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26/01/2012
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Researchers in the US have, for the first time, cloaked a 3D object standing in free space, bringing the much talked about invisibility cloak one step closer to reality.
 Whilst previous studies have either been theoretical or limited to the cloaking of 2D objects, the University of Texas team has been able to demonstrate how ordinary objects of any size and shape can be cloaked in their natural environment in all directions.
Reporting today in the New Journal of Physics, the researchers used a method known as plasmonic cloaking to hide an 18cm cylindrical tube from microwaves. They did this using a different kind of material known as plasmonic metamaterials – composites of metal and non-conductive synthetics made of nanometre-sized structures.
"When light strikes an object, it rebounds off its surface towards another direction, just like throwing a tennis ball against a wall," explained study co-author Professor Andrea Alu. "The reason we see objects is because light rays bounce off materials towards our eyes and our eyes are able to process the information.
"Due to their unique properties, plasmonic metamaterials have the opposite scattering effect to everyday materials. When the scattered fields from the cloak and the object interfere, they cancel each other out and the overall effect is transparency and invisibility at all angles of observation."
To cloak the 18cm cylindrical tube, the researchers coated it with a shell of the plasmonic metamaterial to make it appear invisible. The system was then tested by directing microwaves towards the cloaked cylinder and mapping the resulting scattering both around the object and in the far field.
The cloak was said to demonstrate optimal functionality when the microwaves were at a frequency of 3.1gigahertz and over a moderately broad bandwidth.
"One of the advantages of the plasmonic cloaking technique is its robustness and moderately broad bandwidth of operation," said Alu. "This made our experiment more robust to possible imperfections, which is particularly important when cloaking a 3D object in free space."
The researchers are now looking to demonstrate the cloaking of a 3D object using visible light. The team believes the technique could be used in a variety of different applications and is now investigating the application of these concepts to cloak a microscope tip at optical frequencies. "This may greatly benefit biomedical and optical near-field measurements," Alu concluded.
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Author
Laura Hopperton
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