Engineers
at Purdue University are the first researchers to
create a material that has a "negative index of refraction" in
the wavelength of light used for telecommunications,
a step that could lead to better communications and
imaging technologies.
"This work represents a milestone because it demonstrates
that it is possible to have a negative refractive
index in the optical range, which increases the likelihood
of harnessing this phenomenon for optics and communications," said
Vladimir Shalaev, the Robert and Anne Burnett Professor
of Electrical and Computer Engineering.
The
material consists of tiny
parallel "nanorods" of
gold that conduct clouds of electrons called "plasmons" with
a frequency of light referred to as the near-infrared.
The wavelength size of this near-infrared light is
1.5 microns, or millionths of a meter, the same wavelength
used for fiberoptic communications.
"This is the most important wavelength for communications," Shalaev
said.
Findings are detailed in a paper appearing Dec.
15 in the journal Optics Letters, published by the
Optical Society of America. The paper was written
by Shalaev, his graduate research assistants Wenshan
Cai and Uday K. Chettiar, doctoral student Hsiao-Kuan
Yuan, senior research scientists Andrey K. Sarychev
and Vladimir P. Drachev, and principal research scientist
Alexander V. Kildishev.
The
nanorods are an example
of materials that are able
to reverse a phenomenon
called refraction, which
occurs as electromagnetic waves, including light,
bend when passing from one material into another
and is caused by a change in the speed of light as
it passes from one medium into another. Scientists
measure this bending of radiation by its "index of
refraction." Refraction causes the bent-stick-in-water
effect, which occurs when a stick placed in a glass
of water appears bent when viewed from the outside.
Each material has its own refraction index, which
describes how much light will bend in that particular
material and defines how much the speed of light
slows down while passing through a material. All
natural materials, such as glass, air and water,
have positive refractive indices.
In
the late 1960s, researchers
hypothesized what would
happen if a material had
a negative refractive index,
causing it to bend light
in the opposite direction
from ordinary materials. In 2000, researcher John
Pendry at the Imperial College London theorized that
slabs of such material might be used to create a "superlens" that
would drastically improve the quality of medical
diagnostic imaging and other technologies. Such lenses
theoretically could compensate for the loss of a
portion of the light transmitting an image as it
passes through a lens. Lenses and imaging systems
could be improved if this lost light, which scientists
call "evanescent light," could be restored. An imaging
system that used a combination of positive and negative
refraction might restore the lost evanescent light.
Harnessing
materials that have a negative
index of refraction could
make it possible to take
optical images of objects
that are smaller than the
wavelength of visible light,
including molecules such
as DNA, for research and
medical imaging; the development
of "photo-nanolithography," which would make it possible
to etch smaller electronic devices and circuits,
resulting in more powerful computers; new types of
antennas, computer components and consumer electronics
such as cell phones that use light instead of electricity
for carrying signals and processing information,
resulting in faster communications.
A
major obstacle now hindering
development of opto-electronic
devices is that wavelengths of light are too large
to fit into the tiny features needed for miniature
circuits and components. "Plasmonic nanomaterials," however,
could make it possible to squeeze light waves into
much smaller spaces, Shalaev said.
Various
research groups have fabricated "metamaterials" made
of tiny metal rings and rods, which have a negative
index of refraction. No metamaterials have yet been
created that have negative refraction indices for
visible light, but now the Purdue researchers have
created the first metamaterial with a negative refractive
index in the near-infrared portion of the spectrum.
This is just beyond the range of visible light, demonstrating
the feasibility of applying the concept to communications
and computers.
"The challenge was to fabricate a structure that
would have not only an electrical response, but also
a magnetic response in the near-infrared range," Shalaev
said.
The
gold nanorods conduct 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.
Light from a laser or other source was shined onto
the nanorods, inducing an "electro-optical current" in
the tiny circuit. Each of the rods is about as wide
as 100 nanometers, or 100 billionths of a meter,
and 700 nanometers long.
"These rods basically conduct current because they
are a metal, producing an effect we call optical
inductance, while a material between the rods produces
another effect called optical capacitance," Shalaev
said. "The result is the formation of a very small
electromagnetic circuit, but this circuit works in
higher frequencies than normal circuits, in a portion
of the spectrum we call optical frequencies, which
includes the near-infrared. So we have created a
structure that works as kind of an optical circuit
and interacts effectively with both of the field
components of light: electrical and magnetic."
The research has been funded by the U.S. Army Research
Office and the National Science Foundation and is
affiliated with Purdue's Birck Nanotechnology Center
at Discovery Park, the university's hub for interdisciplinary
research.
"Although many researchers are skeptical about developing
materials with a negative index of refraction in
optical wavelengths and then using them in practical
technologies, I think the challenges are mainly engineering
problems that could eventually be overcome," Shalaev
said. "There is no fundamental law of physics that
would prevent this from happening."
Writer: Emil Venere, (765) 494-4709, venere@purdue.edu
Source: Vladimir Shalaev, (765) 494-9855, shalaev@ecn.purdue.edu
Related Web sites: Vladimir Shalaev: http://ECE.www.ecn.purdue.edu/ECE/People/Faculty/shalaev
Optical Society of America: http://www.osa.org/
Story and photo can be found at: http://news.uns.purdue.edu/UNS/html4ever/2005/051130.Shalaev.negative.html
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