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BBC: Current invisibility techniques use metamaterials that effectively cloak an object from only very specific wavelengths of electromagnetic radiation.The materials are passive in that they require no additional energy. However, they also produce a greater scattering of light across the spectrum than does the uncloaked object. Andrea Alù of the University of Texas at Austin and his colleagues have proposed an active system that significantly increases the range of wavelengths at which cloaking is possible. Their system would use amplifiers to cover the surface of an object with an electrical field that would absorb and diffuse incoming light at most wavelengths. It would also be much thinner than the metamaterials used in passive systems. The researchers have not yet tested such a device but the proposal has received praise from some of the researchers responsible for the previous passive invisibility devices.
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"Active" invisibility cloak may work across more wavelengths Free
12 November 2013