Scientists and curiosity seekers who want to know what an object would look like if it were partially or completely cloaked using metamaterials can now get their wish--although only virtually, rather than in real life. Researchers at the Karlsruhe Institute of Technology (Karlsruhe, Germany) have created a new visualization tool that can render a room containing such an object, showing the visual effects of such a cloaking mechanism and its imperfections.1
These images show a museum nave with a large bump in the reflecting floor covered by a metamaterial-based invisibility device known as the carpet cloak. They reveal that even as an invisibility cloak hides the effect of the bump, the cloak itself is apparent due to surface reflections and imperfections. The researchers call this the "ostrich effect" (in reference to the bird's mythic penchant for partial invisibility).
In 2006, scientists at Duke University (Durham, NC) experimentally demonstrated that an object made of metamaterials can be partially invisible to particular wavelengths of microwave radiation. A few other groups, including one at the University of California, Berkeley, have created a microscopic carpet cloak at optical wavelengths. These and other studies have suggested to the popular press that the Hollywood fantasy of invisibility may one day be reality (however, people in the optics field take this idea with a grain of salt).
Visualizing partial invisibility
While such invisibility has been achieved so far in the laboratory, it is very limited. It works only for a narrow band of wavelengths. Nobody has ever made an object invisible across the visible band, and doing so remains a challenge. Another challenge has been visualizing a cloaked object. It is very likely that any invisibility cloak would remain partly visible because of imperfections and optical effects. Up to now, nobody has been able to show what this would look like, even on a computer.
The problem is that metamaterials may have optical properties that vary over their length. Rendering a room with such an object in it requires building hundreds of thousands of distinct volume elements that each independently interact with the light in the room. The standard software that scientists and engineers use to simulate light in a room only allows for a few hundred volume elements, which is nowhere close to the complexity needed to handle many metamaterials such as the carpet cloak, says Halimeh.
So he and his colleagues built the software needed to do just that. Wanting to demonstrate it, they rendered a virtual museum niche with three walls, a ceiling, and a floor. In the middle of the room, they place the carpet cloak--leading the observer to perceive a flat reflecting floor, thus cloaking the bump and any object hidden underneath it.
1. Jad C. Halimeh et al, Optics Express, Vol. 17, Issue 22, October 26, 2009; abstract at http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-17-22-19328.