Defense Intelligence Reference Document Invisibilty Cloaking Theory And Experiments

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The split-ring resonators are
electromagnetic circuits; they respond to
the electromagnetic field of microwave
radiation. Their response depends on
their shapes. For example, in the
cloaking device shown in Figure 11, the
double stripes in the middle of the split-
ring resonators vary from ring to ring. As
these stripes form an electric capacitor,
the capacitance of the resonators also
varies. The colored curves show how the
electromagnetic functions change over
the distance from the center of the
cloaking device as a result of the varied
capacitance. As they are always positive,
negative refraction is not required for
cloaking. At the inner ring, the red curve
reaches zero, defining the boundary of
the cloaking device. The rings with their
split-ring structures are designed to
perform an approximation of the
Figure 11. Cloaking Device for Microwaves 14
coordinate transformation shown and explained in the previous section. How is this
possible? The split-ring resonators act like the atoms or molecules of a normal optical
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or electromagnetic material: they absorb electromagnetic waves and re-emit them with
a phase delay or advance that depends on their electromagnetic response. Like atoms
or molecules, they are much smaller than the electromagnetic wavelength-3-mm cell
size versus 3-cm wavelength in the case of the microwave cloaking device 15-such that
the waves cannot resolve them individually but, rather, react to them as if they were a
bulk material with electromagnetic properties that may differ from point to point. Unlike
atoms or molecules, the electromagnetic response of each split-ring resonator is tailor-
made because it depends on the shape of the resonator that can be easily modified.
For example, in the case of the microwave-cloaking device, 16 the electromagnetic
response depends on the capacitance that is varied by changing the length of the
double stripes in the resonators. An unstructured circuit board reacts completely
differently to the microwave radiation: it would simply reflect it like the mesh in the
window of a microwave oven. A material with electromagnetic or optical properties that
depends on structures much smaller than the wavelength is called a meta material.
Metamaterials per se are nothing new; the ancient Romans invented the first optical
metamaterial: ruby glass. The Romans probably did not know it, but their recipe for
ruby glass contained one crucial ingredient: 17 tiny gold droplets, typically 5-60
nanometers (nm) in size. These gold particles color the glass in an extraordinary way,
as demonstrated by the exquisite Lycurgus Cup shown in Figure 12. In daylight, the cup
appears a greenish color, but illuminate it from the inside, and it glows ruby. The gold
particles act like the split-ring resonators of the microwave-cloaking device, 18 but here
on light, not on microwave radiation. Light consists of electromagnetic waves as well,
but with significantly smaller wavelengths of around 500 nm. The gold particles are
thus much smaller than the wavelength of light, and they turn out to be resonators as
well: in them, electric currents flow in a way that is dictated by their shapes and sizes.
When the light wave hits the resonance of the gold particle, most of its energy is
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