Editor’s note: On August 17, 2021, Sichuan Daily published on page 12 (“Science and Innovation Future”) an article titled “Cultivating the Last ‘Virgin Land’ of Electromagnetic Waves”, an in-depth report on Academician Shenggang Liu — known as the “father of terahertz in China” — and his work and achievements in terahertz research. The full text follows.
On August 9, by the Shahe River in Chengdu, at the home of 87-year-old Academician Shenggang Liu, the “father of terahertz in China”, our reporter met this towering figure of the field.
As the “Olympics” of the infrared-millimeter-wave-terahertz community, the 46th International Conference on Infrared, Millimeter and Terahertz Waves was scheduled for August 29 to September 3 in Chengdu. Affected by the Delta variant, with sporadic COVID-19 flare-ups in China, the event had to move online.
“The effect may be compromised, but there is no alternative. I believe science will ultimately defeat the virus, and every scientist must work for it,” the 87-year-old said with resignation.
Three years earlier, at 84, he had led a delegation to Japan to bid for the conference, edging out Australia by two votes to win hosting rights for China. After Beijing, Shanghai and Hong Kong, Chengdu became the conference’s fourth stop in China. The event has helped the world better understand China’s terahertz research strength — and the force behind its arrival in Chengdu is none other than Liu and his Sichuan Provincial Key Laboratory of Terahertz Science and Technology at UESTC.
Home of the “flag bearer” of Chinese terahertz research
Mention terahertz and Liu’s eyes light up: his journey with it spans 31 years.
Terahertz is the last “virgin land” of the electromagnetic spectrum. Electromagnetic waves are periodic oscillations of electric and magnetic fields and, like other waves, can carry energy. They are everywhere in daily life — mobile communication uses microwaves, another stretch of the spectrum, and visible light is an electromagnetic wave too.
On the spectrum, the terahertz band’s two “brothers” are the infrared used in remote controls and the microwaves in ovens.
It sees things more clearly than microwaves; compared with visible light it is “X-ray vision”; and compared with X-rays it is safer and does not damage biological tissue. With high transmission rate, large capacity, strong directionality, high safety and good penetration, it holds huge promise in physics, chemistry, biology, electronics, communication, aerospace and defense — called “yet another frontier technological revolution of this century”.
But because terahertz lies in the transition from classical macroscopic theory to microscopic quantum theory, and from electronics to photonics — fully fitting neither optical nor microwave theory — and is hard to generate, terahertz technology remained an almost blank field for a long time.
In the early 1990s, Liu realized that terahertz would underpin a new generation of industry, and that China must make its mark in this “vacuum zone” to seize the initiative in global competition. In 2001, after stepping down as president of UESTC, he personally led a small team to launch the earliest terahertz research in China.
At this point, Liu pulled from his study a small yellow-covered booklet: the report of the 270th Xiangshan Science Conference of 2005, a milestone in China’s terahertz development. That year, domestic media reported that Japan had listed terahertz technology at the top of its “ten key national strategic goals”, mobilizing the whole country, which drew great attention from Chinese academia and ministries. Entrusted by the nation, Liu organized experts and scholars at the Fragrant Hill Hotel in Beijing to discuss China’s direction. The consensus: terahertz is both a “scientific frontier” and a “major national need”.
Advocated by Liu and more than ten other academicians, many Chinese universities and institutes, including UESTC, set up terahertz research centers, pushing domestic research deeper. By statistics, nearly a hundred research teams in China now work in step with the world, creating an upsurge in terahertz research.
Academia and media call Liu the “father of terahertz in China”, but he prefers to be known as the “flag bearer and pioneer” of Chinese terahertz research.
In 2006, UESTC formally established its Terahertz Research Center; at the end of 2010, the Sichuan Provincial Key Laboratory of Terahertz Science and Technology was founded. Today it is the supporting platform for all of China’s terahertz science and technology endeavors.
Birthplace of a revolutionary idea for generating terahertz waves
Across the river from Liu’s home stands the Yifu Building on UESTC’s Shahe campus. It was here that Liu led his team to original, foundational achievements recognized by the international academic community, pushing UESTC’s terahertz research to a leading position worldwide.
On the first floor, the “Terahertz Radiation Source Research Division” is packed with instruments worth tens of millions of yuan. The lab is preparing to move to the Qingshuihe campus to expand.
Instruments hum — experiments are still running. “That is the sound of the superconducting-magnet cryocooler,” said Prof. Min Hu, deputy director of the laboratory, pointing to a gyrotron on the bench — the source of terahertz waves.
You cannot make bricks without straw. High-power, high-efficiency terahertz radiation sources are the foremost challenge in terahertz research and application — and one of this laboratory’s key targets.
The sources are these terahertz gyrotrons, which look like ordinary water pipes. In 2011, China’s first terahertz gyrotron oscillator was born here.
Hu revealed that the laboratory’s devices now reach a maximum frequency of 0.7 THz with kilowatt-level power — the highest output power in the terahertz band.
Megawatt-level high-power output at 0.14 THz is also used in China’s “artificial sun”. Controlled nuclear fusion requires temperatures of hundreds of millions of degrees Celsius — and these gyrotrons serve as its heating devices.
The attainment of high-power, high-efficiency terahertz sources stems from the “revolutionary idea of terahertz generation” born here.
There are many ways to generate terahertz waves, and China has a foundation in using free electrons to produce high-frequency electromagnetic waves. But in the terahertz band, such vacuum-electronic devices face size-effect limitations.
Hu gives an example: the microwave oven at home uses a fist-sized tube emitting at 2.45 GHz. A 2.45 THz device would have to be a thousandth of that size in every dimension — extraordinarily difficult to design and fabricate.
In 2012, Liu published a paper in the world’s top physics journal, Physical Review Letters, unveiling a new discovery — a new type of terahertz radiation source was born.
He discovered a new physical phenomenon and proposed a new theory of terahertz radiation combining electronics and photonics: using free electrons to excite surface plasmon polaritons to generate Cherenkov radiation. The work received a featured commentary in Nature Physics.
In 2014, Liu’s team exploited the surface plasmon waves of the two-dimensional material graphene to propose a new radiation source covering the entire terahertz band. Breaking beyond the framework of conventional vacuum electronics, it produces strong terahertz radiation with hundred-watt-level output power — three orders of magnitude above existing devices — greatly advancing terahertz development worldwide.
How far are we from terahertz applications?
Terahertz has suffered a fate similar to quantum technology and graphene. While the technology is still transitioning from laboratory research to commercialization, “terahertz beds”, bracelets, “energy shoes” and “therapy devices” already flood shopping websites.
Hu explains it to his mother this way: “Anything you can afford is not a genuine terahertz product.” Most terahertz applications are still in development, with instruments costing hundreds of thousands of yuan, and have not yet entered the consumer market.
But in academia, results born from combining terahertz with other fields make headlines. In 2019, astronomers used terahertz detectors to “photograph” the doughnut-shaped black hole.
Beyond fundamental research and device development, the laboratory also works on application scenarios. “To break the terahertz diffraction limit, we have done a great deal of work,” Hu said. Terahertz near-field technology based on atomic force microscopy now reaches 20-nanometer resolution. “With terahertz we can see viruses and bacteria. Using our self-developed high-power source, we achieve better imaging than a five-million-yuan German instrument.”
Driven by terahertz waves, the laboratory has developed a terahertz dynamic nuclear polarization nuclear magnetic resonance (DNP-NMR) system, improving existing NMR resolution by two orders of magnitude, with broad applications in physics, chemistry, materials and biomedicine.
Terahertz waves penetrate strongly and can image through opaque objects. A group in the laboratory is using terahertz waves for qualitative and quantitative analysis of Chinese herbal medicine.
For oral-disease diagnosis, the laboratory is cooperating with West China Hospital of Sichuan University to develop terahertz dental demineralization instruments. Without extraction, non-destructive terahertz testing can detect early caries and the depth of tooth decay.
Terahertz communication draws the most attention. In 2011, under Liu’s impetus, the 863 Program project “Millimeter-wave and Terahertz Wireless Communication Technology Development”, led by UESTC, was launched — the university’s first research project exceeding 100 million yuan.
With that support, the laboratory built China’s first 0.1-THz high-speed wireless communication system based on optoelectronic hybrid technology, reaching 10 Gbps, and China’s first 220 GHz all-electronic terahertz high-speed wireless communication system.
Today, the United States, the EU and Japan are all accelerating terahertz communication for 6G. The ITU has designated 0.12 to 0.22 THz as the band for next-generation terrestrial wireless communication. Many believe terahertz will underpin future 6G communication, ushering in a terahertz communication era.
But terahertz has an inherent shortcoming: severe atmospheric attenuation, meaning terahertz communication cannot reach far. It may be applied to short- or even ultra-short-range communication.
Prof. Shaoqian Li, director of UESTC’s anti-jamming laboratory and pioneer of the university’s terahertz communication research, says it is currently one of the most important candidate technologies for 6G, potentially used for high-capacity terrestrial transmission.
Another possible scenario is communication in space, where terahertz propagation loss is far lower than in urban environments — making terahertz inter-satellite links an actively explored area. In November 2020, the “UESTC” satellite was launched aboard a Long March 6 rocket to test the adaptability of terahertz communication equipment in orbit.
Walking across campus with Hu, the reporter saw a bicycle flash past. “That is my PhD student,” Hu smiled. He has just overcome the key challenges of the first domestically made terahertz-band near-field microscopy system — with terahertz, we can see ever smaller objects.
Report link: https://epaper.scdaily.cn/shtml/scrb/20210817/260124.shtml
Source: UESTC News (via Sichuan Daily) Original article