Nanotubes tune in light
Technology Research News
antenna transmits and receives electromagnetic waves at wavelengths that
are close to the length of the antenna, and it does so by converting electrical
current to electromagnetic waves and vice versa. The electromagnetic spectrum
spans radio waves, microwaves, heat waves, visible lightwaves, ultraviolet
waves, x-rays, and gamma rays.
Carbon nanotubes, which are rolled-up sheets of carbon atoms that
can be smaller than a nanometer in diameter, can act as antennas, but instead
of transmitting and receiving radio waves, which are at the longest end
of the electromagnetic spectrum, antennas of their size pick up the nanoscale
wavelengths of visible light. A nanometer is one millionth of a millimeter.
In contrast, radio wave wavelengths are measured in meters.
Researchers at Boston College, the U.S. Army Natick Soldier Center,
Mega Wave Corporation and Florida International University have demonstrated
the light antenna effect using multiwalled carbon nanotubes. "Aligned carbon
nanotubes receive and transmit light just like radio frequency antennas
receive and transmit radio waves," said Zhifeng Ren, a physics professor
at Boston College.
The method could be used to convert optical signals to electrical
signals in communications equipment, to carry out optical computing, to
detect different wavelengths of light including the infrared wavelengths
used in telecommunications equipment, and to convert sunlight to electricity
in solar energy applications, said Ren.
Antennas convert electromagnetic waves into electric current when
electromagnetic waves that have a wavelength equal to or a small multiple
of an antenna's length cause electrons in the metal to move. Antennas also
do the reverse: an electric current in an antenna generates electromagnetic
waves that have a wavelength equal to the length of the antenna.
To make the light antennas, the researchers grew arrays of multiwalled
carbon nanotubes that were 50 nanometers in diameter and varied in length
from 200 to 1,000 nanometers. Visible lightwaves range from 400 to 700 nanometers
long from crest to crest, which is about ten times smaller than a red blood
Because the frequency of visible lightwaves is so fast, oscillations
in the electric current produced by visible light antennas are too quick
to be measured using today's electronics. The researchers measured the behavior
of the carbon nanotube arrays by instead recording the lightwaves that the
minuscule antennas reradiated.
The researchers demonstrated two signature effects of antennas --
polarization and length matching. They showed that a nanotube antenna's
response to an electromagnetic wave diminishes as the wave's electric field
is rotated perpendicular to the nanotube -- the polarization effect. And
they demonstrated that a nanotube antenna's response is strongest when its
length is a multiple of half the wavelength of the electromagnetic wave
-- the length-matching effect.
The researchers also showed that nanotube antennas are high quality,
with electrons in the nanotubes scattering at a low rate comparable to conducting
metals like copper.
Devices that incorporate nanotube antennas could be tuned to very
specific light frequencies using nanotubes grown to specific lengths.
One key to realizing practical applications of nanotube antennas
is coming up with fast nanoscale diodes capable of capturing the high-speed
electrical signals the antennas produce. A diode allows electrical current
to flow in only one direction.
Practical nanotube antennas could be developed in two to five years,
according to Ren.
Zhifeng's research colleagues were a Yang Wang, Krzysztof Kempa,
Thomas Kempa, Jakub Rybczynski and Andrzej Herczynski of Boston College,
Brian Kimball and Joel Carlson of the U.S. Army Natick Soldier Center, Glynda
Benham of Mega Wave Corporation, and Wenzhi Li of Florida International
University. The work appeared in the September 27, 2004 issue of Applied
Physics Letters. The research was funded by the U.S. Army and the National
Science Foundation (NSF).
Timeline: 2-5 years
TRN Categories: Optical Computing, Optoelectronics and Photonics;
Story Type: News
Related Elements: Technical paper, "Receiving and Transmitting
Light-like Radio Waves: Antenna Effect in Arrays of Alignment Carbon Nanotubes,"
Applied Physics Letters, September 27, 2004
November 17/24, 2004
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