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Professor Hongfei Wang: Light the Weakest Signal with the Most Intense Laser

07, 2022

Email: zhangchi@westlake.edu.cn
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In the Nonlinear Spectroscopy and Ultrafast Dynamics Laboratory at the School of Science led by Professor Hongfei Wang, visible and invisible laser beams are directed into a carefully composed labyrinth of mirrors, where the laser interacts with a thin platform to produce a spectrum for precise measurement.

Surfaces and interfaces exist everywhere in nature and our daily life: the soil under our feet, the aerosols in the air, the battery in our digital gadgets, cosmetics, even cell membranes. Wherever objects are in contact with each other, like the sky and the sea, a surface or interface is formed.

This is what Professor Wang devotes himself to, “We are detecting the faintest signal with the strongest light.”

Wang describes his work as “in the most intense laser light, we shall see the weakest light”, a modification from the motto of Columbia University where he was once a student.

Research: pathfinder at the frontline

You will find this on the homepage of Wang’s research group: Our team intends to carry out research on modern physical chemistry and chemical physics related to surface/interface and condensed phase with ultrafast and nonlinear spectroscopy and dynamics.

“Ultrafast laser light” generally refers to laser pulse with the width of 10 -12 second (picoseconds, that is, one trillionth of a second) or shorter, such as 10-15 (femtoseconds, that is, a trillionth of a second). The brightness of picosecond or femtosecond laser light beam is about ten orders of magnitude higher than natural light, earning their name of the “strongest” light. Wang’s team was the first to synchronize an infrared laser beam of tens of femtoseconds with a visible laser beam about 100 picosecond, and controlled them to simultaneously interact with the molecular interface to measure the high-resolution sum-frequency generation vibrational spectroscopy (SFG-VS) of the interface.

In the mid-80s, physicist Yuen-Ron Shen from UC Berkeley was the first to report SFG-VS research method on molecular surfaces using picosecond lasers. Since then, research on surface nonlinear optics has been making great advances, with broad applications in material and life sciences.

Researching on nonlinear spectroscopy on surfaces and interfaces requires the help of cutting-edge ultrafast lasers and single-photon detection technologies. This field is highly interdisciplinary, and imposes a high-bar for engaging in such research Thus, even though the research is critical and with broad applications, this area of research cannot become very popular. So far, there are more or less than 50 independent research groups doing active research in the world. Started his research in the 90s, Wang has made important contributions and is now one of the few most senior scientists in the field.

Professor Wang has been focusing on the fundamental development of the technology and methodology. “Our work focuses on the systematic development of SFG-VS methods, including how to accurately measure and how to perform reliable analysis based on basic physical and chemical principles,” he explained.

“Developing new research tools and methodology is the key for making  original scientific research. It is only with improved research tools and methodology can we discover phenomena that were unknown in the past – in other words, better tools and equipment help us to see and understand what cannot be  seen before, or even to see what was not expected to see,” Wang added.

“I believe in cultivating students and young scholars through true innovative research. Only they would serve as the pathfinders in the future development of basic science and high-tech innovations,” Wang stated.

Life story: research and idealism behind a “random” life

Unlike the carefully orchestrated laser paths in his lab, Wang has been jumping between universities, research institutes, industry and national laboratory over the past three decades. He has a rich, shiny resume with seemingly random experiences.

Graduated from the Department of Chemical Physics at the University of Science and Technology of China (USTC), Wang received his PhD degree in physical chemistry from Columbia University, and did his post-doctorate work in the Department of Chemistry and the Laboratory for Research on the Structure of Matter, University of Pennsylvania, and DuPont’s Marshall Laboratory. He worked as a research professor at the Institute of Chemistry, Chinese Academy of Sciences, and served as the director of the Molecular Reaction Dynamics Laboratory of the Institute of Chemistry and the deputy director of the State Key Laboratory of Molecular Reaction Dynamics. After that, he joined the Environmental Molecular Sciences Laboratory (EMSL) and the Division Physical and Computational Sciences (PCSD) as chief scientist. In 2017, Wang joined Fudan University as a full-time distinguished professor; in 2019, he started building a new lab at the School of Science of Westlake University as a full-time professor.

He is as much as a chemist as a physicist. At the age of 15, Wang was accepted by China’s only chemical physics undergraduate program at the University of Science and Technology, where he studied “physics, chemistry, and quite a bit of math”. “I didn’t even know such a major existed before filled out my application,” he recalled.

“I almost never planned my academic career; the jobs were either offered to me or I came across them by chance. The only thing I planned and worked for was getting into a PhD program in the US, because they had the best Ph.D. programs back then.”

Wang has always loved teaching and doing research. In his PhD thesis written in 1996, he revealed his plan to pursue “an academic career” in China, where he dreamt to join a research-oriented university and spend the rest of his life teaching and researching.

There was almost no true research university in China back in 2000. Wang joined the Institute of Chemistry of the Chinese Academy of Sciences, where he was supported to conduct high-quality research, and his work there earned him recognition in the international academic community. Around 2006, he was offered a professorship in the Department of Chemical Physics of the University of Science and Technology of China, but then received a job offer from the Pacific Northwest National Laboratory (PNNL) of the US Department of Energy. This was proven a difficult decision to make. To further advance the frontiers of his research, Wang once again postponed his dream to teach in a Chinese university.

At PNNL, he managed to increase the spectral resolution of the ultrafast sum-frequency vibration spectrum from a dozen wavenumbers to an unprecedented 0.6 wavenumber, and he discovered a series of new phenomena with his new tool that was using the recently developed ultrafast laser synchronization technology. These works also led him being elected as a Fellow of the American Physical Society in 2012.

In January 2017, Wang accepted the invitation from Professor Xinhe Bao, then standing vice president of Fudan University and joined as a full-time professor in the department of chemistry.

Westlake University: fate and serendipity

In May 2018, Wang met Professor Li Deng in Shanghai and this led him to visit and then to join Westlake University. Deng was to take on the post as the executive dean of the School of Science with the mission to oversee the building of faculty.

“Professor Deng convinced me that the School of Science at Westlake University was highly aligned with what I wished for. It’s not easy to establish a new research university, especially in China, and that’s exactly what China needs. I am happy that I can contribute to this mission, and Westlake can also  support better fundamental research,” Wang said.

Currently there are 4 doctoral students, 2 postdoctoral fellows, 1 assistant researcher, and 1 administrative assistant in Wang’s team. “I usually take in one or two doctoral students each year, and that is quite enough! Quality matters more than quantity,” Wang said with a smile.

“We will continue to work on new methodology and technology to make easier and more accurate measurement with interfacial spectroscopy.. Along the way, our research and discoveries are unique…”