What invisible objects will actually look like
Over the last few years we’ve covered the development of “invisibility cloaks” using metamaterials – man-made structured composite materials exhibiting optical properties not found in nature that can guide light to achieve cloaking and other optical effects. In 2006, scientists at Duke University demonstrated in the laboratory that an object made of metamaterials can be partially invisible to particular wavelengths of light - not visible light, but rather microwaves. A few groups have even managed to achieve a microscopically-sized carpet-cloak. Now researchers have developed software that can show what such a cloaked object will actually look like.
Although limited invisibility has been achieved so far in the laboratory, it only works for a narrow band of light wavelengths. Nobody has ever made an object invisible to the broad range of wavelengths our eyes can see, and doing so remains a challenge. For this reason it has also been difficult to visualize a cloaked object. Although you might think it is difficult to visualize such objects because they will be invisible, it seems invisibility can be a relative term. It is very likely that any invisibility cloak would remain partly seen because of imperfections and optical effects. Nobody has been able to show what this would look like – until now.
A team of researchers at the Karlsruhe Institute of Technology in Germany has created a new visualization tool that can render a room containing a partially or completely cloaked object, showing the visual effects of such a cloaking mechanism and its imperfections. The software, which is not yet commercially available, is a visualization tool designed specifically to handle complex media, such as metamaterial optical cloaks.
One of the difficulties the researchers had to overcome was the fact that metamaterials – the man-made structured composite materials that can guide light to achieve cloaking - 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 Jad C. Halimeh, a Master of Science graduate of the Karlsruhe Institute of Technology in Germany who wrote and tested the new software as part of his Master's thesis.
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. The images produced by the software 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.
The research team’s paper, Photorealistic images of carpet cloaks, appears in the journal Optics Express.