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.
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
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