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Negative Refractive Index Metamaterials

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.

Metamaterial Simulations with COMSOL - RF CafeIn 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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