Terahertz remote sensing is our primary research focus. Using a type of electromagnetic wave, we can see many kind of matters on the Earth, or even on Mars and Jupiter. Here we show what terahertz waves are, how we make observations with them, and what the benefits of terahertz-based research are.
Terahertz (THz) waves are the band of electromagnetic waves that lies between radio waves and lights. There is no clear definition of terahertz waves yet. Approximately, in terms of frequency, they range from about 0.3 to 30 THz. In terms of wavelengths, they range from about 1 to 0.01 millimeters (mm).
Traditionally, radio waves have been treated in electronics and lights in optics or photonics. Terahertz waves are between them and have not been used much so far. However, recent technological advances have made it possible to use terahertz waves by taking advantage of their properties, such as high-resolution transparent sensing and high-capacity wireless communication.
Using visible lights or infrared rays, we can observe a wide range of wavelengths and produce a clear image. Using radio waves, on the other hand, high sensitivity, high frequency resolution and long-range observations are possible. By using terahertz waves, we can take both of advantages.
Remote sensing is a technology to "see" what human cannot see. We do it with electromagnetic waves. When we see at something, we use a kind of electromagnetic wave called visible light, from red to purple. We see by collecting the light with “lenses” in our eyes, receiving the signals from sensors in our retinas, and processing them in our brains.
In the same way, we can see with other electromagnetic waves, such as infrared, ultraviolet, X-ray, microwaves or terahertz waves. Using wave collectors and sensors that correspond to each frequency, we can see the world through the eyes of each frequency.
Using different frequencies, the world seems completely different. Even transparency depends on the frequency.
For example, our skin appears opaque with visible light, but it is transparent with X-rays. On the other hand, the UV-cut glass used in car windows appears transparent with visible light, but it is opaque with ultraviolet light.
All materials emit electromagnetic waves of their own frequencies. When we emit light or radio waves to a material, it reflects only the frequency of their own. This property can be used to guess what kind of material the object is made of.
With terahertz waves we can observe absorption peaks caused by intermolecular vibrations so that we can detect low-mass molecules, ions and short-lived radicals (*) in the atmosphere.
*Radicals: Substances that are chemillay highly reactive and unstable and
therefore have short lifetime, such as OH, HO2, ClO, BrO.
Terahertz waves are ideal for remote sensing of planets. With terahertz waves we can simultaneously detect many types of molecules such as water, oxygen and carbon dioxide in a planet's atmosphere and on its surface. Securing water and oxygen, which are essential for the human, is an important issue for human activities in space. Among all electromagnetic waves, THz waves have the highest sensitivity to water molecule.
Terahertz waves allow us to make our equipment smaller. The observation accuracy is proportional to the size of antenna and inversely proportional to the wavelength. Terahertz waves have a shorter wavelength than microwaves and millimeter waves, so we can build smaller antennas with the same accuracy. The space inside a spacecraft or ISS is extremely valuable. The ultra-small size allows us to share a small space and increase opportunities for exploration.
Terahertz remote sensing is also useful on the Earth. Water vapor (H2O) and Oxygen (O2) molecules, various airpollutants such as so-called PM2.5, O3 (ozone) which forms the ozone layer, are all low-mass molecules and are suitable for terahertz wave observations. Ground-based observations are subject to limited observation points and weather conditions. Remote sensing, using micro instruments on a satellite and making observation from space, allows us to obtain incomparably more precise data.
Terahertz waves have a mixture of characteristics of both light and radio waves. For example, terahertz waves have a high spatial resolution (a feature of light) and can penetrate matter (a feature of radio waves). These allows us to make ultra-compact and lightweight sensors (a feature of high frequency waves) for remote sensing. Terahertz waves are also expected to be used in other areas, such as high-speed wireless communications, non-destructive testing, security and medicine
The most familiar and most promising application of terahertz waves is high-speed wireless communication. Communications using electromagnetic waves, such as mobile phones and wireless LAN, have become indispensable in our lives.
The higher the communication speed, the higher the frequency of radio waves required. The highest frequency band used for 4G (current mobile phones) is 3.6GHz; for 5G, 28GHz is also used. If it increased at this rate, 10 years later we will use 300GHz, which is in the terahertz band. Beyond 5G is envisioned to start around 2030. To get us there, there are a lot of ideas proposed around the world right now.
In wireless communications, a radio propagation model (channel modeling) is indispensable. Because of extremely short wavelengths, terahertz waves’ propagation can change slightly depending on the movement of objects around them. It makes channel modeling very complex. NICT have some expertise in this area because of the roots as a research institute for radio waves. We have well-developed channel models down to millimeter waves.
Fast wireless is useless if wired transmissions are not fast and accurate. By combining it with optical fiber at 100 gigabits per second or higher, we are developing a terahertz radio signal transmission/reception platform.
We have been conducting research and development of fundamental technologies for terahertz waves in anticipation of the widespread use of terahertz waves. In order to use electromagnetic waves, it is essential to improve the accuracy of detection and transmission technologies. In the terahertz band, especially above 1 THz, these fundamental technologies are not yet fully developed.
We have developed Terahertz Spectral Detector (Hot Electron Bolometer Mixer, HEBM) from the device through collaboration with other laboratories. Using this, we are conducting communication experiments at high frequencies above 2 THz. We also have a terahertz quantum cascade laser (THzQCL). Using this, we have demonstrated phase locking of THz-QCL.
We provide a free web service to easily calculate the terahertz wave attenuation rate at each frequency under standard conditions. Terahertz wave attenuation in the atmosphere is large and its rate of attenuation depends greatly on the frequency. We can estimate in advance the range of terahertz waves for development of communication devices or other related products.
