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'Nanoantennas' Could Bring Sensitive Detectors,
Optical Circuits |
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by: Emil
Venere
September 3, 2002
WEST
LAFAYETTE, Ind., Sept. 3 -- Researchers have shown how tiny
wires and metallic spheres might be arranged in various shapes
to form "nanoantennas" that dramatically increase the
precision of medical diagnostic imaging and devices that
detect chemical and biological warfare agents.
Engineers from Purdue University have
demonstrated through mathematical simulations that
nanometer-scale antennas with certain geometric shapes should
be able to make possible new sensors capable of detecting a
single molecule of a chemical or biological agent. Such an
innovation could result in detectors that are, in some cases,
millions of times more sensitive than current technology.
The nanoantennas in the simulations are
made of metal wires and spheres only about 10 nanometers
thick, or roughly 100 atoms wide. They are an example of
"left-handed" materials, meaning they are able to reverse the
normal behavior of visible light and other forms of
electromagnetic radiation.
Ordinary
materials, such as glass, plastic, air and water, are called
"right-handed" because of the way light bends as it penetrates
a material. Left-handed materials have the ability to bend
waves of electromagnetic radiation in the opposite direction
of right-handed materials. This unusual property means that
such materials might be used to create a so-called "super
lens" that drastically improves the quality of medical
diagnostic images.
The Purdue
researchers are the first to show precisely how left-handed
materials -- the "nanoantennas" -- could be applied to visible
light and other electromagnetic radiation consisting of small
wavelengths. Scientists at the University of California at San
Diego proved two years ago that left-handed materials could be
applied to devices that use microwaves, which are much larger
than the waves needed for medical imaging, and for sensors
used in spectroscopy to detect chemicals and biological
agents. The phenomenon was first predicted in the late 1960s.
"All of the work in this area so far has
been done in the microwave spectral range," said Vladimir
Shalaev, a professor in Purdue's School of Electrical and
Computer Engineering. "We believe this is the first project
for how these types of materials can be used in the visible
range of the electromagnetic spectrum."
The Purdue researchers have shown in
theory how the same phenomenon could be scaled down to devices
only nanometers wide. The research also shows how nanoantennas
with specific shapes are critical for receiving certain
frequencies of electromagnetic radiation. The findings were
published in the March issue of the Journal of Nonlinear
Optical Physics and Materials. The paper was written by
Shalaev, Viktor A. Podolskiy, a post-doctoral fellow at
Princeton University, and Andrey K. Sarychev, a senior
research scientist at Purdue.
Purdue
researchers plan to take the work a step further by creating
the nanoantennas and conducting experiments to support the
theoretical calculations, Shalaev said.
"Left-handed materials might have loads
of applications," Shalaev said. "We don't know yet the full
potential of these materials, because it's a really new
field."
The researchers showed how the
nanoantennas could be created by arranging pairs of tiny wires
parallel to each other. That arrangement, in theory, enables
the nanoantennas to achieve a "negative index of refraction,"
said Shalaev, a physicist by training.
Light and other forms of radiation bend
as they pass through a material. Physicists measure this
bending of radiation by its "index of refraction." The larger
a material's index, the slower light travels through it, and
the more it bends, or changes direction when going from one
material to a different one. Because left-handed materials
bend light in precisely the opposite direction as right-handed
materials, they are said to have a "negative index of
refraction."
"With these new types of
materials, it may be possible to accomplish better performance
than all existing materials, in terms of making images and
manipulating light," Shalaev said. The nanoantennas work by
using clouds of electrons, all moving in unison as if they
were a single object instead of millions of individual
electrons. These groups of electrons are known collectively as
"plasmons."
Researchers hope to one day
use nanoantennas to create more compact, faster circuits and
computers that use packets of light, called photons, instead
of electrons for carrying signals. Photons travel much faster
than electrons, but, unlike electrons, they do not possess an
electrical charge. This lack of an electrical charge makes it
far more difficult to manipulate photons.
"Because electrons are negatively
charged particles, it's easy to manipulate them," Shalaev
said. "You just apply a field and they start moving.
"It turns out that, by employing these
plasmonic nanomaterials, you should be able to manipulate
light. You can guide light. You can basically simulate all the
basic fundamental properties of electronic circuits, but in
this case photons start to work."
Such
photonic circuits could usher in a new class of ultrasensitive
sensors that detect tiny traces of chemicals and biological
materials, making them useful for applications including
analyzing a patient's DNA for medical diagnostics, monitoring
air quality for pollution control and detecting dangerous
substances for homeland security.
"This
could be a way to dramatically enhance sensitivity in
detecting molecules," Shalaev said. "That's a great goal.
These plasmonic nanomaterials accumulate electromagnetic
energy in extremely small areas, nanoscale areas. It's like
focusing light in areas much smaller than the wavelengths of
light.
"Conventional lenses cannot focus
light in an area smaller than the wavelength of the light.
When you use these plasmonic nanomaterials, which act like
nanoantennas, you do focus light in areas much smaller than
the wavelengths. This means that metallic nanostructures might
be able to detect even a single molecule of a substance, which
is not possible with conventional optics."
The nanoantenna shapes used in the
simulations ranged from single spheres to more complex
geometric configurations called "fractals," in which the same
shape is repeated in smaller and smaller sections.
Using metallic structures only a few
nanometers thick is critical to applying the technique to
visible light.
"Light cannot go into
metals," Shalaev said. "But when you take a very small piece
of metal, the light goes through completely and you very
efficiently excite the whole piece of metal."
The research has been funded by the
National Science Foundation.
For more
information, visit: http://www.photonics.com/clickthru/webclickthru.asp?url=http://www.purdue.edu&codivid=purdueuniversity&placement=FeatureArt
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