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New steps towards a real invisibility cloak

The great science fiction writer Arthur C. Clarke famously noted the similarities between advanced technology and magic. This summer on the big screen, the young wizard Harry Potter will once again don his magic invisibility cloak and disappear. Meanwhile, researchers with Berkeley Lab and the University of California (UC) Berkeley will be studying an invisibility cloak of their own that also hides objects from view.

A team led by Xiang Zhang, a principal investigator with Berkeley Lab’s Materials Sciences Division and director of UC Berkeley’s Nano-scale Science and Engineering Center, has created a “carpet cloak” from nanostructured silicon that conceals the presence of objects placed under it from optical detection. While the carpet itself can still be seen, the bulge of the object underneath it disappears from view. Shining a beam of light on the bulge shows a reflection identical to that of a beam reflected from a flat surface, meaning the object itself has essentially been rendered invisible.

Previous work on invisibility cloaks, including at Duke and UC Berkeley, only worked for light that human eyes can't see, like radio and microwaves. But new research shows silicon nanoparticles, as opposed to aluminum or other meta-materials, can work at shorter wavelengths - even in visible light.

While metallic metamaterials have been successfully used to achieve invisibility cloaking at microwave frequencies, until now cloaking at optical frequencies, a key step towards achieving actual invisibility, has not been successful because the metal elements absorb too much light.

"Our optical cloak not only suggests that true invisibility materials are within reach, it also represents a major step toward transformation optics, opening the door to manipulating light at will," said Xiang Zhang, director of UC Berkeley's Nanoscale Science and Engineering Centre.

The cloaks reflect and bend light in a way that fools the observer into perceiving a flat surface. The cloaks' nanometer-size silicon materials are spaced apart at varying densities, so light flows around the bulge without distorting the view. A person looking head-on at the bulge would see only a flat plane mirror.

The main problem is scale - both cloaks are wee things that might obscure little more than the eye of a needle. The Cornell cloak works for light that's just barely visible to humans; the Berkeley cloak only works for wavelengths barely outside what humans can see.

Zhang said it was easy to make and should be easy to scale up into visible light, however.

"Our next goal is to realise a cloak for all three dimensions, extending the transformation optics into potential applications," he said. Applications like sneaking around where one is not allowed, perhaps?