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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
Spectrum magazine,
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.
Walter Rotman
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.
There 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 result.
Posted June 14, 2012
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