New
T-ray source
could improve airport security, cancer detection
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ARGONNE, Ill. (Nov. 23, 2007) — Going through airport security can be such a
hassle. Shoes, laptops, toothpastes, watches and belts all get taken off, taken
out, scanned, examined, handled and repacked. But "T-rays", a completely
safe form of electromagnetic
radiation, may reshape not only airport screening procedures but also
medical imaging practices.
Scientists at the U.S. Department of Energy's Argonne National Laboratory,
along with collaborators in Turkey and Japan, have created a compact device
that could lead to portable, battery-operated sources of T-rays, or terahertz
radiation. By doing so, the researchers, led by Ulrich Welp of Argonne's Materials
Science Division, have successfully bridged the "terahertz gap" – scientists'
name for the range of frequencies between microwaves (on the lower side) and
infrared (on the higher side) of the electromagnetic spectrum.
While scientists and engineers have produced microwave
radiation using conventional
electric circuits for more than 50 years, Welp said, terahertz radiation could
not be generated that way because of the physical limitations of the semiconducting
circuit components.
"Right around 1 terahertz, you have a range of frequencies where there
have never been any good solid-state sources," he added. "You can
make those frequencies if you are willing to put together a whole table full
of expensive equipment, but now we've been able to make a simple, compact solid-state
source."
Unlike far more energetic X-rays, T-rays do not have sufficient energy to "ionize" an
atom by knocking loose one of its electrons. This ionization causes the cellular
damage that can lead to radiation sickness or cancer. Since T-rays are non-ionizing
radiation, like radio waves or visible light, people exposed to terahertz radiation
will suffer no ill effects. Furthermore, although terahertz radiation does
not penetrate through metals and water, it does penetrate through many common
materials, such as leather, fabric, cardboard and paper.
These qualities make terahertz devices one of the most promising new technologies
for airport and national security. Unlike today's metal or X-ray detectors,
which can identify only a few obviously dangerous materials, checkpoints that
look instead at T-ray absorption patterns could not only detect but also identify
a much wider variety of hazardous or illegal substances.
T-rays can also penetrate the human body by almost half a centimeter, and
they have already begun to enable doctors to better detect and treat certain
types of cancers, especially those of the skin and breast, Welp said. Dentists
could also use T-rays to image their patients' teeth.
The new T-ray sources created at Argonne use high-temperature superconducting
crystals grown at the University
of Tsukuba in Japan. These crystals comprise
stacks of so-called Josephson junctions that exhibit a unique electrical property:
when an external voltage is applied, an alternating current will flow back
and forth across the junctions at a frequency proportional to the strength
of the voltage; this phenomenon is known as the Josephson
effect.
These alternating currents then produce electromagnetic fields whose frequency
is tuned by the applied voltage. Even a small voltage – around two millivolts
per junction – can induce frequencies in the terahertz range, according to
Welp.
Since each of these junctions is tiny – a human hair is roughly 10,000 times
as thick – the researchers were able to stack approximately 1,000 of them on
top of each other in order to generate a more powerful signal. However, even
though each junction would oscillate with the same frequency, the researchers
needed to find a way to make them all radiate in phase.
"That's been the challenge all along," Welp said. "If one junction
oscillates up while another junction oscillates down, they'll cancel each other
out and you won't get anything."
In order to synchronize the signal, Argonne physicist Alexei Koshelev suggested
that the stacks of Josephson junctions should be shaped into resonant cavities,
which visiting scientist Lufti Ozyuzer of the Izmir
Institute of Technology, Turkey, and graduate student Cihan Kurter then
fashioned. When the width of the cavities was precisely tuned to the frequencies
set by the voltage, the natural resonances of the structure synchronized the
oscillations and thus amplified the T-ray output, in a method similar to the
production of light in a laser.
"Once you apply the voltage," Welp said, "some junctions will
start to oscillate. If those have the proper frequency, an oscillating electric
field will grow in the cavity, which will pull in more and more and more of
the other junctions, until in the end we have the entire stack synchronized."
By keeping the length and thickness of the cavities constant while varying
their width between 40 and 100 micrometers, the researchers were able to generate
frequencies from 0.4 to 0.85 terahertz at a signal power of up to 0.5 microwatts.
Welp hopes to expand the range of available frequencies and to increase the
strength of the signal by making the Josephson cavities longer or by linking
them in arrays.
"The more power you have, the easier it is to adopt this technology for
all sorts of applications," he said. "Our data indicate that the
power stored in the resonant cavities is significantly larger than the detected
values, though we need to improve the extraction efficiency. If we can get
the signal strength up to 1 milliwatt, it will be a great success."
Collaborators on this research were Lutfi Ozyuzer, Alexei Koshelev, Cihan
Kurter, Nachappa (sami) Gopalsami, Qing'An Li, Ken Gray, Wai-Kwong Kwok and
Ulrich Welp of Argonne; Masashi Tachiki from the University
of Tokyo; Kazuo Kadowaki,
Takashi Yamamoto, Hidetoshi Minami and Hayato Yamaguchi from the University
of Tsukuba; and Takashi Tachiki from the National
Defense Academy of Japan.
The research was supported by DOE's Office of Basic
Energy Sciences and by
Argonne's Laboratory Directed Research and Development funds.
A scientific paper based on their research,
"Emission
of Coherent THz Radiation from Superconductors," appears in the November
23 issue of Science.
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By Jared Sagoff.
For more information, please
contact Steve McGregor (630/252-5580 or media@anl.gov)
at Argonne.
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