Documents / Report
This Defense Intelligence Reference Document, dated 2 March 2010 and produced by the Defense Intelligence Agency under its Advanced Aerospace Weapon System Applications (AAWSA) Program, is a technical survey of invisibility. It covers camouflage, including stealth aircraft and optical camouflage, then transparency, cloaking by coordinate transformation, metamaterials and non-Euclidean broadband cloaking. It concludes that perfect cloaking is impossible but imperfect microwave cloaks are within reach of present technology. Whether cloaking will work at visible wavelengths remains unclear.
UNCLASSIFIED/ ,'Flilll. lilFFllil_.,le lal!!I!! 8111!¥ Optical Cloaking Cloaking in the optical range of the spectrum poses several challenges. The present design of non-Euclidean cloaking devices still requires materials where, in some parts of the device, the speed of light is larger than in the environment of the device, which, in practice, means larger than the speed of light in vacuum. Most probably, this problem can be circumvented by inventing new designs and new geometrical forms of suitable curved spaces, because there is no mathematical reason why non-Euclidean cloaking should be limited in this way. However, solving this problem takes imagination and mathematical creativity; it cannot be planned by a clear roadmap, but it can be encouraged and stimulated. It could take 1 or 2 years or a much longer time until such designs are invented; truly imaginative research is unpredictable. This research takes a specific mindset, clear mathematical thinking combined with playfulness and physical intuition, a stimulating environment, and freedom. The greatest challenge for turning invisibility from an idea into a workable device is not technology but imagination. The only way to solve this problem is to follow the Solomonic advice to invest in the right people. The technology for cloaking will depend on the design of such advanced cloaking devices. Probably they will require highly anisotropic materials, but perhaps liquid crystals could be sufficient. Maybe metamaterials are not needed after all. In this case, invisibility could become a feasible technology within a generation. If optical metamaterials are needed, they will rely on structuring on extremely short scales, possibly on sub-nanometer distances. The technology for making such structures will be developed because the silicon-electronics industry will need them; but whether large- scale devices with sub-nanometer structures can be made remains to be seen. Another practical challenge is impedance managing. Ideal cloaking devices require materials with equal electric and magnetic response because they implement geometries and geometries are universal-they act on both the electric and the magnetic fields of electromagnetic waves like light. In practice, broadband optical materials mostly respond to the electric field but not to the magnetic one. Optical magnetism has been demonstrated with metamaterials, 39 but only in narrow regions of the spectrum. If the electric response differs from the magnetic response, the electromagnetic impedance is mismatched, which results in reflections. One could reduce such reflections by using smooth refractive-index profiles as appropriate antireflection coatings. Most probably, cloaking devices will be rigid shells; to make them flexible like wearable invisibility cloaks poses a significant challenge. The reason is that their optical properties must be adjusted to their geometrical shapes, as the refractive-index profile of a cloaking device depends on its shape. If the shape changes, the index-profile must follow suit. The required optical properties should be calculated in real time, and the material should change accordingly. Liquid crystals could adjust their optical properties, but controlling a large, complicated array of liquid crystals with possibly several layers appears to be difficult, despite the progress made in liquid-crystal displays. 22 UNCLASSIFIED//Flilll. lilFFlliil,t k llili lil•lk¥
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Report, from the dia 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.