (Fig. 1) allows an electron beam to interact with the copropagating evanescent field of a microresonator waveguide in the object plane of a transmission electron microscope (TEM)
While the length gauge is usually chosen for localized quantum systems in cavity quantum electrodynamics[49], the above velocity gauge is a natural choice for free electrons at finite momentum pin an electromagnetic field with vector potential A
Simultaneous in situ optical and electron measurements show that this cQED-type setting yields a quantitative understanding of the interaction and facilitates electron energy gain spectroscopy at the microelectronvolt level
Summary
Despite the use of phase matching and resonant amplification in dielectric laser accelerators[19], prism geometries[46,47] or free-space coupled whispering-gallery-mode microresonators[48], achieving strong phase modulation of an electron beam has remained out of reach of regular electron microscopes. We overcome this challenge and demonstrate highly efficient electron–photon interactions in the continuous-wave regime using an electron microscope and photonic integrated circuits based on Si3N4. The interaction between the optical mode (a^, a^†: annihilation/creation operator) and an electron
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