6G is on the way, what is the terahertz technology behind it?

With the release of the world's first 6G white paper, it means that 6G research is already on the way, which is somewhat unexpected. 5G has not been used yet, but 6G research has already begun. But this is actually in line with the law of technological development. As early as 2009 when the first version of the 4G LTE standard was completed, major equipment manufacturers began to study 5G, so when the 5G R15 standard was completed, 6G research was also put on the agenda.

Judging from the 6G white paper and the proposals put forward by various countries, one technology is used as a breakthrough point - terahertz. What exactly is terahertz? Let's learn about it together today!

Terahertz is actually a unit of frequency, 1THz=1000GH. People's research on terahertz is mainly between 0.1THz and 10THz.

Terahertz waves, also known as far-infrared waves, have a wavelength between 0.03mm and 3mm, which is shorter than microwaves. Microwaves and infrared rays on both sides of this range have been widely used. It is the last electromagnetic band that has not been fully understood and applied by humans, so this frequency band has a nickname called the "terahertz gap."

Terahertz is a very special existence. From the perspective of the spectrum, terahertz waves are between microwaves and infrared waves in the entire electromagnetic spectrum; from the perspective of optics, terahertz waves are called far-infrared rays; from the perspective of energy, the energy of the terahertz band is between electrons and photons.

It is precisely because of its particularity that it has the characteristics of high frequency, short pulse, strong penetration, small energy, and less damage to matter and the human body. Terahertz was once rated as one of the "top ten technologies that will change the future world", and scientists believe that terahertz has a wide range of application prospects.

For example, in space communications, terahertz waves can be used as a high-speed broadband communication carrier. Terahertz wave communication has extremely high directivity and penetration, so it is suitable for short-distance confidential communication in harsh environments, and also for satellite communications with high bandwidth requirements. The International Telecommunication Union has designated the 0.12 and 0.22 THz frequency bands for the next generation of ground wireless communications and satellite-to-satellite communications, respectively.

In terms of security detection, terahertz waves have strong penetration ability for many non-polar substances, and can perform long-distance detection and high-resolution imaging. It can not only detect metals, but also non-metallic, colloid, powder, ceramic, liquid and other dangerous items carried by the human body can be identified by the system, providing technical support in military reconnaissance, suspicious dangerous goods, toxic and hazardous items detection, etc.

In biomedicine, the terahertz frequency band can directly detect the information of biological molecules, which is unmatched by other electromagnetic bands. Therefore, it can detect waste oil in food, pesticides in vegetables and fruits, melamine in milk powder, etc.

Since terahertz waves are easily absorbed by polar substances such as water molecules or oxygen molecules, the radiation will not penetrate the human skin and is very safe for the human body. At the same time, water and other tissues have different absorption rates for terahertz waves, so it can be widely used in local imaging of the human body and medical diagnosis of diseases, such as the detection of skin cancer and breast cancer.

Not long ago, the Terahertz Technology Research Center of the Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, stated that terahertz waves can directly "see" biological macromolecules such as DNA and proteins, which can help prevent and treat early cancer. For cancer patients, early detection and early diagnosis can help cure cancer.

When a patient is in the early stages of cancer, although the content of cancer cells in the human blood or body fluids is very small, terahertz wave technology can detect it about 6 months earlier than traditional detection technology.

The China Academy of Engineering Physics also achieved the first international research results on the diagnosis and treatment of glioma using terahertz technology. It obtained the refractive index, absorption coefficient, and dielectric constant of glioma in the terahertz band, eliminated the influence of water, and better reflected the changes in the characteristics of glioma and normal brain tissue. It also provided the terahertz frequency range and frequency points suitable for different imaging methods (continuous wave imaging, pulse imaging), which can effectively guide terahertz imaging of glioma and lay the foundation for the application of terahertz spectroscopy and imaging technology in the diagnosis of glioma. In terms of terahertz sources used in biomedical research, terahertz pulse radiation with a peak power of nearly 1MW was obtained, which improved the signal-to-noise ratio of terahertz biomedical diagnostic systems.

It can be said that terahertz technology has very broad prospects in biomedicine.

In communications, terahertz has always been considered a key technology for 6G and the future of wireless communication technology. According to the communication principle of terahertz, the higher the frequency, the larger the bandwidth range allowed to be allocated, and the greater the amount of data that can be transmitted per unit time, which is what we usually call "faster network speed."

If we can really maturely operate terahertz technology in the 6G era, the network speed of 6G will be about 10 times that of 5G in terms of frequency alone. In addition, terahertz also has the advantages of microwave communication and light wave communication, namely high transmission rate, large capacity, strong directionality, high security and strong penetration.

Scientists also want to use terahertz on satellites because in outer space, in a nearly vacuum state, there is no need to consider the influence of moisture. This will be hundreds to thousands of times faster than the current ultra-wideband technology and can achieve a bandwidth of more than 10,000.

This is why all countries want to make breakthroughs in terahertz technology. Once we take the initiative in the 6G era, we will take control of the global economy. This is why the United States has been unhappy that China is the global leader in 5G. The patent data company IPlytics released a 5G patent competition situation report and pointed out that 5G will promote the creation of tens of thousands of new products, new technologies and new services, and will improve productivity and create new industries. The global 5G network will unify mobile communications and connect everything together through the Internet of Things (IoT). 5G technology can connect vehicles, ships, buildings, instruments, machines and other physical objects with electronics, software, sensors and the cloud. Embedded 5G technology will allow machines to exchange information in the physical world and integrate into computer-based systems. In recent years, 3G and 4G patent holders have controlled the way mobile technology is used in the smartphone industry. Therefore, 5G patent holders may also become technology and market leaders by realizing 5G connections in various markets.

It is precisely because the United States is not leading in 5G that it wants to make a comeback in 6G, so the research and development of terahertz technology is very urgent. In 2019, the Federal Communications Commission (FCC) of the United States voted unanimously to open the "terahertz" spectrum for future 6G network services for innovators to conduct 6G technology trials.

In 2005, Japan listed terahertz technology as the first of the "Ten Key Strategic Goals for National Pillars" and mobilized the entire country to carry out research and development.

At present, Japan is already at the forefront of the world in terahertz technology research. Hiroshima University in Japan is the first in the world to realize terahertz communication in the 300GHz frequency band based on low-cost CMOS technology. In addition, Japan is "unique" in the field of terahertz and other electronic communication materials, almost achieving a monopoly, which is its unique advantage in developing 6G.

In summary, terahertz technology can be said to have broad and unlimited prospects. However, due to the immaturity of the current terahertz wave source and terahertz wave detection technology, there is a severe shortage of basic functional devices related to terahertz band transmission control, which are essential components for building the future terahertz application technology framework.

In addition, for terahertz, its very stringent requirements for filters, detectors, etc. also restrict its application prospects.

Although research in these fields has been carried out for more than 20 years, compared with laser technology, many key components required for terahertz technology are still very limited, many technologies are still to be developed, and even some basic theoretical research is in urgent need of development.

The Institute of Solid State Physics (IAF) in Germany, the Physikalisch-Technische Bundesanstalt (PTB) in Germany, the University of Braunschweig, NTT in Japan, Bell Labs in the United States, the University of Toronto in Canada, IEMN in France, Asyrmatos Communications Systems in the United States, etc. have invested huge efforts in the research of terahertz technology.

The multinational TERRANOVA program established by the European Union in 2017, including Germany, Greece, Finland, Portugal, and the United Kingdom, clearly proposed the research and development of ultra-high-speed terahertz innovative wireless communication technology.

At present, terahertz technology is gradually developing towards higher speed, higher atmospheric window frequency, low power consumption, small integration and practicality. Up to now, terahertz communication technology has formed three types of parallel development for different application scenarios: optoelectronic combination based on microwave photonics, all-solid-state mixing electronics, and direct modulation.

The 120 GHz communication system developed by Japan's NTT Corporation a long time ago achieved 10 Gbps wireless communication over a distance of one kilometer and was used in the broadcast of the 2008 Beijing Olympic Games.

China has also spent a lot of energy on the research of terahertz technology. As early as 2016, the University of Electronic Science and Technology of China took the lead in developing the first terahertz communication system with direct modulation in the world, and realized high-definition video transmission at the kilometer level. The system uses an external high-speed modulator to directly modulate the spatial transmission terahertz signal. Compared with the existing terahertz communication method, this modulation method has the advantage of being able to flexibly match medium and high power terahertz radiation sources to achieve long-distance communication, and effectively breaks through the problem of too low transmission power in the current terahertz communication system. At present, the system has achieved high-definition video service data transmission at a working frequency of 0.34 THz gigabits per second.

Therefore, although there are still many problems in the application of terahertz technology, its broad prospects have also made scientists continue to strive to conquer it completely. Let us wait and see!