UT ECE professor Andrea Alù and colleagues at the UT Applied Research Labs have successfully created a cloaking device capable of hiding a 3D object in free space from microwaves. To create the device, the team used a plasmonic metamaterial shell to cover the object being cloaked. The cloak, based on a plasmonic metamaterial, can hide a cigar-sized cylinder from microwaves – it currently only operates for one microwave polarization.

The concept of invisibility cloaks was introduced by Alù and Engheta in 2005, and the first experimental realization, based on a different technique, was presented in 2006 by a team led by David Smith at Duke University in North Carolina, US. This experiment, and following ones, were limited to cloaking a very narrow frequency around an area a few centimetres in diameter inside a closed waveguide. The device was based on a metamaterial comprising an array of resonators that altered the electrical permittivity and magnetic permeability throughout the cloak. Variations in these properties resulted in the microwaves bending round the hidden space like water around a stone, albeit only in 2D and inside a waveguide.

Since then, there has been a great deal of research into invisibility, with one goal being to develop a cloak that can hide a macroscopic object over the broad range of visible light frequencies and in 3D. Last year there was a big step towards this goal when Martin Wegener and colleagues at the Karlsruhe Institute of Technology in Germany developed the first 3D cloak, operating in the near-infrared. But this was a flat, carpet cloak, whereby the hidden object had to be placed on a surface, with the cloak itself laid on top. Ideally, a 3D cloak would allow an object to be positioned away from a surface, in free space.

Now Andrea Alù and his colleagues have realized just such a cloak, operating in free-space and for all angles of incidence and observation. In their design, the wave scattered by an object is cancelled precisely by an exterior metamaterial conformal shell. Plasmonic metamaterials have special properties at certain frequencies where the electromagnetic radiation can excite electron oscillations called plasmons. The shell works because it has a very low permittivity, providing it with a polarization opposite to that of the object. Any scattering off the object is therefore cancelled out, and the object appears transparent.

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