bridges infrared-microwave gap
Technology Research News
Technologies that broadcast radio and television
programming, transmit data, scan the human body and search the stars use
electromagnetic radiation in the form of radio waves, microwaves, infrared
light, visible light, ultraviolet light and x-rays.
These different types of radiation have waves of vastly different lengths.
Various communications technologies, for instance, use waves that measure
several thousand kilometers to only a few thousandths of a millimeter
from crest to crest. More of the shorter waves pass a given point per
second, allowing them to transmit information faster. The newest generation
of cell phones, for example, transmit at 2.2 gigahertz, or billion times
Where infrared, or heat waves, meet microwaves there is a gap in the wavelengths
traditionally harnessed for communications and detection technologies.
Devices capable of producing electromagnetic waves at these frequencies
-- around a few trillion hertz -- could be the key to safer medical imaging,
detectors for toxic gases and explosives, and high-speed wireless communications.
Researchers from Italy and England have taken a big step toward filling
the terahertz wavelength gap by building a chip-based laser that emits
light at 4.4 terahertz. The terahertz semiconductor
laser currently requires cryogenic cooling, but has the potential
to work at higher, more useful temperatures, said Rüdeger Köhler, a physicist
at the Italian National Institute for Condensed Matter Physics (INFM).
Terahertz beams are particularly desirable for medical imaging because
they produce much less energy than x-rays and so are less stressful to
biological tissue, said Köhler. Terahertz radiation could also be tuned
to highlight specific types of tissue to detect, for example, cavities
or early-stage skin cancer, he said.
Terahertz lasers also have security applications. Scanners could penetrate
clothing and plastics to detect hidden metal objects, and chemical detectors
could identify toxic gases and explosives that have characteristic spectral
fingerprints, or vibrations, in the terahertz region, said Köhler.
Scanners and chemical detectors work by using terahertz waves to detect
the signature vibrations of the atoms in a given substance, said Köhler.
"Like balls connected with springs, these atoms vibrate at characteristic
frequencies, which allows [you] to distinguish the compounds," he said.
The terahertz range also promises fast wireless communications. Terahertz
waves are shorter then radio waves, and so more of them are transmitted
per second. Terahertz waves are long enough to penetrate walls like the
radio waves used in some types of local area networks that connect computers
without wiring, said Köhler. By contrast, the infrared waves used in other
types of wireless networks are shorter than terahertz waves and can therefore
carry more data, but are so short they cannot penetrate barriers like
walls and must use unobstructed paths.
Lasers typically produce a beam of light when photons emitted by the electrons
of a laser material bounce between parallel mirrors, causing them to hit
other electrons that in turn release additional photons. This amplification
process produces the laser's intense, monochromatic beam.
The researchers' terahertz laser is a more complicated quantum cascade
device made from a stack of 1,500 alternating layers of semiconductor
materials, some only a few atoms thick. The whole stack measures 12 microns,
or a little more than twice the diameter of a red blood cell. Quantum
cascade lasers usually emit shorter, infrared wavelengths.
The terahertz laser is made up of alternating layers of gallium arsenide
and aluminum gallium arsenide. Fourteen-layer units form 104 steps in
an electronic staircase that individual electrons bounce down, releasing
a photon at each step.
The thicker the step, the longer the wavelength it emits. Even though
some layers in the researchers' laser are extremely thin, the steps as
a whole are relatively thick, which produces the longer terahertz waves.
Key to the researchers design was using a large number of steps so that
many electrons emit photons at once.
The researchers' terahertz semiconductor laser "is the biggest step forward
in semiconductor laser technology since the original development of the
quantum cascade laser at Bell Labs" in 1994, said Paul Harrison, a reader
in quantum electronics and electronic and electrical engineering at the
University of Leeds in England. "The researchers have pushed the technology
of manipulating electron scattering to the extreme, far beyond current
knowledge," he said.
The work is "very significant," said Cun-Zheng Ning, a scientist at NASA's
Ames Research Center. "From past experience with semiconductor lasers,
it should be expected that significant improvements and rapid commercialization
will come soon," he said.
The researchers next steps are to make the terahertz laser work at higher
temperatures, to make it emit light in continuous waves rather than pulses,
and to make it emit light at slightly lower frequencies, said Köhler.
The researchers also plan to build devices using the laser, including
chemical detectors and data communications systems, Köhler said. "On a
somewhat longer time-scale, we plan to implement the laser into a detection
system for chemicals and to set up a... data link using the laser as the
sender and a... semiconductor detector as the receiver," he said.
The terahertz laser could be used in practical astronomy applications
like detecting the chemical compositions of objects like distant stars
in two to five years, said Köhler. All other applications are likely to
take five to ten years, he said. "This compares to the six years it took
for conventional mid-infrared quantum cascade lasers to be commercially
available," he said.
Köhler's research colleagues were Alessandro Tredicucci and Fabio Beltram
of INFM, Harvey E. Beere, Edmund H. Linfield, A. Giles Davies and David
A. Ritchie of the University of Cambridge in England, and Rita C. Iotti
and Fausto Rossi of Torino Polytechnic in Italy. They published the research
in the May 9, 2002 issue of the journal nature. The research was funded
by the European Commission.
Timeline: 2-5 years, 5-10 years
TRN Categories: Optical Computing, Optoelectronics and
Story Type: News
Related Elements: Technical paper, "Terahertz semiconductor-heterostructure
laser," Nature, May 9, 2002
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