New Terahertz Detectors and Light Sources in Japan Could Open practical application of terahertz waves

There are high expectations for the application of terahertz-frequency electromagnetic waves in various fields, including the non-destructive detection of narcotics or stimulants in mail, the identification of foreign matter in food, and investigation of residual chemicals in crops. However, terahertz waves have yet to be used widely because of the difficulty in generating and detecting them. For this reason, terahertz waves are considered to be ‘unexplored’ waves. RIKEN’s Tera-photonics Team has been developing a terahertz light source and detector, and an associated database, to open the way for the application of terahertz waves.

Terahertz wave technology has boundless applications because they have the properties of both light and radio waves.

Terahertz waves are a form of electromagnetic wave, like gamma-rays, X-rays, ultraviolet light, visible light, infrared light and radio waves (Fig. 1). On the frequency spectrum, terahertz waves (0.1–100 THz) fall between infrared light and radio waves. However, this part of the electromagnetic spectrum has been largely ignored. “Terahertz waves have hardly been used because of the difficulty in both generating and detecting them,” says Hiroaki Minamide, deputy team leader of the Tera-photonics Team.

They have successfully generated terahertz waves of 1–3 THz and are completing work on 1-40 THz light sources and detectors.

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The development of the light source is in its second stage. The goal is now to develop a terahertz light source that can generate any frequency in the range from 0.1 to 100 THz, the entire terahertz range. At this stage, one of the key points is what non-linear optical crystal to use. Minamide is currently using an organic non-linear optical crystal called 4-(4-dimethylaminostyryl)-1-methylpyridinium tosylate, or ‘DAST’. Compared with inorganic non-linear optical crystals such as lithium niobate, the organic DAST crystal offers higher conversion efficiency from incident excitation light to terahertz waves. It was Ito who selected the crystal. “DAST is a non-linear optical material invented by Professor Hachiro Nakanishi of Tohoku University,” says Minamide. “‘DAST could be used to generate terahertz waves,’ said Dr Ito, who also worked as a professor at Tohoku University. This intuition was surely based on his wealth of experience. Thus, we also developed a technique for growing large, practical crystals from small pieces of crystal.”

The new light source can generate any terahertz wave in a frequency range from 1 to 40 THz, and the frequency can be changed in as little as one millisecond

One future challenge is to generate terahertz waves below 1 THz and above 40 THz. Why do they pursue a wider frequency range? “Terahertz waves offer great potential in various applications, but in fact we do not know which frequency is suitable for each field. We may miss important applications in which terahertz waves might have been the best choice if our light source provides only a limited range of frequencies. We want to develop a dream light source that can cover all frequencies in the terahertz range

Another reason why terahertz waves have not been developed is that they are difficult to detect. “Even if we develop a dream light source, the application of terahertz waves will not proceed without a user-friendly detector. Thus, we are striving to develop a broadband terahertz detector to accompany the dream light source.”

We are now developing a detector that can detect terahertz waves indirectly by detecting the light generated when terahertz waves enter the DAST crystal. We want to complete the set, a dream light source and detector, within several years. A table-top, compact terahertz system.

Terahertz Substance Fingerprint Database

Minamide believes that a ‘fingerprint spectrum’ is essential for the application of terahertz waves. Some substances may transmit incident terahertz waves, whereas others may absorb them. The frequency components that a substance absorbs are unique to the individual substance. Thus, individual substances could be identified by referring to a set of absorption spectra that indicates which substances absorb particular frequencies. Such an absorption spectrum is called a ‘fingerprint spectrum’ because it can be compared to the fingerprints used to identify individuals