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"This study demonstrates the interaction between free electrons and nonlinear optics, generates optical solitons in an electron microscope, and enables ultrafast gating of electron beams, expanding the application of microcavity optical frequency combs to the new field of free electron regulation."

Regarding his first-author paper in Science, Yang Yujia, an undergraduate alumnus of Zhejiang University, a doctoral graduate from the Massachusetts Institute of Technology, and a postdoctoral fellow at the Swiss Federal Institute of Technology in Lausanne, said.

Photo | Yang Yujia (Source: Yang Yujia)

In the study, they placed the on-chip integrated high-quality factor silicon nitride optical microcavity into a transmission electron microscope.

By utilizing the third-order nonlinear response of the optical microcavity, a series of nonlinear optical states are generated, including dissipative Kerr solitons, Turing patterns, chaotic modulation instabilities, etc.

For these optical states, they correspond to the spatiotemporal modulation of different modes of the light field in the microcavity, and can form coherent or incoherent microcavity optical frequency combs in frequency.

By studying the interaction between free electrons and these nonlinear optical states, Yang Yujia and others detected the characteristic "fingerprints" left by these optical states in the free electron energy spectrum.

In particular, dissipative Kerr solitons can form optical solitons with pulse times below 100fs and repetition frequencies above 100GHz in a microcavity.

At the same time, in this work, he and his team also studied the ultrafast regulation of free electron beams by this optical soliton.

(Source: Science)

It is expected that this achievement will achieve three applications:

First, for nonlinear optical dynamics, especially nonlinear integrated optics, free electron-based detection and characterization techniques can be developed.

This not only effectively supplements traditional photonic measurement methods, but also demonstrates unique advantages such as ultra-high spatial resolution, direct interaction with on-chip or microcavity light fields, and non-invasive measurement.

Second, develop ultrafast electron microscopy technology based on the technology of conventional electron microscopy.

In this work, Yang Yujia and his research group achieved ultrafast light-electron interaction by using femtosecond soliton pulses in an integrated optical microcavity.

Based on this, it is expected that ultrafast electron microscopy technology can be developed based on conventional electron microscopy.

It is expected that this technology will be able to use continuous electron beams, continuous lasers, and integrated optical chips, without the need for more expensive femtosecond mode-locked lasers.

Furthermore, ultrafast electron microscopy technology can be used for ultra-high spatiotemporal resolution imaging of material structure, ultrafast dynamics, and light-matter interactions.

Third, it is used for on-chip dielectric laser electron accelerators.

Integrated optical microcavities have a high free spectral range that can reach GHz-THz.

By utilizing precisely designed microcavity structures and regulating free electrons with the help of optical solitons within the cavity, a small-sized, high-repetition-rate micro-electron accelerator can be realized.

Therefore, it is expected to be used in medical instruments, industrial equipment, scientific devices, etc. that do not require ultra-high electron energy but require a compact structure.

(Source: Science)

The electron microscope that gave rise to two Nobel Prizes

It is reported that free electrons have extensive and profound applications in modern science and technology.

These applications include electron microscopes, particle accelerators, free-electron lasers, microwave generation and amplification, and vacuum tubes.

Especially for electron microscopes, due to the ultra-short de Broglie wavelength of free electrons and their strong interaction with matter, electron microscopes can achieve ultra-high spatial resolution imaging, diffraction and energy spectrum technology at the atomic level.

Currently, electron microscopes have been widely used in fields such as materials science and structural biology.

Relevant scholars have also won the 1986 Nobel Prize in Physics for their results in transmission electron microscopy, and the 2017 Nobel Prize in Chemistry for their results in cryo-electron microscopy.

In recent years, the interaction between free electrons and photons has been achieved by introducing nano-optical structures in electron microscopes.

Based on this, a series of new achievements have been achieved, including ultrafast electron microscopy, quantum coherent free electron control, attosecond electron pulses, on-chip electron accelerators, and new free electron light sources.

However, the nonlinear optical properties of optical materials and structures in free electron-photon interactions have rarely been explored.

So how did Yang Yujia get into this research field? This story starts from his school days.

He received his bachelor's degree from Zhejiang University and his master's and doctoral degrees from the Massachusetts Institute of Technology. During his doctoral studies, he mainly researched nano-optics, ultrafast optics, free electron physics, and quantum physics.

While studying the interaction between free electrons and nano-optical structures, he realized that compared with nano-optical antennas with lower quality factors, integrated optical microcavities with high quality factors are expected to greatly enhance the interaction between free electrons and photons.

Therefore, when considering the research topic for his postdoctoral fellowship, Yang Yujia contacted Professor Tobias J. Kippenberg of the Swiss Federal Institute of Technology in Lausanne, a well-known scholar in the field of integrated optical microcavities.

After that, Yang Yujia also received project funding from the EU "Marie Curie Scholar".

(Source: Science)

Traveling by train between Germany and Switzerland with a suitcase full of instruments

At that time, Professor Kippenberg was working on a collaborative project with Professor Claus Ropers of the Max Planck Institute in Germany.

So Professor Kippenberg invited Yang Yujia to join his research group for postdoctoral research.

In 2021, Yang Yujia's Kippenberg team and the Ropers team jointly developed a new experimental platform.

Through this, they combined a transmission electron microscope with an integrated optical chip and used a high-quality optical microcavity to demonstrate the strong phase control of free electron wave functions by low-power light waves [1]. The relevant paper was published in Nature.

In 2022, they used a similar experimental platform and single-electron and single-photon detection to demonstrate the generation of electron-photon pairs by free electrons in an integrated optical microcavity [2], and the related paper was published in Science.

However, in the above studies, they only used the linear optical response of the integrated optical chip and the optical microcavity, and did not use the nonlinear optical properties of the optical microcavity.

For Yang Yujia's team, most of their research is carried out around nonlinear integrated optics.

Therefore, in the study of free electron-photon interaction, they also want to explore the regulation of free electron beams by the nonlinear optical response of integrated optical chips, thereby filling the gap in the field.

In this study, Yang Yujia first went to the research group of his German collaborator to carry out experiments.

However, he found that the quality factor of the optical microcavity would be reduced in an electron microscope, resulting in the generation of multiple soliton states rather than single soliton states, that is, there was only one optical soliton pulse in the microcavity.

After returning to Switzerland, Yang Yujia and others prepared a new batch of integrated optical microcavity chips with higher quality factors, and decided to use single-sideband modulation to achieve rapid scanning of the laser frequency in order to more easily obtain single soliton states.

In April 2022, Yang Yujia and his colleague Arslan S. Raja once again came from Switzerland to Professor Ropers' research group in Germany and generated a single soliton state in an electron microscope for the first time.

Everyone was very excited about the success of this experiment. However, in the subsequent data analysis, Professor Kippenberg pointed out that the spontaneous radiation noise was not filtered out when the optical amplifier was used to enhance the laser power in the experiment.

Although this small problem does not affect the correctness and scientific nature of the entire experiment, it will affect the interpretation of the experimental results.

In July 2022, Yang Yujia and others came to Germany again, repeated the previous experimental work, properly filtered out the spontaneous radiation noise, and finally completed all the data collection work.

"In order to complete cross-border collaborative experiments, my colleague Arslan and I carried two large suitcases full of experimental instruments and took a 7-10 hour (often delayed) train to and from Göttingen, Germany and Lausanne, Switzerland," said Yang Yujia.

Subsequently, Yang Yujia completed the data processing and data analysis of this study, and used theoretical simulation methods to reproduce the experimental results and explain the underlying mechanisms.

Finally, the related paper was published in Science[3] with the title “Free-electron interaction with nonlinear optical states in microresonators”.

Yujia Yang, Arslan S. Raja, Jan-Wilke Henke, and F. Jasmin Kappert are co-authors.

Yang Yujia, Professor Tobias J. Kippenberg of the Swiss Federal Institute of Technology in Lausanne, and Professor Claus Ropers of the Max Planck Institute in Germany serve as co-corresponding authors.

Figure | Related papers (Source: Science)

At the same time, Science also published an opinion article co-authored by Professor Albert Polman of the Institute of Atomic and Molecular Physics in the Netherlands and Professor F. Javier Garcia de Abajo of the Institute of Photonic Sciences in Spain [4], praising this as a disruptive innovation that combines free electrons and nonlinear optics.

In the next step, Yang Yujia and others will conduct free electron detection on other nonlinear integrated optical devices and dynamics, such as detecting on-chip lasers, optical amplifiers, dark solitons and supercontinuum spectra.

At the same time, he also hopes that after completing his postdoctoral research, he can return to China to establish a cross-disciplinary research laboratory that can reach world-leading levels to explore electron microscopes and photonic chips.

References:

1. Henke, J.-W. et al. Integrated photonics enables continuous-beam electron phase modulation. Nature 600, 653–658 (2021).

2. Feist, A. et al. Cavity-mediated electron-photon pairs. Science 377, 777–780 (2022).

3. Yang, Y. et al. Free-electron interaction with nonlinear optical states in microresonators. Science 383, 168–173 (2024).

4. Polman, A. & García de Abajo, F. J. Electrons catch light pulses on the fly. Science 383, 148–149 (2024).

Typesetting: Liu Yakun

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