Defense Intelligence Reference Document Invisibilty Cloaking Theory And Experiments

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converted into the electromagnetic oscillation of the gold particle in much the same way
a tuning fork responds to sound of the right frequency.
However, the electromagnetic oscillation
is damped out by the electric resistance
in the metal, and its energy is absorbed
and ultimately turned into heat. The
color of light corresponds to the
frequency or wavelength. If one of the
frequencies is absorbed, the
corresponding color is missing in the
spectrum. For the gold particles in the
Lycurgus Cup, this color is green; a
spectrum with green missing appears
red, which produces the cup's exquisite
color. (Light is also scattered in the
material, and this scattering is enhanced
near the green of the resonance; hence
the greenish color of the cup seen in
daylight.)
What is new about metamaterials is the
degree of control on their structures
achieved by applying modern technology
and the level of theoretical
Figure 12. Lycurgus Cup (British Museum; fourth
century AD). This Roman cup is made of ruby glass.
When viewed in reflected light-for example, in daylight-
it appears green. However, when a light is shone into the
cup and transmitted through the glass, it appears red.
The cup illustrates the myth of King Lycurgus. He 1s seen
being dragged into the underworld by the Greek nymph
Ambrosia, who is disguised as a vine.
understanding of their workings. The Romans most probably never understood why
ruby glass is neither golden like gold nor transparent like glass, its ingredients, but
ruby. They did not know that light is an electromagnetic wave, nor did they know the
basic laws of electromagnetism. And they would not have had the technological tools to
use this knowledge in the design of novel optical metamaterials.
Optical Metamaterials
As light is simply an electromagnetic wave with shorter wavelengths than microwave
radiation, one could imagine an optical cloaking device as the microwave cloak but with
much smaller cells, fitted to the smaller wavelength. However, this simple idea is too
simple, for two different reasons. One is that metals like the copper of the circuit board
or the gold of ruby glass are more electrically resistant to currents oscillating with the
frequency of visible light than to currents in the microwave range of the spectrum.
Second, and more important, the cells of a metamaterial also emit electromagnetic
radiation in an incoherent way, not just as a coherent response to the incoming
electromagnetic wave, similar to the spontaneous emission of light by atoms and
molecules. The spontaneous emission is significantly stronger in the optical range of the
spectrum. In short, metamaterials do not scale; they must be designed differently for
visible light, and the loss of light by absorption and incoherent scattering usually is
greater for visible light than for microwaves. Figure 13 below illustrates the idea 19 for
an optical cloaking metamaterial. Instead of split-ring resonators, nano-scale metal
wires are embedded in a transparent host material, for example glass. The wires
replace the split-ring resonators on the circuit board of the microwave-cloaking device.
They act similarly to the gold particles embedded in ruby glass; their optical properties
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