Invisibility cloak built for diffusive media

Researchers have developed a method that will allow objects to vanish when placed behind frosted glass or a similar light-scattering material.
The laws of physics prevent an optical invisibility cloak from making objects in air invisible for any directions, colours, and polarisations, said researchers at the Karlsruhe Institute of Technology (KIT), Germany.
"Ideal optical invisibility cloaks in air have a drawback," said Martin Wegener at the KIT Institute of Applied Physics and the KIT Institute of Nanotechnology.
"They violate Albert Einstein's theory of relativity that prescribes an upper limit for the speed of light. In diffuse media, in which light is scattered several times, however, the effective speed of light is reduced. Here, ideal invisibility cloaks can be realised," Wegener said.

The technology takes advantage of how materials such as fog or frosted glass scatter light instead of letting it move in lines as normal, according to 'factor-tech.Com'.
"This property of light-scattering media can be used to hide objects inside. The new invisibility cloaks have a rather simple structure," said Robert Schittny, first author of the study.

In an experiment, Schittny used an extended light source to illuminate a Plexiglas tank of a few centimetres in width from the back. The tank was filled with a white, turbid liquid. Objects inside cast a visible shadow onto the tank wall.
Simple metal cylinders or spheres of a few centimetres in diameter were used as test objects. To hide them, they were first coated with a white dispersion paint, such that the light was reflected in a diffusive manner.

To pass the light around the object, the researchers applied a thin shell made of the transparent silicon material PDMS, to which a certain concentration of light-scattering melamine microparticles was added.
The silicon/melamine shell caused a quicker diffusion than in the environment and, thus, passed the light around the objects. Hence, they did no longer cast a shadow.
Disappearance of the shadow indicates successful cloaking, researchers said.
"We will have to wait a long time for real applications, but with the help of the principle found, it might be possible to produce frosted glass panes for bathrooms with integrated metal bars or sensors against burglary. These sensors or bars would be invisible from the inside and outside," Schittny said.

The results are published in the journal Science.