Recently, the team of Wu Xiaojun from Beihang University together with the team of Li Yutong from the Institute of Physics, Chinese Academy of Science (CAS) made a significant breakthrough in the high-energy strong-field Terahertz (THz) source. The breakthrough was referred to as a new world record made through pulse front tilting in the keynote report given by Prof. Hebling, the inventor of the titled-pulse-front technology, in the 9th International Symposium on Ultrafast Phenomena and Terahertz Waves (ISUPTW 2018) during April 23 to 26.
Fig.1 (Left) Dr. Wu Xiaojun presented the latest findings in the 9th ISUPTW. (Right) Dr. Wu Xiaojun and Prof. Hebling
Terahertz, the Radiation of Life
Terahertz radiation, also known as submillimeter radiation, falls in between microwaves and far-infrared waves in the electromagnetic spectrum with its frequency ranging from 0.1 to 30THz. Being an unexploited and undeveloped frequency band, Terahertz has significant academic value and useful applications. Terahertz radiation is also called the “radiation of life” because not only some particular frequencies in THz range corresponds to collective vibrational modes of biological molecules such as DNA and proteins but also the hydrogen-bonded network of water which makes up 70% of body weight is fluctuated on a picosecond timescale (1 THz = 1012 Hz = 1 ps-1).
Fig. 2 The electromagnetic spectrum
The THz features ubiquity, uniqueness, broad bandwidth and instantaneity. Signals of the THz frequency band exists both in the human body and photons emitted since the Big Bang in abundance and THz waves serve as an effective tool for man to investigate the nature. The uniqueness is reflected in the strong penetrability and the high resolution of the THz radiation. Besides, different materials have unique spectrum in the terahertz range. Thus the THz is likely to play an important role in the fields of security and the biological detection. Moreover, the THz is also widely applied in high-speed data transmission and ultrafast dynamics of thermochemical reactions. Thanks to the ultrashort THz pulse intervals, chances are that chemical reactions can be steered at the atomic level.
Strong-field Teraherz Radiation Sources
Many difficulties and challenges confronted the scientists when they tired to adopt optical and electronic approaches to access the THz frequency band from low frequency and high frequency respectively. The THz domain therefore became an unexplored frontier of the electromagnetic spectrum. The main obstacle in the THz science and technology development is the lack of highly efficient radiation sources, high-sensitivity detectors and THz functional devices, among which the highly efficient THz sources are in the greatest demand.
Dr. Wu’s research focused on high-energy strong field when she worked as a postdoctoral researcher in German. Having participated in the researches on EU’s electron accelerator and other accelerators, she is quite experienced in this field. After exhaustive investigations and field visits, she got down to building a experiment platform in Beihang University, introducing the advanced foreign technologies to China.
The building of the experiment platform was not an easy task, so Dr. Wu and her team had to shuttle back and forth between the Beihang University and the Institute of Physics, which had ready-made laser devices. Under such unfavorable conditions, Dr. Wu and her team made a crucial breakthrough in the research of strong-field THz source in less than a year after she returned to China.
Because of the extremely short pump laser pulse and the damage threshold limit of crystals, it is difficult to maintain the original efficiency in the realization of high-energy strong-field THz source, let alone improve it. Therefore, the team led by Dr. Wu broadened the pulse by chirping pump laser pulses. The method not only extended the effective interaction length for phase matching but also reduced the peak power of pump laser pulses, achieving the highly efficient generation of THz sources. The output energy of THz pulses obtained in the experiment was 0.2 mJ and the electric field strength reached 4 MV/cm, which is 400 times that of the lightning (10 KV/cm) normally seen in daily life.
Fig. 3 The low-frequency high-energy strong-field THz source (the energy is 0.2mJ and the electric field strength is 4MV/cm) generated through the techonologies of chirped broadened pulses and spectrum-cutting as well as its experiment devices
Serve the Country and Cultivate Talents
Prof. Miao Jungang, Director of Beijing Key Laboratory of Microwave Sensing and Security Applications, introduced the laboratory and the significance of Dr. Wu’s work. “I want to make more contribution.”—these words were most frequently mentioned when Prof. Miao and Dr. Wu talked about their personal motivations. They have gone through the hard times in the past and witnessed the prosperity of the country now, and their experiences of studying abroad make them realize that there is still a long way to go before China can reach the international advanced level in some fields. Therefore, they make up their minds to make contribution to the country.
Beijing Key Laboratory of Microwave Sensing and Security Applications is built on the basis of the Electromagnetic Engineering Laboratory of Beihang University, which pioneered the research into stealth aircraft in China and has a complete electromagnetic wave measurement system and a test site. Many breakthroughs have been made in the laboratory in the key technologies for various kinds of military or civil equipment and products in the fields of communication, security and so on. It is sure that the THz source technology developed by the team of Dr. Wu will add luster to the laboratory.
Besides achieving a new scientific breakthrough, Dr.Wu Xiaojun greatly supports undergraduates in participating in scientific research activities. She attended the 9th ISUPTW with four undergraduates, among whom Fang Zhaoji (a junior) from the School of Electronic and Information Engineering gave an oral report in the symposium and the other three students were given the chance to made poster presentations.
Fig. 4 Dr. Wu (the first one on the right) and her team
At present, the all-solid-state millijoule-level THz system has not been invented yet in the world, and the team of Dr. Wu will explore new mechanism and technologies in this field to develop the all-solid-state high-energy strong-field THz source and lay a good foundation for the follow-up applications.
Reported by Tan Lisha and Cao Jiahui
Planned and written by Cao Jiahui
Designed by Yang Zhihan
Translated by Xiong Ting
Special thanks to the School of Electronic and Information Engineering
Reviewed and Released by GEOOS
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