Two independent groups of physicists have built invisibility cloaks that can shield large objects lying on a plane. These “carpet cloaks” are far closer to the intuitive idea of an invisibility cloak than devices previously built, they argue, because they hide objects that can be seen with the naked eye and do so at visible wavelengths. The cloaks are also relatively cheap and easy to make, being constructed from the natural material calcite.
Carpet cloaks were proposed in 2008 by John Pendry of Imperial College, London as a way of extending the operating range of invisibility cloaks, which were mostly limited to microwave wavelengths. These devices are placed over an object sitting on a reflective plane and alter the path of light bouncing off the object in such a way that the light appears to have bounced straight off the plane.
The team used a technique known as transformation optics to design their cloak. They calculated the optical parameters that were needed to transform the space between a small and a large triangle, such that light passing through this space would do so as if it were passing through all of the larger triangle, thereby effectively rendering the smaller triangle – the wedge – invisible. Having calculated these parameters the researchers were able to construct their cloak using conventional lens fabrication, with the cross section of the cloak being equal to the space between the two triangles, minus the top of the larger triangle.
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