The wizards invisibility cloak
Research groups all over the world have spent years trying to research the possibility to bend light around objects so that they appear not to be there. As a sort of “invisibility cloak”. So far – this has possible only in the world of fairy tales or science fiction – as in Harry Potter or Star Trek. But now, researchers appear to come some way on the road to invisibility. The prestigious Physical Review Letters of American Physical Society will publish an article based on a collaboration between researchers at KTH (The Royal Institute of Technology) in Kista and Zhejiang University in China.
“We feel excited! It is always a good thing that our research is acknowledged by the research world, says the research team leader Min Qiu at KTH ICT (KTH School of Information and Communication Technology).”
Researchers in UK and USA have recently taken one step toward reproducing Harry Potter’s invisibility cloak, and have shown that a cloak made of metamaterials and shaped into a column could make the object inside completely invisible. The Swedish-Chinese collaboration has resulted in a theoretical study affirms that the proposed ideal column design will allow perfect invisibility, if metamaterials can be made to the right specifications.
“We are not the one who came up the ideas of realizing an invisibility cloak, Min Qiu explains. We have theoretically proved that the proposed invisibility cloak is truly perfect, for the given wavelength. Another important result is that the perfect invisibility is not really feasible as small distortion will give rise to big problems: the hidden object might be seen.”
The key is that the metamaterials force light to follow a particular path. “When electromagnetic waves pass through the invisibility cloak, the cloak will deflect the waves, guide them around the object, and return them to the original propagation direction without perturbing the exterior field,” the physicists write. As the authors say, “a cloak with the ideal material parameters is a perfect invisibility cloak.” The researchers have showed that a cloak made to ideal specifications could render an object (or wizard) hidden inside perfectly invisible.
“It is theoretical so far, Min Qiu points our. A rough demonstration of invisibility cloak was done last year by Prof. David Smith’s group at Duke University, USA. The cloak works at microwave region, not visible light yet. And their experiment is flawed as they did not really make an real invisibility cloak, but a simplified version of it, which can not truly hide an object.”
There are a few hurdles before we can manufacture what would surely be first on every child’s (and criminal’s) wish list. The most daunting is that if the metamaterials had even the slightest flaw, or part of the cloak removed, light would scatter off the cloak, rendering it visible. For instance, with a thin layer moved from the inner wall of the cloak, the wizard inside would be visible though appear only as a thin line, and the background would be slightly distorted. As more of the inside is removed, the wizard would become more apparent and the background would become more distorted. Physicists haven't yet worked out exactly how these distortions would appear to human eyes.
“We have not done any experiments yet, though we are thinking of it. And it is really an engineering challenge. The current proposals of invisibility cloaks are still non-realistic. We are now working at new ways of designing realistic cloaks, which can be possibly made in the practical world. It is not an easy task, for sure, Min Qiu concludes”
The research group
Min Qiu is the team leader,and Min Yan and Curtis W Neff are both post-docs. Zhichao Ruan, is a joint Ph.D. student between KTH and Zhejiang University. He spent his early part of Ph.D. study in China, and since 2005 with KTH ICT. The team works within the lab Photonics and Microwave Engineering.
The research group is interested in all kind new optical materials including photonic crystals and metamaterials, and their applications. When the proposal of invisibility cloak using metamaterials came across, in particular, after the experiments in Science (mentioned above), the group decided to have a look on their optical properties, and to see if we can apply them for other optical applications.
"Ideal Cylindrical Cloak:Perfect but Sensitive to Tiny Perturbations," in the 14 September 2007 issue of Physical Review Letters (Vol.99, No.11):
Science Daily: http://www.sciencedaily.com/releases/2007/09/070905203843.htm
Min Qiu’s homepage http://web.it.kth.se/~min/
The lab Photonics and Microwave Engineering http://www.imit.kth.se/info/OPQ/FMI/Standard.php