In the past few decades, breakthroughs and innovations in the field of optics have been transforming human life.

About 20 years ago, the combination of optics and micro-nano processing gave birth to the entirely new field of micro-nano photonics, bringing forth new types of on-chip optical devices.

After 20 years of development, a multitude of chip-level optical device applications have enabled significant breakthroughs in various fields such as optical frequency combs and radar.

At present, micro-nano photonic devices are primarily based on simple on-chip structures, such as single waveguides and single resonant microcavities.

Compared to these on-chip structures, coupled microcavities can provide multiple reconfigurable photonic energy levels, generating efficient photon-electron interactions.The process of expanding from a single waveguide and a single microcavity to a coupled microcavity is akin to the process of expanding a physical system from a single atom to multiple atoms, which will lead to the emergence of a multitude of new optical phenomena and functional devices.

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Therefore, coupled microcavities are considered to be one of the most promising photonic devices of the next generation.

However, the complexity of coupled microcavities is significantly higher than that of current conventional on-chip optical devices, which makes it difficult for researchers in the field to control and explore them.

The research of Assistant Professor and Researcher Yaowen Hu from the School of Physics at Peking University is dedicated to the development of an electro-optic coupled microcavity platform based on lithium niobate thin films, focusing on the application of optoelectronic integrated chip technology. Through this platform, he has realized a series of electro-optic control devices based on coupled microcavities, including electro-optic frequency shifters and optical frequency combs. These devices either surpass the world's highest level in performance or demonstrate unprecedented functions.

With his leadership in the research of optoelectronic integrated chips based on the lithium niobate photonic platform, achieving high-speed and efficient electro-optic control on-chip, and providing a new development path for the realization of future all-optical integrated chips, Yaowen Hu has become one of the Chinese inductees of the "35 Innovators Under 35" by MIT Technology Review in 2023.Develop a lithium niobate-based coupled microresonator platform to achieve the world's highest performance electro-optic frequency shifter and optical frequency comb.

The ideal coupled microcavity needs to meet many stringent conditions, including materials that can provide extremely low optical loss, high-efficiency coupling strength, extremely high bandwidth for optical/electrical control, and good scalability, among others.

However, previous on-chip coupled microcavity systems could not achieve these conditions.

In Hu Yaowen's view, the advantages of thin-film lithium niobate, such as strong light-electric interaction, low loss, and the ability to scale up, are exactly what the coupled microcavity needs.Therefore, over the past few years, he has successfully constructed an electro-optic coupling microcavity platform based on lithium niobate thin films. This platform is capable of providing a photonic multi-level system and can impose transitions under the strong coupling scale through the electro-optic effect.

At the same time, he also proposed for the first time the generalized critical coupling theory for controlling the flow of energy in the optical field.

Based on the aforementioned platform, in 2021, he applied the generalized critical coupling theory to a system where a multi-level system is coupled with a continuous spectrum, achieving an electro-optic frequency shifter that surpasses the world's highest level [1].

"In simple terms, this device can efficiently and rapidly change the color of light through frequency modulation," said Hu Yaowen.

In terms of performance, this electro-optic frequency shifter can not only change the optical frequency by 10 to 30 gigahertz but also has a translation efficiency of more than 99% and an on-chip loss of only 0.45 decibels.In addition, by utilizing this platform, Hu Yaowen also discovered a phenomenon of cascaded frequency shift that previously did not exist in photonic devices. That is, by using only a 30-GHz microwave, the frequency of light was changed by 120 GHz.

In this process, the unidirectional flow of light has no reverse conversion in the energy levels of the frequency domain.

"The key significance of this achievement lies in the fact that by using only a low-frequency microwave (several tens of GHz), one can obtain an ultra-high bandwidth microwave (>100 GHz, millimeter wave), which greatly reduces the dependence on expensive equipment required for research and application of ultra-high bandwidth millimeter wave scales, and provides more possibilities for future device development," said Hu Yaowen.

From the perspective of application, this electro-optic frequency shifter can provide the world's best frequency shift in the GHz frequency band, and the cascaded frequency shift can directly obtain a frequency shift of >100 GHz using low-frequency microwaves.

Therefore, this achievement can be applied to all applications that require optical frequency control, which includes not only basic physics such as laser cooling of atoms, but also covers the fields of communication, quantum computing, radar, etc.According to Hu Yaowen, the research took about two years, going through three stages: theoretical construction, device fabrication, performance measurement, and optimization. To achieve better device performance, they spent about more than a year on optimizing the process.

"For a technology to have a significant impact, on the one hand, it depends on conceptual innovation, and on the other hand, it requires the process to be as refined as possible. Although it is a very tedious process, it is a necessary path," said Hu Yaowen.

In this process, he inevitably encountered many insurmountable problems. But as long as he saw a glimmer of possibility, he would resolutely try and actively seek advice from the people around him.

"Once, basically everyone in our research team of more than thirty people knew I had this problem because I had asked each of them. Finally, based on their experience and my repeated attempts, I successfully solved that problem," said Hu Yaowen.

In addition, by leveraging the lithium niobate-based electro-optic coupled microcavity platform, in 2022, he applied the coupled microcavity and the generalized critical coupling theory to the field of electro-optic frequency combs, developing an optical frequency comb with ultra-high performance [2]."Compared to the best electro-optic frequency combs in the world before, this comb has improved the conversion efficiency by 100 times, and the bandwidth has also been increased by 2.2 times. So far, the performance of this achievement is still second to none globally," said Hu Yaowen.

From the perspective of application, due to the significant performance improvement over single microcavity electro-optic frequency combs, this comb can completely replace single microcavity electro-optic frequency combs for mass production as a standard paradigm.

In addition, he also applied the above platform to the field of optical synthetic dimensions, demonstrating a four-dimensional frequency crystal [3] and a synthetic mirror in the frequency space (reflectivity > 0.9999) [4].

In fact, the aforementioned two studies both belong to the research of optoelectronic integration chips based on the lithium niobate thin-film photonic platform.

Speaking of the connection and difference between these studies, Hu Yaowen said that they all involve photon-electron interaction, which is essentially equivalent to controlling photons through electrons, and this is also the biggest advantage of the platform.The differences between them lie in that a frequency shifter changes the color of light from one to another; the generation of an optical frequency comb, on the other hand, converts one color of light into many colors, with these colors being equidistant from each other, he said.

The plan is to elevate the research from the device level to the system level, aiding the field of optoelectronic integration chip to achieve more breakthroughs.

In 2013, Yaowen Hu was admitted to the Department of Physics at Tsinghua University. During his time at university, he received several honors, including the Tsinghua University Undergraduate Special Scholarship and the Tsinghua University Student of the Year.

After graduating with a bachelor's degree in 2018, he went to Harvard University in the United States to pursue a doctoral degree. The aforementioned achievements were all realized during this period.

Starting from April 2023, he engaged in postdoctoral research at Harvard University, continuously advancing the development of new on-chip photonic devices. Ten months later, he joined the School of Physics at Peking University, serving as an assistant professor and researcher.Judging from the above resume, Hu Yaowen is clearly an outstanding student and a talented scientific researcher. However, he admits that he has never been considered the "other people's child" that others talk about from childhood to adulthood.

"Whether in junior high, high school, or university, every time I entered a new environment, I was basically at the 'bottom' of the list. But thanks to step-by-step efforts and persistence, as well as the resilience in the face of difficulties, I was always able to reach the 'Top' position in the current environment before moving on to the next one," he said.

When it comes to innovation, he said: "At first, I thought innovation was about creating something out of nothing. But after so many years of scientific research, I found that the vast majority of innovations in the world actually come from transfer, imitation, and integration. For example, combining existing things from field A and field B and placing them in field C can bring great significance to field C. This is my understanding of innovation."

Thinking along this dimension, Hu Yaowen generally does not attribute innovation to a flash of inspiration, but prefers to accumulate a large amount of knowledge persistently in a certain field.

"When the accumulation reaches a certain level, it naturally leads to some very important innovations," he said.As mentioned above, before joining Peking University, Hu Yaowen mainly completed the development of the coupled microcavity platform and demonstrated the potential of the platform through many devices.

In the next phase, he plans to elevate his research from the device level to the system level.

For example, expanding the coupled microcavity frequency shifter to the system level, combining it with a single-photon source based on periodically poled lithium niobate, to prepare a frequency-multiplexed deterministic single-photon source, or using a photonic circuit composed of a large number of optoelectronic integrated devices for general computing, optical quantum computing, etc.

"The coupled microcavity platform is just a subset of the optoelectronic integrated chip. In the future, I hope to make more breakthroughs in this field and push the development of optoelectronic integrated chips further," said Hu Yaowen.