DIA, 2010: estudos do AAWSAP avaliam capas de invisibilidade e metamateriais e concluem que ocultar da luz visível ainda é incerto

UNCLASSIFIED//[FOR OFFICIAL USE ONLY, struck through] Defense Intelligence Reference Document Acquisition Threat Support 2 March 2010 ICOD: 1 December 2009 DIA-08-1003-001 Invisibility Cloaking: Theory and Experiments

Invisibility Cloaking: Theory and Experiments Prepared by: Acquisition Support Division (DWO-3) Defense Warning Office Directorate for Analysis Defense Intelligence Agency Author: 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) Program. Comments or questions pertaining to this document should be addressed to [AAP Person 1], AAWSA Program Manager, Defense Intelligence Agency, ATTN: CLAR/DWO-3, Bldg 6000, Washington, DC 20340-5100.

Invisibility Cloaking: Theory and Experiments Introduction 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.

Camouflage 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 surround ings. 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 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.

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. Summary A cloaking device is a passive device made of a transparent material that guides light around any object in its interior as if the light has passed through empty space. The cloaking device conceals the object and hides the act of hiding itself. Perfect cloaking devices are impossible because they require materials where the speed of light approaches infinity. Imperfect cloaking devices could be made. Such devices implement suitable curved-space geometries. For electromagnetic microwaves, cloaking devices are definitely within reach of the present technology. Whether invisibility in the visible range of the spectrum will become a reality is not entirely clear yet. Most probably, this will depend more on the new theoretical research than on advances in new materials, and on the application of mathematical intelligence, intuition, and imagination.

UNCLASSIFIED//[FOR OFFICIAL USE ONLY, struck through] Defense Intelligence Reference Document Acquisition Threat Support 6 April 2010 ICOD: 1 December 2009 DIA-08-1004-006 Metamaterials for Aerospace Applications

Metamaterials for Aerospace Applications Prepared by: Acquisition Support Division (DWO-3) Defense Warning Office Directorate for Analysis Defense Intelligence Agency Author: AAP Person 78 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) Program. Comments or questions pertaining to this document should be addressed to [AAP Person 1], AAWSA Program Manager, Defense Intelligence Agency, ATTN: CLAR/DWO-3, Bldg 6000, Washington, DC 20340-5100.

Definition of Metamaterials A metamaterial is defined as an artificial medium whose properties (mechanical, optical, magnetic, or other) cannot be found in naturally-occurring materials. The emphasis of this study will be on electromagnetic and optical metamaterials. Such metamaterials can exhibit rather extreme properties, such as negative refractive index, which implies that both electric permittivity and magnetic permeability must be negative (£ < 0 µ < 0) (Reference 1). Such metamaterials used to be called " left-handed" because of the unusual phase relationship between the electric and magnetic fields. Specifically, in most (positive index, including vacuum) media one uses the right-hand rule to define the relationship between electric field (E) magnetic field (H), and the propagation wavenumber (f). The physical basis of the right-hand rule is that the direction of energy propagation defined by the Poynting vector S = cE×H/4π' and the direction of the phase velocity (defined by the wavenumber k) must coincide. That does not hold true for negative index metamaterials where the two directions are opposite, therefore, the left-handed relationship must hold for the three vectors. Nevertheless, the "left-handed" designation did not withstand the test of time because it was causing confusion and creating irrelevant allusions to helical (a.k.a. chiral) structures. Although chiral structures can indeed exhibit negative index behavior (Reference 2), chirality is not necessary. A typical metamaterial consists of resonant elements such as Split Ring Resonators (SRR). An example of an SRR is shown in Figure 1. The main function of the SRR is to enable strong magnetic response of the structure. A simple empirical formula exists for the magnetic permeability of a metamaterial comprised of the SRRs: μ = 1 - Fω_M^2/(ω^2 - ω_M^2) < 0, (1) where ω_M is the resonant frequency of the SRR, and F is proportional to the volume filling factor of SRRs. It is noteworthy that SRRs are designed in such a way that it has a large capacitance. As the result, the resonant frequency of an SRR is small, (that is, the SRR-containing cell is very sub-wavelength). In the example shown in Figure 1 (taken from Reference 6), the unit cell operated at ω/2π = 10 GHz is λ/10. In fact, the sub-wavelength size of the metamaterial is what distinguishes them from their close cousins: photonic crystals. By properly designing magnetic SRRs, it is possible to achieve any value of µ for any given frequency. Special challenges exist for optical structures, though, as will be explained below.

Figure 5. Schematic of the Super-Lens with n=-1 Refractive Index Corresponding to (ε = -1, µ = -1) Surrounded By Vacuum. Super-lens' presence enables imaging sub-diffraction objects such as the periodic grating shown here. There are, however, interesting circumstances when it is very important to transport the image towards the scanning device. One such special circumstance is spatially-resolved spectroscopy of small (for example, cellular) structures. One can envision space expeditions to other planets that could, potentially, result in finding some evidence of primitive cellular-level life. It would then be highly desirable to examine the structure of the living cell in its natural environment. In all likelihood, that environment would be liquid. Therefore, it would be very desirable to examine the cell without actually touching it with a tip of a near-field optical microscope. Thus, the sub-surface imaging of a small object which is buried underneath a liquid layer would be necessary. No such experiments have so far been conducted. However, several years ago there was an experiment demonstrating imaging of sub-diffraction objects buried under the layer of silicon dioxide. The schematic and experimental results from the experiment (Reference 19) are shown in Figure 6. In this experiment the sub-wavelength objects were simple holes that were milled in the metal using an FIB. They were buried underneath the super-lens consisting of SiC (negative epsilon material for mid-infrared frequencies) and silicon dioxide (positive epsilon material). Note that this configuration (materials with ε1 > 0 and ε2 ≈ -ε1 < 0 joined together: sandwiched or positioned next to each other) is typical for a near-field super-lens. The difference between the near-field super-lens shown in Figure 6 and the "ideal" super-lens shown in Figure 5 is that the ideal also requires a material with a negative value of magnetic permeability.

Summary and Conclusions While it is difficult to pinpoint the exact applications that metamaterials will find in advanced aerospace industry, metamaterials possess several features that uniquely suit them for aerospace applications. First, they enable miniaturization of a variety of optical components. Making space-born devices small and light-weight is essential. These opportunities have been covered in detail. Second, metamaterials enable new modalities for sub-diffraction imaging: super-lenses, hyperlenses, and far-field superlenses. Those modalities dispense with the near-field scanning microscopes, which are complex, slow-scanning, large devices that are not appropriate for advanced aerospace platforms. Harvesting infrared photons, whether from coherent laser sources on Earth (for guidance, energy recharging, and so forth), from thermal Earth glow, or from the stars, is likely to be important for aerospace platforms. Metamaterials offer unique opportunities for making efficient wavelength-tunable, wide-angle absorbers. As discussed in the numerous examples in this report, metamaterials are going to revolutionize the way light is captured, manipulated, and used for imaging. Although metamaterials are still an academic area of research, these examples illustrate that there is great potential for practical applications.