Deputy Team Leader
RIKEN Advanced Science Institute
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.
What are terahertz 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. “Electromagnetic waves have various frequency-dependent characteristics. When you view an object using X-rays or visible light, you will see a different picture. In the same way, terahertz waves make it possible to observe a different world. Terahertz waves are expected to find applications in areas such as security, agriculture, medical services, food science and the semiconductor industry. We are striving to explore terahertz waves, which have remained untouched for so long even though they have great potential. We want to pave the way for practical applications.”
In October 1990, RIKEN founded the Photo-Dynamics Research Center in Sendai, Miyagi Prefecture. From October 1998, the Research Center’s activities entered the second stage. RIKEN started full-scale studies on terahertz waves through the selection of “A Study on the Technology of Coherent Terahertz Waves” as a major research subject for the second stage. In 1999, Minamide joined the development team, led by Hiromasa Ito, as a research scientist to develop a terahertz light source. “Our aim was to develop a ‘dream’ light source. We aimed to develop a single light source that has a broad frequency range, variable frequency and quick frequency switching. We have strived to develop an ultra-wideband, wavelength-variable terahertz light source with these three features, since such a light source is essential for the development of terahertz wave applications.”
Looking to future applications
“To begin with, I attempted to develop a light source that can generate terahertz waves of 1–3 THz, because the generation of terahertz waves in this frequency range was considered to be the most difficult, and because the real joy of development is to solve difficult challenges,” said Minamide. He was confident that he could develop the new light source, “At Tohoku University, Dr Ito suggested generating terahertz waves efficiently using a special crystal called a non-linear optical crystal.”
Minamide took only one year to successfully generate terahertz waves of 1–3 THz. What was the secret that enabled him to solve the challenge so quickly? He cited the selection of lithium niobate for the non-linear optical crystal as a major factor behind his success. This crystal has been used in many areas of optics, but no other research group had considered it as a source material for generating wavelength-variable terahertz waves.
The non-linear optical crystal has a unique characteristic in that it can generate light waves of different frequencies based on the wavelength of excitation light. Several methods were known to generate new light waves, but he chose to use parametric oscillation for generating terahertz waves of 1–3 THz. Incident light generates two light beams when it passes through the non-linear optical crystal: a beam known as a Stokes beam, and a terahertz beam with energy equal to the difference between the incident light and the Stokes light. The angles of the three light beams are controlled by a specially developed optical device that allows a terahertz wave with arbitrary frequency to be generated.
Minamide attributes the possibility of success to Team Leader Ito. “Dr Ito believed that RIKEN and the university should play different roles. In his view, the university should address the basic research issue of how to generate terahertz waves, whereas RIKEN should focus on studying ways to apply the results. This is why I managed to start developing the terahertz light source in full swing immediately after setting up the research team.”
The light source developed by Minamide and his team generates monochromatic light—light with a single frequency. Most terahertz light sources developed previously by other research groups generate broadband terahertz waves having a dispersed frequency composition with a wide frequency range. Why did Minamide develop a monochromatic light source? “Laser light has become indispensable in our lives and in industry. Laser light is monochromatic and thus has unique features, such as excellent directivity, light collection and brightness. In terms of application and usability, terahertz waves should be monochromatic. We should try not only to solve difficult challenges, but also to consider future applications. Recently, other research groups have been focusing on monochromatic light and developing light sources as well as high-power light sources. But our research is far ahead of the other groups.”
Just one more step toward a dream light source and detector
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.”
Terahertz waves are generated as shown in Figure 2. First, a green laser beam (wavelength, 532 nm) passes through a potassium titanyl phosphate (KTP) crystal, also a non-linear optical crystal. In this way two beams of near-infrared light with different frequencies are generated by parametric oscillation. The two beams are input simultaneously into the DAST crystal as incident excitation light, which in turn generates a terahertz wave with energy equivalent to the energy difference between the two input beams. This occurs by a mechanism known as difference-frequency generation. The frequencies of the near-infrared beams can be changed by turning the KTP crystal so as to change the incident angle of the green laser beam, resulting in a corresponding change in the frequency of the generated terahertz wave. The technologies for green and infrared light are well established, as green light is visible and infrared light is used in optical communications. “We are striving to explore the terahertz world, where technological development lags behind, by utilizing well-developed optical technology,” says Minamide.
The 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 (Fig. 3). “Unquestionably, this is the world’s top terahertz light source,” smiles Minamide. Other research groups are working hard to catch up with his team using commercial DAST crystals. “Here, we have crystal growth experts, who use their own techniques to form many large DAST crystals. We also have an expert who can skillfully polish crystal surfaces. He is a veteran engineer, over 80 years of age, who can recognize the deformation of a crystal just by holding it. We are leading the world in this field because we have these experts.”
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.” Their terahertz detector is based on a simple idea: if terahertz waves can be generated from light at more conventional wavelengths, they can also be converted back to those wavelengths. In the world of optics, there is a range of high-speed, sensitive detectors that operate at room temperature. “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.”
The ‘fingerprint spectrum’ database
The Tera-photonics Team is also preparing for the application of terahertz waves. 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. “In order to identify substances using terahertz waves, we need to prepare many fingerprint spectra beforehand. Research institutes have investigated fingerprint spectra for their own research work, but most information is only known by the researchers within particular research institutes. I decided that we needed to establish a common database for fingerprint spectra that everybody could use freely.”
In 2007, the Tera-photonics Team opened a database for fingerprint spectra, starting with about 200 substances including test reagents and biological molecules that had been investigated during the course of their research. “When the article reporting our new database was published in Nature Photonics in February 2008, we received many inquiries and responses from across the world, proving that there is strong demand for a terahertz wave database.”
In September 2008, the team opened a new database that combines their own database with a database of pigments created by the National Institute of Information and Communications Technology (NICT) in Germany (http://www.thzdb.org). “It is important for a database to include not only a sufficient quantity of data, but also a wide variety of data. The data provided by RIKEN are related to purely chemical substances, whereas the data provided by the NICT are mainly related to paint substances such as pigments. Thus, we managed to establish a database without redundancy that covers a wide range. This is the world’s first integrated database for terahertz waves.” Everybody can use the database and search it using keywords. Initially, about 500 entries were registered, but the number of entries has now increased to about 2,000. The team is working to increase the number of registered entities in cooperation with experts responsible for measuring fingerprint spectra. The integrated database can be accessed from outside for new data to be registered. Thus, the team aims to establish a standard database for use by the world.
The identification of substances based on terahertz fingerprint spectra is on the verge of practical use. One example is non-destructive customs inspection. Terahertz waves can be used to detect narcotics or stimulant drugs without opening bags because the waves can penetrate through paper (Fig. 4). Terahertz waves can also be used to investigate residual chemicals in agricultural products before shipment, to identify foreign matter in food, and for quality control by checking to see whether the uniformity of a surface coating during the process of manufacturing tablets, thus preventing medical agents from melting and leaking unintentionally. “In particular, we are focusing on applications in the quality control of semiconductor substrates,” says Minamide. The semiconductor industry, through the fabrication of light-emitting diodes and solar panels, will play an important role in the move toward a ‘greener’ society. The development of high-performance, next-generation semiconductor devices requires more accurate measurements of the basic properties of semiconductor substrates. “Terahertz waves will allow us to measure the carrier density related to semiconductor substrates and their in-plane uniformity. Thus, we are now jointly developing a practical measurement method with a semiconductor company. We need to switch between terahertz wave frequencies at high speed for this measurement, and our broadband wavelength-variable terahertz light source can do this.”
Minamide is positive about the appeal of terahertz waves. “They have boundless applications because they have the properties of both light and radio waves. Terahertz waves are still an uncharted territory to be explored, and new light sources will surely allow us to observe new phenomena. Thus, we look forward to observing the world anew using terahertz waves generated by our dream light source. I am particularly interesting in seeing the world of water.”
About the Researcher
Hiroaki Minamide was born in Wakayama, Japan, in 1969. He received his bachelor degree from Tohoku University in 1993, and his PhD in electronic engineering from the same university in 1999. He joined the Photo Dynamics Research Center of RIKEN as a frontier researcher in 1999, and a new project of the RIKEN Sendai Tera-photonics Team as a researcher in 2005. In 2007, he was promoted to deputy team leader. His research interests include widely tunable terahertz wave generation, detection and applications using nonlinear optics.