DOW-UAP-D122, AAWSAP DIRD, Invisibility Cloaking Theory and Experiments, March 2010

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UNCLASSIFIED/j FOR OFFICIAL USE O14Lf Invisibility Cloaking: Theory and Experiments Prepared by: Acquisition Support Division (DW0-3) Defense Warning Office Directorate for Analysis Defense Intelligence Agency AAP Person 68 Administrative Note COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not authorized. This product is one in a series of advanced technology reports produced in FY 2009 under the Defense Intelligence Agency, Defense Warning Office's Advanced Aerospace Weapon System Applications (AAWSA Pro ram. Comments or uestions pertaining to this document should be addressed to AAP Person 1 AAWSA Program Manager, Defense Intelligence Agency, ATTN: CLAR/DWO-3, Bldg 6000, Washington,

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Figure 3. The Secret Optical Camouflage................................................................ 2 Figure 4. H. G. Wells's The Invisible Man: Invisibility by Transparency .................. 3 Figure 5. Complementary Media Invisibility Cloak .................................................. 4 Figure 6. Cloaking Shell.......................................................................................... 5 Figure 9. An Optical Material Deforms the Coordinates of Space ............................ 8 Figure 10. Light Waves at Cloaking Device............................................................. 9 Figure 11. Cloaking Device for Microwaves .......................................................... 10

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Figure 12. Lycurgus Cup (British Museum; AD Fourth Century)........................... 11 Figure 13. Idea for a Cloaking Device for Visible Light ......................................... 12 Figure 14. Advances in Metamaterials.................................................................. 13 Figure 15. Demonstration of Negative Refraction With "Bulk" Optical Metamaterials Made of Nano-Fishnets................................................. 14 Figure 16. Demonstration of Negative Refraction With "Bulk" Optical Metamaterials Made of Nanowires....................................................... 15 Figure 17. Fundamental Problem of Transformation-Based Cloaking Devices ...... 16 Figure 18. Wave Packets are Made by Combining Waves With Different Figure 20. Non-Euclidean Cloaking Device in Two Dimensions ............................. 19 Figure 21. Three-Dimensional Cloaking................................................................ 20 Figure 22. Coordinate Transformation Implemented by a Ground-Plate Cloak..... 21 Figure 23. Implementation of the Ground-Plate Cloak ......................................... 21

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Invisibility Cloaking: Theory and Experiments The idea of invisibility has fascinated people for millennia, inspiring many myths, novels, and films. Invisibility cloaking has recently become a subject of science and technology. This paper describes the important current theoretical and experimental developments and tries to project into the future.

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Invisibility may be achieved through three principal methods: camouflage, transparency, and cloaking. Many animals and some plants use camouflage to disguise themselves from predators-for example, by assuming the shapes and colors of objects in their surroundings. The military has long used forms of camouflage; a recent military application of camouflage is stealth technology. Stealth planes have aerodynamically unusual, edgy shapes and are coated with a special material. Both features serve the same purpose: to make the plane "invisible" to radar. How does it work? In radar, electromagnetic microwaves are emitted by a source, and their reflection by an object-an airplane, for example-is detected. From the direction and the time delay of the reflected waves, the direction and distance of the object are inferred. If the object does not reflect the electromagnetic microwaves back to the source, it will not appear on the radar. This is precisely what stealth technology achieves. Owing to the edgy shape of Figure 1. B2 Stealth Bomber the stealth plane, most of the incident electromagnetic waves are reflected in different directions; the coating of the plane absorbs the rest. In this way, the stealth plane has become completely black in the spectral range of radar. As for radar waves, the sky is black, not blue, and the plane has assumed the color of the background: the stealth plane is camouflaged.

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Another recent example of camouflage is optical camouflage, developed by the University of Tokyo's Tachi Laboratory. Figure 2 shows an example of optical camouflage. A camera captures the background scene behind the person. The image is processed and projected onto the person via a semitransparent mirror. The person wears an "invisibility cloak" made of a retroreflective material that reflects light back in the direction whence it came, like cats-eye-reflectors do. As the cloak carries the projected image of the background, the person seems to disappear, but surely the equipment standing around the person is clearly visible. In addition, optical camouflage works only in one direction; seen from the side, the person is visible. Nevertheless, optica l camouflage may become a useful tool in some situations where obstacles are in the way of sight- Figure 2. Optical Camouflage (Tachi Laboratory, for example in surgery, where the Tokyo} surgeon's hands and instruments may obstruct the view. Figure 3 shows how optical camouflage works. The S~cret of Trollnsparent croalc Figure 3. The Secret Optical Camouflage (Tachi Laboratory, Tokyo)

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H. G. Wells's novel The Invisible Man represents another strategy for becoming invisible: transparency. In Wells's novel, the invisible man, a disgruntled college professor, invents a substance that somehow changes the refractive index1 of his body. Most transparent substances, like glass, air, or water, modify the speed of light, because the atoms or molecules of these substances absorb and re-emit light, which takes time. The delay caused by the atoms and molecules results in a reduced speed of light and hence in a refractive index larger than 1. It is, however, also possible to achieve a refractive index smaller than 1, although only in narrow bands of the spectrum. In these cases, the atoms or molecules advance the wave fronts of light because Figure 4. H. G. Wells's The Invisible Man: Invisibility by Transparency they are excited such that their electron clouds oscillate ahead of the light. If the refractive index is uniform in a material, light is reflected and refracted at the boundary but otherwise is traveling straight through. On the other hand, if the refractive index varies, light is scattered at the index inhomogeneities and gets lost. Most white substances appear white because of such scattering . Milk, for example, consists of minuscule oily droplets-fat-in water. The refractive index of the droplets differs from water, and hence light is scattered at them; it does not penetrate the substance, and the diffused light appears as white. Now, human bodies are visible, because they absorb light. Most of the absorption is due to the scattering of light in biological tissue, in the cells of which the bodies are made. If