Electron energy-gain spectroscopy of optical excitations in integrated photonic structures

BIO Web of Conferences 129, 09009 (2024) EMC 2024 https://doi.org/10.1051/bioconf/202412909009 Electron energy-gain spectroscopy of optical excitations in integrated photonic structures Jan-Wilke Henke1,2, Dr. Yujia Yang3,4, F. Jasmin Kappert1,2, Dr. Arslan S. Raja3,4, Germaine Arend1,2, Guanhao Huang3,4, Dr. Armin Feist1,2, Zheru Qiu3,4, Rui Ning Wang3,4, Aleksandr Tusnin3,4, Dr. Alexey Tikan3,4, Prof. Dr. Tobias J. Kippenberg3,4, Prof. Dr. Claus Ropers1,2 1Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany, 2University of Göttingen, 4th Physical Institute, Göttingen, Germany, 3Institute of Physics, Swiss Federal Institute of Technolog Lausanne (EPFL), Lausanne, Switzerland, 4Center for Quantum Science and Engineering, Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne, Switzerland Background Optical shaping of electron beams, e.g. in the form of longitudinal attosecond bunching that promises increased temporal resolution [1], significantly extends the range of experiments possible in transmission electron microscopes (TEM). Based on inelastic scattering with an optical field, the momentum and energy of the electron are modified by absorption or emission of photons [2]. This inelastic interaction can, in turn, be employed to investigate the nano-optical response of samples with high spatial resolution in photon-induced near-field microscopy (PINEM) [3]. However, due to the weak coupling of free electrons and photons, inelastic field probing and beam shaping techniques so far required intense optical pulses and short electron pulses available in ultrafast TEMs. Methods & Results Here, we present the efficient modulation of a continuous electron beam by integrated photonics microresonators made from silicon nitride (Si3N4) that are optically pumped with a continuous-wave (CW) laser [4]. The fibercoupled, chip-based resonator is placed inside a TEM, as illustrated in Figure 1a, such that the continuous electron beam can pass over the chip parallel to its surface before being analysed with an imaging spectrometer. Swift electrons interacting with the resonator's guided optical mode can absorb or emit photons from the laser field coupled to the resonator. This leads to the formation of electron energy sidebands spaced by the photon energy (∼0.8eV, corresponding to ∼1550nm) in the spectrum as shown in Figure 1b. The inelastic electron-light scattering is facilitated by the velocity matching of the electrons to the optical phase velocity as well as the high-Q resonant field enhancement. We characterise the latter by employing electron energy-gain spectroscopy (EEGS). To this end, the frequency of the CW pump laser is scanned across the cavity resonance at a low input power while electron spectra are recorded in parallel. We retrieve the laser detuning-dependent electron-light coupling strength (Fig. 1c) that exhibits a linewidth of 390 MHz © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (https://creativecommons.org/licenses/by/4.0/). BIO Web of Conferences 129, 09009 (2024) EMC 2024 https://doi.org/10.1051/bioconf/202412909009 corresponding to a spectral feature of only 3.1µeV width. From this EEGS trace, we infer a cavity quality factor of 7.7*105. Increasing the optical pump power coupled to the microring resonator, the inherent nonlinearity and anomalous dispersion cause the parametric generation of new optical frequencies via four-wave mixing. We observe the formation of various nonlinear optical intracavity states whose spectral and temporal properties strongly depend on the laser detuning from the cavity resonance frequency. When scanning the laser across the resonance at a power > 100mW, we can thus perform an EEGS measurement on the nonlinear optical states by recording electron energy spectra in parallel [5]. The resulting electron spectral trace, shown in Figure 1d, exhibits prominent changes when entering different nonlinear optical states (marked by dashed white lines). For stable and chaotic intensity modulations (regions 1 and 2), resulting from the superposition of different optical wavelengths, averaging over different instantaneous interaction strengths leads to a smoothening of the electron spectra. However, the interaction of electrons with dissipative Kerr solitons (region 3), self-stable short optical pulses with a broad spectrum, yields a broad, low-intensity plateau and a strong central peak since only a fraction of electrons interacts with the high-intensity pulse and scatters to high energy changes. Conclusions In conclusion, we characterise the inelastic interaction between electrons and the optical mode of an integrated photonics microresonator. By performing EEGS on one of the cavity resonances, we achieve an unprecedented energy resolution that might be transferred to both the study of material excitations as well as the probing of quantum optical excitations with free electrons. The observed strong interaction of a continuous electron beam with a low-power CW laser, moreover, enables efficient longitudinal electron beam modulation with optical fields in a conventional TEM setup. Harnessing the toolbox of optical waveform shaping in integrated photonics, we employ the multicolour fields of optical frequency combs and their impact on the electron energy spectra upon interaction to further extend these beam-shaping capabilities. 2 BIO Web of Conferences 129, 09009 (2024) EMC 2024 https://doi.org/10.1051/bioconf/202412909009 Graphic: Keywords: EEGS, UTEM, Inelastic Electron-Light Scattering Reference: [1] K.E. Priebe et al., “Attosecond electron pulse trains and quantum state reconstruction in ultrafast transmission electron microscopy”, Nature Phot. 11, 793-797 (2017) [2] B. Barwick et al., “Photon-induced near-field electron microscopy”, Nature 462, 902-906 (2009) [3] A. Polman, M. Kociak & F.J. Garcia de Abajo, “Electron-beam spectroscopies for nanophotonics”, Nature Materials 18, 1158-1171 (2019) 4 J.-W. Henke et al. “Integrated photonics enables continuous beam electron phase modulation”, Nature 600, 653-658 (2021) [5] Y. Yang et al., “Free-electron interaction with nonlinear optical states in microresonators”, Science 383, 168-173 (2024) 3  (...truncated)