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An easy-to-produce material made from the stuff of
computer chips has the rare ability to bend light in the opposite
direction from all naturally occurring materials. The
semiconductors that constitute the Princeton invention are grown
from crystals using common manufacturing techniques, making it
less complex, more reliable and easier to produce than other
metamaterials.
Image by Keith Drake
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Previous metamaterials were two-dimensional
arrangements of metals, which limited their usefulness. The Princeton
invention is the first three-dimensional metamaterial constructed
entirely from semiconductors, the principal ingredient of microchips
and optoelectronics.
"To be useful in a variety of devices, metamaterials need to be
three-dimensional," said Princeton electrical engineering professor
Claire Gmachl, one of the researchers on the study. "Furthermore, this
is made from semiconductors, which are extremely functional materials.
These are the things from which true applications are made."
The research team, led by Princeton engineering graduate student
Anthony Hoffman, published its findings online
Oct. 14, 2007, in the journal Nature Materials.
Other Princeton researchers on the team include graduate students
Leonid Alekseyev, Scott Howard and Kale Franz; former Council of
Science and Technology fellow Dan Wasserman, now at the University of
Massachusetts-Lowell; and former electrical engineering professor
Evgenii Narimanov, now at Purdue University. The team also includes
collaborators from Oregon State University and telecommunications firm
Alcatel-Lucent.
Light waves and other forms of electromagnetic radiation bend whenever
they pass from one medium to another. This phenomenon, called
refraction, is readily observable when a straw placed into a glass of
water appears to be bent or broken. Lenses in reading glasses or a
camera work because of refraction.
All materials have an index of refraction, which measures the degree
and direction that light is bent as it passes through them. While
materials found in nature have positive refractive indices, the
material recently invented by Princeton researchers has a negative
index of refraction.
In the case of the straw in a glass, normal water would make the
underwater portion of the straw appear to bend toward the surface. If
water were able to refract light negatively, as the newly invented
semiconductor does, the segment of straw under the water would appear
as if it were bending away from the surface.
Far more than a neat optical illusion, negative refraction holds
promise for the development of superior lenses. The positive
refractive indices of normal materials necessitate the use of curved
lenses, which inherently distort some of the light that passes through
them, in telescopes and microscopes. Flat lenses made from materials
that exhibit negative refraction could compensate for this aberration
and enable far more powerful microscopes that can "see" things as
small as molecules of DNA.
In addition, the Princeton metamaterial is capable of negative
refraction of light in the mid-infrared region, which is used in a
wide range of sensing and communications applications. Its unique
composition results in less lost light than previous metamaterials,
which were made of extremely small arrangements of metal wires and
rings. The semiconductors that constitute the new material are grown
from crystals using common manufacturing techniques, making it less
complex, more reliable and easier to produce.
"Currently, the typical infrared lens is a massive object - the setups
are bulky," Hoffman said. "This new material may enable more compact
mid-infrared optics because we now have a new material with an
entirely new set of optical parameters in our toolkit."
The research is part of a multi-institutional research center called
Mid-Infrared Technologies for Health and the Environment (MIRTHE).
Researchers at MIRTHE are developing compact sensors that detect trace
amounts of gases in the atmosphere and human breath. These could one
day be used in devices that monitor air quality and enhance homeland
security, as well as in non-invasive and on-the-spot medical tests for
diabetes and lung disease.
The research relies on a new type of laser that emits mid-infrared
light. Gmachl, who directs the MIRTHE project, said the new material
could be used to make the lasers better and smaller.
Next, the team plans to incorporate the new metamaterial into lasers.
Additionally, the researchers will continue to modify the material in
attempts to make features ever smaller in an effort to expand the
range of light wavelengths they are able to manipulate.
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