Field: Material/Biomaterial Science
Session Topic:
Extreme Photonics
Speaker:
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 strong
absorption; 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.