Large Broadband Invisibility Cloak for Visible Light
Vera Smolyaninova (Towson University)
[This is an invited article based on recent work of the authors. -- 2Physics.com]
Authors: Vera Smolyaninova1 and Vlad Shalaev2
1Dept. of Physics Astronomy and Geosciences, Towson University, Towson, MD, USA
2Birck Nanotechnology Center, School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN, USA
Most researchers believe that sophisticated artificially engineered materials are required to build an invisibility cloak. Such “metamaterials” exhibit high losses and work for only one color. The resulting invisibility cloaks are tiny, and cannot hide anything if another color of light is used. Our team, Igor Smolyaninov from BAE Systems, Vera Smolyaninova from Towson University, Alexander Kildishev and Vlad Shalaev form Purdue University demonstrated a different approach to cloaking.
Vlad Shalaev (Purdue University)
Instead of sophisticated metamaterials, we used a waveguide, which is curved to mimic the metamaterial properties. This approach leads to all-color invisibility cloak with much lower losses. As a result, we built a large optical cloak, which is about hundred times larger than the cloaks built previously. This “see-through” cloak bends light around itself and thus differs from the “invisibility carpet,” which camouflages bumps on a metal surface. We believe that further size increase is possible, and that the same technique may be applied to other tasks, which require the use of metamaterials, such as building new “hyperlenses” which considerably surpass the resolution limit of conventional lenses.
This work is reported in the May 29 issue of Physical Review Letters . In the experiments, conducted at Towson University, electromagnetic cloaking is achieved using a specially tapered waveguide. An area with a radius ~100 times larger than the wavelengths of light shined by a laser into the device has been cloaked, an unprecedented achievement. This is the first experiment on optical cloaking performed with normal visible light.
Previous experiments with metamaterials, which require complex nanofabrication, have been limited to cloaking regions only a few times larger than the wavelengths of visible light [2,3]. The new design is a far simpler device: waveguides represent established technology - including fiber optics - used in communications and other commercial applications. Because the new method enabled us to dramatically increase the cloaked area, the technology offers hope of cloaking larger objects. All previous attempts at optical cloaking have involved very complicated nanofabrication of metamaterials containing many elements, which makes it very difficult to cloak large objects. Here, we showed that if a waveguide is tapered properly it acts like a sophisticated nanostructured material. The waveguide is inherently broadband, meaning it could be used to cloak the full range of the visible light spectrum. Unlike metamaterials, which contain many light-absorbing metal components, only a small portion of the new design contains metal.
Igor Smolyaninov (BAE Systems)
Theoretical work for the design was led by Purdue, with BAE Systems and Towson University leading work to fabricate the device and demonstrate its cloaking properties. The cloaking device is formed by two gold-coated surfaces, one a curved lens and the other a flat sheet. We cloaked an object about 50 microns in diameter, or roughly the width of a human hair, in the center of the waveguide. Instead of being reflected as normally would happen, the light flows around the object and shows up on the other side, like water flowing around a stone.
This research falls within a new field called transformation optics, which may usher in a host of radical advances, including cloaking; powerful "hyperlenses" resulting in microscopes 10 times more powerful than today's and able to see objects as small as DNA; computers and consumer electronics that use light instead of electronic signals to process information; advanced sensors; and more efficient solar collectors.
Alexander Kildishev (Purdue University)
Unlike natural materials, metamaterials are able to reduce the "index of refraction" to less than one or less than zero. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another. It causes the bent-stick-in-water effect, which occurs when a stick placed in a glass of water appears bent when viewed from the outside. Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material. Natural materials typically have refractive indices greater than one. Metamaterials, however, can be designed to make the index of refraction vary from zero to one, which is needed for cloaking. The precisely tapered shape of the new waveguide alters the refractive index in the same way as metamaterials, gradually increasing the index from zero to 1 along the curved surface of the lens. Previous cloaking devices have been able to cloak only a single frequency of light, meaning many nested devices would be needed to render an object invisible.
We reasoned that the same nesting effect might be mimicked with the waveguide design. Subsequent experiments and theoretical modeling proved the concept correct. We do not know of any fundamental limit to the size of objects that could be cloaked, but additional work will be needed to further develop the technique.
Recent cloaking findings reported by researchers at other institutions have concentrated on a technique that camouflages features against a background. Those works, which use metamaterials, are akin to rendering bumps on a carpet invisible by allowing them to blend in with the carpet, whereas our work concentrates on enabling light to flow around an object.
The work was funded by the ARO-MURI and the National Science Foundation.
 “Anisotropic metamaterials emulated by tapered waveguides: application to electromagnetic cloaking”, I.I. Smolyaninov, V.N. Smolyaninova, A.V. Kildishev, and V.M. Shalaev,
Phys. Rev. Letters, 102, 213901 (2009). Abstract.
 “Metamaterial electromagnetic cloak at microwave frequencies”, D. Schurig, J.J. Mock, B.J. Justice, S.A. Cummer, J.B. Pendry, A.F. Starr, D.R. Smith, , Science 314, 977-980 (2006). Abstract.
 “Two-dimensional metamaterial structure exhibiting reduced visibility at 500 nm”, I.I. Smolyaninov, Y.J. Hung, and C.C. Davis, Optics Letters 33, 1342-1344 (2008). Abstract.