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Optical Invisibility

Figure 1 - Scattering of light by the objects embedded into 2- and 3-dimensional layered ENZ-metamaterial. The objects are larger than wavelength. a), b) – the scattering on infinite and finite cylinders. c), d) – the cross-section of fig. b. f), g) – scattering by large object of arbitrary shape and the difference between field amplitudes in the case of empty metamaterial and metamaterial with the object.

Since 2006 quite considerable attention is drawn towards realization of cloaking devices, suggesting placing an object into carefully designed camouflage cover. 

Among the large span of existing proposals, periodic shells with alternating refractive index enables incident wave bending around the object and without introducing scattering channels. Another proposal, coined by the name of ‘Carpet Cloak’, relaxes some tight constrains on perfect cloaking conditions and just emulate an object by a flat surface, e.g. hides things under the carpet.

Figure 2 - Scheme of multi-layered cloaking coating. Distribution of amplitude and phase of electric field for different coatings.

Our original proposal on cloaking takes another approach to invisibility and requires just scattering suppression from an object. Foe majority of applications weaker condition of scattering suppression is sufficient and, in contrary to perfect cloaking schemes, does not require extremely complex realizations. Our device utilizes layered metal dielectric composite, operating at the regime of vanishing dielectric permittivity along the layers. This property controls radius of curvature of scattering waves, namely maintain the same profile as of the incident wave. It was shown, that the wave front of transmitted wave does not depend on the size or shape of an object, embedded inside the layered structure. It means, that it is undetectable by an observer, and the invisibility application is achieved this way (7% wave front distortion was estimated in the worst case). 

All-dielectric covers were also investigated. The main impact of this approach is that it does not supper from additional losses, always present in metal-based structures. Our numerical routine utilizes stochastic optimization algorithms and shows 2-fold scattering suppression from bigger-than-a-wavelength object. For this application only 4 layers of dielectric were used.