Broadband Array of Invisibility Cloaks in the Visible Frequency Range
Affiliation: Dept. of Physics Astronomy and Geosciences, Towson University, MD, USA
Ever since the first experimental demonstrations in the microwave and visible ranges [1,2], invisibility cloaks have stimulated progress in the fields of metamaterials and transformation optics. Very recently, Farhat and co-workers  suggested that arrays of invisibility cloaks may have interesting electromagnetic properties, and suggested some potential applications in noninvasive probing, sensing and communication. Our team, Vera Smolyaninova and Kurt Ermer from Towson University, and Igor Smolyaninov from the University of Maryland demonstrated the first experimental realization of an invisibility cloak array.
Our experiment is based on the recent demonstration of broadband invisibility cloak, which relies on a curved waveguide mimicking the metamaterial properties necessary for cloaking . Since a gap between a gold-coated spherical lens touching a gold-coated planar glass slide provides a good approximation of the required waveguide shape, such geometry can be easily transformed into a large array of broadband invisibility cloaks using commercially available microlens arrays. This work is reported in the May issue of the New Journal of Physics . In the experiments, conducted at Towson University, very large arrays of roughly 25000 cloaks were used to “hide” approximately 20% of the surface area. This is the first experimental arrangement which lets you study mutual interactions of a very large number of invisibility cloaks.
Igor Smolyaninov of University of Maryland
Unlike the so-called “carpet cloaks” which hide objects on the metallic mirror background, every cloak in our array guides light around the cloaked area. While the former approach is akin to rendering bumps on a carpet invisible by allowing them to blend in with the carpet, classical cloaking concentrates on enabling light to flow around an object. Typically, such classical cloaks [1,2] require sophisticated metamaterial nanofabrication. Each material has its own refractive 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. Refraction occurs as electromagnetic waves 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.
Kurt Ermer of Towson University
Unlike natural materials, metamaterials are able to produce the index of refraction ranging from very large positive values of the order of 100 to less than zero. In particular, artificial metamaterials needed for cloaking must have the index of refraction, which varies from zero to one. Unfortunately, such artificial metamaterials have very large losses. In our cloak array the precisely tapered shape of the waveguide around each cloak alters the refractive index in the same way as in metamaterials, gradually increasing the index from zero to 1 along the curved surface of each microlens. Since light propagates mainly through the air gap, losses in this design are very low, and the resulting structure is broadband. It works across the whole visible light spectrum.
Theoretical work for the design was led by the University of Maryland, with Towson University leading work to fabricate the device and demonstrate its cloaking properties. The cloaking array device is formed by two gold-coated surfaces, one surface being a commercially available microlens array, and the other a flat glass slide. Individual cloaks in the array were separated by about 30 microns, or roughly the width of a human hair, so that a 5 by 5 millimeter squared microlens array would make approximately 25000 individual invisibility cloaks. Instead of being reflected as normally would happen, the light flows around each cloak and shows up on the other side, like water flowing around an array of stones.
[Click on image to see a higher resolution version]
Building and studying the arrays of invisibility cloaks offers more refined experimental tools to test individual cloak performance. Compared to the characterization of individual cloaks, the angular performance of cloak arrays appears to be more sensitive to cloak imperfections. For example, cloak arrays perform better when light is sent in along the row directions. These findings may be useful in such related areas as acoustic and surface-wave cloaking, as well as in the potential practical applications listed above.
On the other hand, since light is “stopped” near each cloak, and the cloak radius depends on the light wavelength, the cloak array produced in our study may be used in the spectrometer on the chip applications. The “trapped rainbow” effect observed near each cloak  may find applications in such fields as biosensing and testing for genetic decease. Stopping light at the cloak boundary leads to considerable enhancement of fluorescence near each cloak in the array .
The work was funded by the National Science Foundation.
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 “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.
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 “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 103, 213901 (2009). Abstract. 2Physics Article.
 “Experimental demonstration of a broadband array of invisibility cloaks in the visible frequency range”, V.N. Smolyaninova, I.I. Smolyaninov, and H.K. Ermer, New J. Phys. 14, 053029 (2012). Full Article.
 “Trapped rainbow techniques for spectroscopy on a chip and fluorescence enhancement” V.N. Smolyaninova, I.I. Smolyaninov, A.V. Kildishev, and V.M. Shalaev, Applied Physics B 106, 577-581 (2012).Abstract. arXiv:1101.336.