Materials Science

Extreme Photonics
Kodo Kawase, Nagoya University

1. Introduction
The terahertz (THz) wave region, bordered by the far-infrared and the millimeter waves, refers to the radiation within the approximate limits 0.3 THz – 10 THz, corresponding to wavelengths between 30 µm and 1 mm. While both sides of the spectrum have had a long history of research and development, leading to widely available sources, detectors, meters, and many additional devices, the THz range is still in its infancy, representing the last unexplored part of the electromagnetic spectrum. This delayed development was mainly caused by the difficulty of producing reliable and strong THz wave generators, as well as the unavailability of sensors that can conveniently detect this exotic radiation.

In recent years, however, several research groups have been able to fill the gap that once separated the radio waves and the light waves. Although still at a state-of-the-art level, sources, detectors, and specific optics have become less rare and more affordable. Research such as that carried out in our group has led to the development of various applications that used to be, not many years ago, simply unthinkable.

2. Properties of the THz waves
There are two key points around which much of the THz research and applications turn:

1) Spectral specificity: Most chemical substances present characteristic absorption features in the THz range. Such features are almost absent under 0.3 THz, in the millimeter and microwave region.

2) Transmission properties: A wide range of materials are transparent or partially transparent to THz waves. These materials generally become opaque as you go beyond 3 THz into the infrared.

The combination of these two properties is what makes THz radiation so interesting and unique for noninvasive inspection. You can see through packaging and identify the contents. The ability of the THz wave to pass through many packaging materials, such as paper and cardboard, textiles, plastics, wood, ceramics, semiconductors, dried or frozen materials, and so on, will allow the nondestructive and noninvasive inspection of mail packages in post offices, luggage and personal belongings in airports and border crossing points, and others.

Compared to the inspection techniques made possible by X-ray imaging, the THz radiation promises some advantages such as highly reduced risk of irradiation, increased image contrast to differentiate between various soft materials, and the possibility of chemical identification. This comes from the fact that the THz wave is more sensitive to the nature of the materials it passes through, being more selective than X-rays. By analyzing the frequency dependence of the transmission or reflection intensity, each substance presents a particular behavior, which allows what is called “fingerprinting,” that is, assigning a spectral characteristic to each chemical. Spectral fingerprints are essential in the process of identifying the chemicals in an unknown target.

Some materials limit the applicability of the THz waves, by being either highly reflective or highly absorptive. Metals for instance reflect most of the incoming THz radiation; this means that containers made of metal are opaque to THz probing. Another substance that blocks THz waves is water and materials containing a significant amount of water. This time the THz waves are stopped by a strongabsorption; a layer as thin as 100 µm of water transmits about 10% of the incident power. The problem posed by metals and water is not limited to THz waves but is also common for the neighboring ranges of the electromagnetic spectrum, including the microwaves and the infrared.

Infrared radiation is also spectrally sensitive to chemicals and could be used for identification purposes. However, much fewer materials are transparent to infrared than to THz. Besides simple absorption, in many practical cases infrared radiation is strongly scattered by the target, which prevents its use for security applications. On the other side of the THz range, the millimeter waves have the disadvantage that they are generally not chemically sensitive, very few substances having fingerprint spectra in the millimeter range. In addition, for imaging applications, their longer wavelength translates into poor image resolution.
In exchange for the obvious advantages offered by the THz frequency range for nondestructive inspection, some practical drawbacks must be mentioned. The novelty of the field and the lack of a well-developed specialized industry lead to prohibitive prices for most devices used in this field. Much of the equipment used in THz research has large dimensions and weight, and special conditions have to be supplied, such as liquefied gasses, controlled temperature and humidity, and so on, which make it hard to implement THz systems in real-life applications. However, continuous research is being done to decrease the size and weight of the sources and detectors, as well as to make THz systems more accessible and less demanding of special conditions. On the other hand, novelty and inaccessibility represent an obstacle for the individuals and organizations that would attempt to pose a security threat.

Our group has been conducting research activities in several directions within the THz field. We introduced the THz-wave parametric generator as a widely tunable source, and we suggested a whole range of real-life applications. Among our research activities we can mention: i) Noninvasive detection of illicit drugs using spectral fingerprints; ii) Laser-THz emission microscope for semiconductor device inspection; iii) Printable metal mesh sensor for DNA/protein chip. iv) Real-time detection of micro-leak defects in the seal of flexible plastic packages; v) Water content measurement in plants and seeds; vi) Monitoring of the freezing state in food stuffs;

At the workshop, I would like to introduce several of our research results.

Acknowledgments

I would like to thank the following individuals for their valuable support: Adrian Dobroiu, Chiko Otani, Masatsugu Yamashita of RIKEN; Yuichi Ogawa and Shin’ichiro Hayashi of Tohoku University; Masayoshi Tonouchi of Osaka University; Hiroyuki Inoue and Tatsuyuki Kanamori of the National Research Institute of Police Science.

This work was partially supported by the Grant-in-Aid for the Scientific Research (18206009) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). This work was also partially supported by the Asian Office of Aerospace Research and Development through Grant AOARD-06-4014.

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Materials Science

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