Negative Refractive Index Metamaterials
are receiving as much attention in university research departments these days as graphene
and carbon nanotubes. What makes metamaterials so desirable is its negative refractive
index. It causes waves - be they electromagnetic or mechanical - to bend (refract) in
the opposite direction as nearly every material found in nature. If water droplets had
a negative index of refraction, rainbows would display color in the opposite direction
with red on the bottom and violet on the top. If a negative refractive index was the
norm in nature, our resistor color code would probably be reversed: 0=black, 1=brown,
2=violet, 3=blue=4, etc. So, why is a negative refractive index a big deal? It allows
waves (signals) to be bent (focused) across wider bandwidths without dispersing (spreading
out) the beam spatially. It can also be used to steer waves around an object in a manner
that renders the object invisible within the bandwidth of interest, a prime requirement
for cloaking. While cloaking is most often thought of as a military application for hiding
soldiers and equipment, it is also being studied for uses such as directing earthquake
waves and wind around buildings and bridge abutments.
In the June 2012 edition of IEEE's
COMSOL had a large special advertising section promoting uses of
their analysis software for metamaterials work. The thumbnail above is a microwave Rotman
lens (description) whose size was able to be significantly reduced thanks
to the integration of magnetic metamaterials into the steering array.
developed the lens named in his honor that allows multi-beam capability in radars and
other radio systems without having to physically move the antenna. Fine angle increments
require a large number of antenna elements, requiring a larger switching lens assembly.
The size can be made smaller with the aid of a negative refractive coefficient material
in the path. Rotman created pseudo metamaterials back in 1962 in his paper titled, "Plasma
Simulation by Artificial Dielectrics and Parallel-Plate Media"
(IEEE Xplore subscription or purchase required). It deals with
a negative refractive index within a plasma, such as that which engulfs an orbital vehicle
during atmospheric reentry. Rotman proposed exploiting that property for use with his
"lens" in what is now being accomplished with compact solid metamaterials. The current
crop of metamaterials has a fairly narrow bandwidth of operation, with most work being
done below the visible light spectrum. With military-grade detection equipment spanning
from kHz to THz, creating a Star Trek-like cloaking field that is undetectable at any
frequency will remain in the realm of science fiction for quite some time.
are currently no known naturally occurring materials with a negative refractive index
other than the aforementioned plasma states. Astronomers have postulated the existence
of interstellar plasmonic material that could help explain otherwise paradoxical findings
on galactic formation and universe expansion. This is a prime example of how investigations
on specific phenomena by researchers with largely disparate goals can result in a synergistic
Posted June 14, 2012
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