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Scientists develop invisibility cloak

January 27, 2012 3:09pm
The invisibility cloak - the stuff of movies from "Harry Potter" to "Star Trek" - may be one step closer to reality, after US scientists cloaked a three-dimensional object in free space.
An article posted on the website of London-based Institute of Physics said this is one step further than previous studies that were either theoretical or were limited to the cloaking of two-dimensional objects.
In their study the researchers from the University of Texas at Austin used a method known as "plasmonic cloaking" - using artificial plasmonic metamaterials - to hide an 18-cm cylindrical tube from microwaves, the article said.
"Due to their unique properties, plasmonic metamaterials have the opposite scattering effect to everyday materials," it said.
The paper was published in the IOP and German Physical Society's New Journal of Physics.
Normally, light rebounds off its surface towards another direction when it strikes an object.
Light rays bouncing off materials reach the eyes, and the brain process the visual information.
"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," study co-author Professor Andrea Alu said.
Alu added an advantage of the plasmonic cloaking technique is its "robustness and moderately broad bandwidth of operation, superior to conventional cloaks based on transformation metamaterials."
"This made our experiment more robust to possible imperfections, which is particularly important when cloaking a 3D object in free-space," Alu said.
Microwave testing
In their test, the US researchers cloaked the cylindrical tube with a shell of plasmonic metamaterial to make it appear invisible.
They then directed microwaves towards the cloaked cylinder and mapped the resulting scattering both around the object and in the far-field.
"The cloak showed optimal functionality when the microwaves were at a frequency of 3.1 gigahertz and over a moderately broad bandwidth," IOP said.
On the other hand, the researchers said the shape of the object is irrelevant, saying oddly shaped and asymmetric objects can both be cloaked using this technique.
However, one of the main challenges for the researchers will be to demonstrate the cloaking of a 3D object using visible light.
"In principle, this technique could be used to cloak light; in fact, some plasmonic materials are naturally available at optical frequencies. However, the size of the objects that can be efficiently cloaked with this method scales with the wavelength of operation, so when applied to optical frequencies we may be able to efficiently stop the scattering of micrometer-sized objects," Alu said.
"Still, cloaking small objects may be exciting for a variety of applications. For instance, we are currently investigating the application of these concepts to cloak a microscope tip at optical frequencies. This may greatly benefit biomedical and optical near-field measurements," he added.
Limitations of the technique
A separate article on tech site CNET said there appear to be some limitations on what kinds of materials the technique will work on.
CNET cited cited a description from the University of Texas that the experiment worked on a dielectric object but not on metals.
But it said that in the future, portions of aircraft could be covered or optical instruments could use the material to correct effects.
"We believe that our results pave the way to realistic, practical applications of 3D stand-alone cloaks for radar evasion and non-invasive radio frequency probing," it cited the paper as saying. — TJD, GMA News

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