Alumni of Zhejiang University achieve ultra-fast light on photonic chips, enabli

This study demonstrates the interaction between free electrons and nonlinear optics, generating optical solitons in an electron microscope, and achieving ultrafast gating of the electron beam, expanding the application of microcavity optical frequency combs to the new field of free electron control.

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

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

Using the third-order nonlinear response of the optical microcavity, a series of nonlinear optical states were produced, including dissipative Kerr solitons, Turing patterns, and chaotic modulation instabilities.

For these optical states, they correspond to different spatial and temporal modulations of the light field inside 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 have detected the characteristic "fingerprints" that these optical states leave in the free electron energy spectrum.

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In particular, the dissipative Kerr soliton, which can form an optical soliton with a pulse duration of less than 100 femtoseconds and a repetition frequency of more than 100 gigahertz in a microcavity.

At the same time, in this work, he and his team also studied the ultrafast control of this optical soliton on the free electron beam.

It is expected that the results of this work will achieve applications in three aspects:

Firstly, for nonlinear optical dynamics, especially nonlinear integrated optics, free-electron-based detection and characterization techniques can be developed.This can not only effectively supplement traditional photonic measurement methods, but also demonstrate unique advantages such as ultra-high spatial resolution, direct action with on-chip or microcavity optical fields, and non-invasive measurement.

Secondly, based on the technical foundation of conventional electron microscopy, develop ultrafast electron microscopy technology.

In this work, Yang Yujia and his research group achieved ultrafast photon-electron interactions by using femtosecond optical soliton pulses in integrated optical microcavities.

Based on this, it is expected to develop ultrafast electron microscopy technology on the basis of conventional electron microscopy.

It is anticipated that this technology will be able to use a continuous electron beam, a continuous laser, and integrated optical chips, without the need for more expensive femtosecond mode-locked lasers.Furthermore, ultrafast electron microscopy technology can be applied to high spatial and temporal resolution imaging of material structure, ultrafast dynamics, and light-matter interactions.

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

Integrated optical microcavities have a high quality factor and can achieve a free spectral range of GHz-THz.

By utilizing precisely designed microcavity structures and leveraging the control of free electrons by optical solitons within the cavity, it is possible to achieve miniature electron accelerators with small size and high repetition frequency.

This is expected to be used in medical instruments, industrial equipment, and scientific devices that do not require ultra-high electron energy but need a compact structure.The Electron Microscope That Has Spawned Two Nobel Prizes

It is reported that free electrons, which have a wide and profound application in modern science and technology, include electron microscopes, particle accelerators, free-electron lasers, microwave generation and amplification, as well as vacuum electron tubes, among others.

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 atomic-level ultra-high spatial resolution imaging, diffraction, and spectroscopy techniques.

At present, electron microscopes have been widely used in the fields of materials science and structural biology.Scholars have successively won the Nobel Prize in Physics in 1986 for their achievements in transmission electron microscopy, and the Nobel Prize in Chemistry in 2017 for their achievements in cryo-electron microscopy.

In recent years, by introducing nano-optical structures into electron microscopes, people have achieved the interaction between free electrons and photons.

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

However, the nonlinear optical properties of optical materials and structures in the interaction between free electrons and photons have rarely been explored.

So, how did Yang Yujia step into this research field? It has to be traced back to his school days.He graduated from Zhejiang University with a bachelor's degree, and earned his master's and doctoral degrees from the Massachusetts Institute of Technology in the United States. During his doctoral studies, he mainly focused on research in 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 to nano-optical antennas with lower quality factors, integrated optical microcavities with high quality factors are expected to significantly enhance the interaction between free electrons and photons.

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

After that, Yang Yujia also received funding from the European Union's "Marie Curie Fellows" program.Carrying a suitcase full of instruments, he traveled back and forth between Germany and Switzerland by train.

At that time, Professor Kippenberg was just collaborating with Professor Klaus Ropers from the Max Planck Institute in Germany on a research project.

So Professor Kippenberg invited Yang Yujia to join his research group as a postdoctoral fellow.

In 2021, the Kippenberg research group, in conjunction with the Ropers research group, jointly developed a new experimental platform.

Through this, they combined a transmission electron microscope with an integrated optical chip, and used high-quality optical microcavities to demonstrate strong phase control of low-power optical waves on the wave function of free electrons [1]. The related paper was published in Nature.In 2022, they used a similar experimental platform, as well as single-electron and single-photon detection, to demonstrate the electron-photon pairs generated by free electrons in integrated optical microcavities [2], with the related paper published in Science.

However, in the aforementioned research, they only used the linear optical response of the integrated optical chip and optical microcavity, and did not use the nonlinear optical characteristics of the optical microcavity.

For Yang Yujia's team, the vast majority of their research is centered around nonlinear integrated optics.

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

In this study, Yang Yujia first went to the laboratory of German collaborators to conduct experiments.However, he found that the quality factor of the optical microcavities would decrease under an electron microscope, resulting in the generation of multi-soliton states instead of single-soliton states, meaning that there is only one optical soliton pulse in the microcavity.

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

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

The success of this experiment excited everyone. However, in the subsequent data analysis, Professor Kippenberg pointed out that when using an optical amplifier to enhance the laser power in the experiment, the spontaneous emission noise was not filtered out.

Although this minor issue 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 his colleagues once again visited Germany to repeat the previous experimental work, properly filtered out the spontaneous emission noise, and ultimately completed all the data collection work.

"In order to complete the collaborative experiment across countries, my colleague Arslan and I have carried two large suitcases full of experimental equipment on several occasions, taking 7-10 hours (often delayed) train trips between Göttingen, Germany, and Lausanne, Switzerland," said Yang Yujia.

Subsequently, Yang Yujia successively 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."

Yang Yujia, Arslan S. Raja, Jan-Wilke Henke, and F. Jasmin Kappert are co-first authors.Yang Yujia, along with Professor Tobias J. Kippenberg from the Swiss Federal Institute of Technology in Lausanne and Professor Claus Ropers from the Max Planck Institute in Germany, served as co-corresponding authors.

In the same issue, Science also published a perspective article co-authored by Professor Albert Polman from the Dutch Institute for Atomic and Molecular Physics and Professor F. Javier Garcia de Abajo from the Spanish Institute of Photonic Sciences [4], praising it as a disruptive innovation that combines free electrons with nonlinear optics.

Next, Yang Yujia and his team will conduct free-electron detection on other nonlinear integrated optical devices and dynamics, such as on-chip lasers, optical amplifiers, dark solitons, and supercontinuum spectra.

At the same time, he also hopes to establish a world-leading interdisciplinary research laboratory exploring electron microscopy and photonic chip technology after completing his postdoctoral research and returning to his home country.