An undergraduate at the University of St. Andrews has overcome a major hurdle in the development of invisibility cloaks by adding an optical device into the cloak’s design that not only remains invisible itself, but also has the ability to slow light. The optical device, known as an “invisible sphere,” has the potential for an invisibility cloak wearer to move around among ever-changing backgrounds of a variety of colors. Under the guidance of professor Ulf Leonhardt, Janos Perczel determined the potential of the invisible sphere, then fine-tuned it to be a suitable background for cloaking. Traditional invisibility cloaks work on the basis of bending light around a target object to be concealed, preventing the light from hitting the object and revealing its presence. Much like water flowing over a rock in a riverbed, the bent light engulfs the object and carries on its path as if nothing were there. Light, however, can be accelerated only to a speed faster than it would travel in space under certain conditions, restricting invisibility cloaks to work in a limited part of the spectrum – essentially, only one color. This proved problematic if the invisibilty cloak wearer wanted to move throughout a space because the scenery would distort and reveal the person under the cloak. (a) Shown is a typical light trajectory (red) within an invisibility cloak. The cloak’s color is related to the speed with which light must propagate in a given region – purple and orange correspond to the fastest and slowest speeds, respectively. The light rays enter the device, complete a loop, bounce off the spindle-shaped mirror twice (dark cyan) and leave the cloak with its original direction restored. (b) A closer view of the vicinity of the inner region of the cloak. Objects placed within the white region are invisible. Figure reprinted with permission from J. Perczel et al (Aug. 9, 2011). Invisibility cloaking without superluminal propagation. New J Phys 13, 083007. ©2011 by the New Journal of Physics. Perczel discovered that, when all of the light was slowed down with an invisible sphere, it did not need to be accelerated at such high speeds and could work in all parts of the spectrum. “Our work shows that invisibility is possible without superluminal propagation,” Perczel said. “This result clears a major theoretical hurdle in the development of invisibility cloaks and will hopefully inspire further research in this direction.” The breakthrough research was published in the Aug. 9 issue of New Journal of Physics (doi: 10.1088/1367-2630/13/8/083007). In the long term, Perczel said his findings might help the practical implementation of invisibility cloaks. “As the next step, the material parameters of the cloak would have to be optimized to make the implementation of the device feasible. Then the optimized material parameters would have to be engineered into the appropriate metamaterials to obtain a functioning cloak in the lab.”