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.
Invisibility cloaks shield the large and visible
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.
However, all visible-light carpet cloaks built so far were demonstrated under a microscope, hiding objects no larger than 100 wavelengths across (about 50 μm). In addition, these cloaks were difficult to make, since they consisted of complex, artificially engineered materials. And they were not portable because the cloak, object and surrounding medium all tended to be made from a single structure.
The latest devices follow on from the work of Yu Luo of Zhejiang University in China and colleagues who realized last year that carpet cloaks can be built from homogeneous – rather than more complex inhomogeneous – materials, as long as those materials are anisotropic. Both devices in fact are built from the naturally occurring crystalline material calcite, the refractive index of which depends on the relative orientation of an incoming light wave’s polarization axis and the calcite’s optical axis.
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