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Invisibility cloaks, Prepared by : Sameh Hamzawy Egypt

This document summarizes the experimental demonstration of an array of 2-D broadband invisibility cloaks that operate in the visible light spectrum. The array was fabricated by coating a microlens array with a thin gold film and pressing it against a gold-coated glass slide. Testing showed the cloaked areas appeared as dark circles surrounded by rings when visible light was passed through. However, the angular performance decayed due to a reduction in symmetry from the light not coinciding with the main axes of the array. This was the first experiment to realize an array of invisibility cloaks operating in the visible frequency range.

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Experimental demonstration of a broadband array of invisibility cloaks in the visible frequency range
 What are Metamaterials ?  History of Metamaterials  Principle and Fabrication  Applications
May 1959
What are Metamaterials ?  Metamaterials are artificial materials engineered to have properties that may not be found in natural materials.  They gain their properties not from their composition, but from their exactingly-designed structures.
 Theoretical design by Victor Veselago in 1968.  Predicted the possibility of opposite properties for materials by getting negative refractive index. V.G.Veselago  Dr. John Pendry showed practical method of making metamaterials in 1999 .  In 2009,Vera Smolyaninova and her colleagues at Towson University and the University of Maryland have been built a 2-D invisibility-cloak array which was the first of its kind and operates in the visible frequency range. Dr. John Pendry V.Smolyaninova
Metamaterials can :  create negative index of refraction  guide wave in desired direction
Fabrication Process:  As a first fabrication step, microlens array was coated on the microlens side with a 30 nm gold film.  The array was placed with the gold-coated side facing down on top of a flat glass slide coated with a 70 nm gold film.  Two gold-coated surfaces were pressed against each other using a mechanical arrangement.  Figure(b) shows the deal case scenario light would propagate through such an array without scattering.
 When a 514nm light propagates through the array, the cloaked areas appear as dark circles surrounded by concentric rings in the experimental images.  However, the angular performance of this dense hexagonal cloak array clearly shows signs of decay/damage  This decay may be due to the light direction does not coincide with one of the three main symmetry axes of the array which leads to a reduction in the symmetry of the cloaking arrangement
 This paper reported the first experimental realization of an array of 2-D broadband invisibility cloaks that operates in the visible frequency range.  The broadband performance appears to be similar to the performance of individual cloaks in the array.  Almost 20% of the surface area is cloaked in this experiment,  the angular performance of a dense array shows signs of decay/damage due to a reduction of the symmetry of the cloaking arrangement.
• V.N Smolyaninova, I I Smolyaninov, and H K Ermer1, New Journal of Physics 14 (2012) 053029,1367 2630/12/053029. • Farhat M, Chen P-Y, Guenneau S, Enoch S, McPhedran R,Rockstuhl C and Lederer F 2011 Phys. Rev. B84 235105. • Smolyaninov I I, Hung Y J and Davis C.C 2008 Opt.Lett.33 1342 44.
   Although we have thorough knowledge of theoretical concept of invisibility but we have not enough practical implementation of these concepts. Security concern is another serious issue. The tiny structures embedded in the metamaterial would have to be smaller than the wavelength of the electromagnetic rays you wanted to bend.