A new invisibility cloak technology conceals objects in diffusive media such as fog, clouds or frosted glass. Optical cloaks have been challenging to achieve for all situations; most only work at specific colors or polarizations, or from very specific directions. Diffusive media admit light but scatter it, obscuring its source. "This property of light-scattering media can be exploited to hide objects in it," said lead researcher Robert Schittny of the Institute of Applied Physics at Karlsruhe Institute of Technology (KIT). "The new [cloaks] are actually quite simple." Traditional invisibility cloaks cast a shadow on an object (depicted at left), while an optical cloak that uses diffusive light-scattering media does not (depicted at right). Courtesy of KIT. In the experiment, an extended light source back-lit a narrow (just a few centimeters wide) Plexiglas tank that was filled with a white, cloudy liquid. Test objects such as simple metal cylinders or spheres produced visible shadows on the vessel's wall. To disguise this, the researchers first painted them with white emulsion paint, so that they would reflect light diffusely. In diffuse media, the light is scattered several times, which reduces the effective speed of light, so the researchers mixed light-scattering melamine microparticles into a thin shell of transparent silicone material (PDMS). The silicone/melamine mixture causes faster diffusion than in the surroundings, so that the objects no longer cast shadows. "Ideal optical invisibility cloaks in the air ... violate Albert Einstein's theory of relativity, which prescribes an upper limit for the speed of light," said Dr. Martin Wegener, a professor at the KIT Institute of Applied Physics and the Institute of Nanotechnology. This study is proof of principle, Wegener and Schittny said. Real-world applications are still a long way off but could include frosted bathroom windows that obscure built-in security sensors to detect intruders, for example. The work was funded by the DFG Center for Functional Nanostructures. The research was published in Science (doi: 10.1126/science.1254524). For more information, visit www.kit.edu