09 Sep 2008
In a follow-up to recent breakthroughs in cloaking devices, a team of Chinese scientists has devised a way to cancel the effects of invisibility cloaks for the first time.
Researchers from Shanghai Jiao Tong University have proposed an anti-cloak that can counteract the negative refractive properties of invisibility cloaks. An anti-cloak placed underneath an invisibility cloak would allow invisible objects to be seen again as well as enable the wearer to see the outside world (Optics Express 16 14603).
"An invisibility cloak makes an object invisible to the outside world, but it also makes the outside world invisible to the object," Huanyang Chen, a researcher in the Department of Physics, told optics.org. "This means that an invisible observer would also be blind. We wanted to give the observer an option to see the outside world by simply putting a layer of anti-cloaking material in contact with the invisibility cloak."
Research into cloaking has hit the headlines repeatedly in the last couple of months with reports in July of the first demonstration of cloaking behaviour in the visible by a team from the University of Maryland, US. This was closely followed by two separate groups of scientists from the University of California, US, building the first optical metamaterial that exhibits negative refraction in three dimensions.
Now, the Chinese team has developed a way to unmask such invisibility cloaks. "Cloaking to achieve invisibility is the ultimate form of light manipulation and has been demonstrated recently to be conceptually possible," commented Chen. "Anti-cloaking is the follow-up to these approaches. Our anti-cloak can successfully defeat the cloaking achieved from transformation optics."
While an invisibility cloak is designed to bend light around an object so that it becomes invisible, the anti-cloak will guide some light into the shielded domain so that it becomes visible.Impedance-matching
The team designed the anti-cloak as an anisotropic negative refractive shell that is impedance-matched to the cloak outside, which has a positive refractive index. The negative refractive index medium serves to cancel the effects of a positive index medium that has the same impedance.
"The anti-cloak annihilates the functionality of the interior part of the invisibility cloak, and effectively shifts the enclosed region outwards to make contact with the outer part of the cloaking shell that is not cancelled. This leads to a finite cross section," explained Chen.
According to Chen, the anti-cloak can be designed to operate at any frequency and contains many small metallic objects embedded in an insulating medium. The metallic objects have special shapes that are purposely designed to give the required response to external electric and magnetic fields.
The group admits that engineering a suitable anti-cloaking material is the key problem it faces and that commercialization is still a way off. "The research we are conducting shows what is possible in principle," concluded Chen. "The commercialization of these new ideas requires further research."
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