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This Defense Intelligence Reference Document, dated 2 March 2010, was prepared by the Defense Intelligence Agency's Defense Warning Office under the Advanced Aerospace Weapon System Applications program. It reviews invisibility through camouflage, transparency, and cloaking, covering metamaterials, transformation optics, and non-Euclidean broadband cloaking designs. It concludes that perfect cloaking is impossible, but imperfect devices could be made. Microwave cloaking is within reach of present technology, while visible-light invisibility remains uncertain and depends mainly on new theoretical research.
From the source: Release of 2026-09-18 Incident: 3/2/10, Las Vegas, Nevada. Released with redactions. This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed. This DIRD surveys the theory and early experiments behind invisibility cloaking, describing several ways an object might be hidden from visual or sensor detection, including camouflage, transparency effects, and optical cloaking that bends light around an object. It focuses mainly on metamaterials, negative refraction, and transformation optics, and reviews experiments that had already demonstrated limited cloaking at microwave frequencies. The report argues that “imperfect” cloaking is physically achievable in some parts of the electromagnetic spectrum, especially for microwaves, but that “perfect” cloaking is not practical because it would require material properties that conflict with the underlying physics. Its overall conclusion is that cloaking is a scientific field with plausible narrow applications, but that useful visible-light cloaking depends more on future theoretical breakthroughs than on conventional advances in materials science.
UNCLASSIFIED/ /FOR: OFFl@IAL l!ISI!!! 9HL'f converted into the electromagnetic oscillation of the gold particle in much the same way a tun ing fork responds to sound of the right frequency . However, the electromagnetic oscillation is damped out by the electric resista nce 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 correspond ing color is missing in the spectrum. For the gold particles in the Lycurgus Cup, th is 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 t he degree of control on their struct ures achieved by applying modern technology and the level of theoretica l Figure 12. Lycurgus Cup (British Museum; fourth century AD). This Roman cup is made of ruby glass. When viewed in reflected light- fo r example, in daylight it appea rs green. However, when a light is shone into the cup and transmitted thro ugh the glass, it appea rs red . The cup illustrates t he myt h of King Lycurgus. He is seen bei ng dragged into the underworl d by the Gree k nymph Am brosia, wh o 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 technolog ical tools to use this knowledge in the design of novel optical metamaterials. Optical Metamaterials As light is simply an electromagnetic wave with shorter wavelengths t han microwave radiation , one could imagine an optica l cloaking device as the microwave cloak but with much smaller cells, fitted to the smaller wavelength. However, this simple idea is t oo 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 rad iation 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 ci rcuit board of the microwave-cloaking device . They act similarly to the gold particles embedded in ruby glass; their optica l properties UNCLASSIFIED/ /EAR OFFICIO! I 155 ODIL¥ 11
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Official release, from the pursue collection. The PDF is mirrored here; the original link is above. 29 pages are in the text index: search them above, or from the library's search.