Continuous-Wave Optical Phase Modulation of Electron Beams Using Chip-based High-Q Microresonators

Armin Feist^a^,^c, Arslan Sajid Raja^b, Jan-Wilke Henke^a^,^c, Guanhao Huang^a^,^c, Germaine Arend^a^,^c, Yujia Yang^b, F. Jasmin Kappert^a^,^c, Rui Ning Wang^b, Marcel Möller^a^,^c, Jiahe Pan^b, Junqiu Liu^b, Ofer Kfir^a^,^c, Tobias J. Kippenberg^b, Claus Ropers^a^,^c

^aUniversity of Göttingen, IV. Physical Institute, 37077 Göttingen, Germany

^bSwiss Federal Institute of Technology Lausanne (EPFL), Institute of Physics, Lausanne, Switzerland

^cMax Planck Institute for Biophysical Chemistry (MPIbpc), 37077 Göttingen, Germany

Proceedings of Electron Beam Spectroscopy for Nanooptics 2021 (EBSN2021)

Online, Spain, 2021 June 14th - 15th

Organizers: Mathieu Kociak and Nahid Talebi

Oral, Armin Feist, presentation 023
Publication date: 8th June 2021

The optical shaping of free-electron beams enables a broad range of applications, from free-space acceleration [1] and attosecond bunching of electrons [2] to the implementation of laser-driven phase plates [3,4] and beam splitters [5]. Despite recent progress towards phase-matched and high-efficiency coupling [8,9], inelastic electron light scattering (IELS) and electron energy gain spectroscopy (EEGS) [10,11] typically require femtosecond high-intensity laser pulses, precluding broad usage in state-of-the-art continuous-beam electron microscopes.

Here, we show IELS on a CW-pumped Si₃N₄ microresonator with a Q-factor of >10⁵, demonstrating μeV-EEGS spectroscopy and achieving an unprecedented high coupling to a continuous electron beam.

In a custom modified Schottky-field-emission TEM [12], a continuous electron beam interacts with the optical whispering gallery mode confined in a fiber-coupled Si₃N₄ microresonator chip (fabricated in the photonic Damascene process [13], linewidth of ~390 MHz and free spectral range of ~1 THz for the quasi-TM fundamental mode). When the CW laser is tuned to a resonance of the cavity, the initially narrow energy distribution is significantly broadened. At an electron energy of 115 kV, a spectral width of ~160 eV is observed for only 4 mW of optical power coupled to the microresonator. Detuning the laser frequency enables spectral characterization of the resonance, yielding a 3.1-μeV effective linewidth in EEGS. Finally, the interaction strength between the electrons and the evanescent cavity field is mapped by energy-filtered imaging, revealing a rich spatial interaction pattern, resulting from the interplay of phase-matched electron-light interaction and the three-dimensional mode profile.

Combining electron microscopy with integrated photonics opens up new experimental pathways, ranging from versatile light-driven electron phase plates to free-electron cavity quantum optics.

References:

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[5] Feist, A.; Yalunin, S. V.; Schäfer, S.; Ropers, C. High-Purity Free-Electron Momentum States Prepared by Three-Dimensional Optical Phase Modulation. Phys. Rev. Research 2020, 2 (4), 043227

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[12] Feist, A.; Bach, N.; Rubiano da Silva, N.; Danz, T.; Möller, M.; Priebe, K. E.; Domröse, T.; Gatzmann, J. G.; Rost, S.; Schauss, J.; Strauch, S.; Bormann, R.; Sivis, M.; Schäfer, S.; Ropers, C. Ultrafast Transmission Elec-tron Microscopy Using a Laser-Driven Field Emitter: Femtosecond Resolution with a High Coherence Electron Beam. Ultramicroscopy 2017, 176, 63–73.

[13] Liu, J.; Raja, A. S.; Karpov, M.; Ghadiani, B.; Pfeiffer, M. H. P.; Du, B.; Engelsen, N. J.; Guo, H.; Zervas, M.; Kippenberg, T. J. Ultralow-Power Chip-Based Soliton Microcombs for Photonic Integration. Optica 2018, 5 (10), 1347.

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