A new terahertz light source and detector could open the way for the 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.
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.
Boundless applications
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."
Original article
