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A triple layer of carbon nanotube arrays on a sapphire base are the basis for a new type of terahertz polarizer invented at Rice University. The polarizer could lead to new security and communication devices, sensors and non-invasive medical imaging systems. (Credit: Lei Ren/Rice University) |
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Researchers
at Rice University are using carbon nanotubes as the critical component
of a robust terahertz polarizer that could accelerate the development
of new security and communication devices, sensors and non-invasive
medical imaging systems as well as fundamental studies of
low-dimensional condensed matter systems.
The
polarizer developed by the Rice lab of Junichiro Kono, a professor of
electrical and computer engineering and of physics and astronomy, is the
most effective ever reported; it selectively allows 100% of a terahertz
wave to pass or blocks 99.9% of it, depending on its polarization. The
research was published in the online version of the American Chemical
Society journal, Nano Letters.
The
broadband polarizer handles waves from 0.5 to 2.2 THz, far surpassing
the range of commercial polarizers that consist of fragile grids wrapped
in gold or tungsten wires.
Kono
said technologies that make use of the optical and electrical regions
of the electromagnetic spectrum are mature and common, as in lasers and
telescopes on one end and computers and microwaves on the other. But
until recent years, the terahertz region in between was largely
unexplored. "Over the past decade or two, people have been making
impressive progress," he said, particularly in the development of such
sources of radiation as the terahertz quantum cascade laser.
"We
have pretty good terahertz emitters and detectors, but we need a way to
manipulate light in this range," Kono said. "Our work is in this
category, manipulating the polarization state—the direction of the
electric field—of terahertz radiation."
Terahertz
waves exist at the transition between infrared and microwaves and have
unique qualities. They are not harmful and penetrate fabric, wood,
plastic and even clouds, but not metal or water. In combination with
spectroscopy, they can be used to read what Kono called "spectral
fingerprints in the terahertz range"; he said they would, for instance,
be useful in a security setting to identify the chemical signatures of
specific explosives.
The
work by Kono and lead author Lei Ren, who recently earned his doctorate
at Rice, makes great use of the basic research into carbon nanotubes
for which the university is famous. Co-authors Robert Hauge, a
distinguished faculty fellow in chemistry, and his former graduate
student Cary Pint developed a way to grow nanotube carpets and to
transfer well-aligned arrays of nanotubes from a catalyst to any
substrate they chose, limited only by the size of the growth platform.
While Hauge and Pint were developing their nanotube arrays, Kono and his team were thinking about terahertz. Four years ago,
they came across a semiconducting material, indium antimonide, that
would stop or pass terahertz waves, but only in a strong magnetic field
and at very low temperatures.
At
about the same time, Kono's lab began working with carbon nanotube
arrays transferred onto a sapphire substrate by Pint and Hauge. Those
aligned arrays—think of a field of wheat run over by a
steamroller—turned out to be very effective at filtering terahertz
waves, as Kono and his team reported in a 2009 paper.
"When
the polarization of the terahertz wave was perpendicular to the
nanotubes, there was absolutely no attenuation," Kono recalled. "But
when the polarization was parallel to the nanotubes, the thickness was
not enough to completely kill the transmission, which was still at 30 to
50%."
The
answer was clear: Make the polarizer thicker. The current polarizer has
three decks of aligned nanotubes on sapphire, enough to effectively
absorb all of the incident terahertz radiation. "Our method is unique,
and it's simple," he said.
Kono
sees use for the device beyond spectroscopy by manipulating it with an
electric field, but that will only become possible when all of the
nanotubes in an array are of a semiconducting type. As they're made now,
batches of nanotubes are a random mix of semiconductors and metallics;
recent work by Erik Hároz, a graduate student in Kono's lab, detailed
the reasons that nanotubes separated through ultracentrifugation have
type-dependent colors. But finding a way to grow specific types of
nanotubes is the focus of a great deal of research at Rice and
elsewhere.
Co-authors
are former Rice postdoctoral researcher Takashi Arikawa and research
associate Iwao Kawayama and Professor Masayoshi Tonouchi of the
Institute of Laser Engineering at Osaka University, Japan.
The Department of Energy, the National Science Foundation and the Robert A. Welch Foundation supported the research.
Broadband Terahertz Polarizers with Ideal Performance Based on Aligned Carbon Nanotube Stacks
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