A practical invisibility cloak, the imaginative stuff of science fiction, fantasy and comic books, has not been invented quite yet. However, it is becoming a realistic possibility thanks to research with electromagnetic meta-materials, which are materials especially designed to bend electromagnetic waves, including light.
The theory behind invisibility is that the process of seeing an object involves receptors in the eyes that are triggered by the light reflected back from that object. The eyes then transmit information to the brain, where it is interpreted as an image. If light flows around an object instead of being reflected back, the object will not reflect light into the eyes to trigger the light receptors. It will therefore become, for all intents and purposes, invisible because the eyes will have no information to communicate with the brain. Consequently, in the search for an invisibility cloak, physicists have been experimenting with ways to use meta-materials to bend light waves.
In 2006, physicists at Duke University in North Carolina discovered a way to bend microwaves around an object using concentric rings of copper split-ring resonators. However, the procedure only worked in two dimensions. Although the shielded object was invisible at eye level, it could be seen by looking into the cylinder from above.
In 2008, Jensen Li and John Pendry of Imperial College, London theorized the possibility of creating a “carpet cloak.” They proposed hiding an object beneath the surface of a smooth material where it would create a bump in the same way that a surface bump is caused by something hidden under a carpet. A layer of metamaterial could then be used to smooth out the bump by reflecting light off the surface as if the bump were not there. Then in 2010, Pendry and colleagues from Karlsruhe University demonstrated the first viable carpet cloaking device. They were able to hide microscopic objects in three dimensions using unpolarized light in the near infra red spectrum by placing a polymer-filled stack of silicon wafers on a reflective gold surface.
However, experiments with a naturally occurring material, calcite, currently seem to be yielding the most promising results. Calcite is birefringent, meaning it bends differently polarized light rays in different ways. Subsequent to Pendry and Li’s carpet cloaking experiments, researchers at the University of Birmingham managed to cloak a paperclip using a lump of calcite. Then in February of 2011, MIT physicists made a roll of paper disappear by placing it in a space beneath a pair of calcite crystal prisms.
While the MIT calcite cloak can only hide objects up to 2mm in height, it is theoretically possible to hide larger objects by using larger calcite crystals, or by using metamaterials in the form of artificial crystals.
Proposed practical applications include the use of transformation optics, the study of the light bending properties of materials, in the design of antennas and optical resonators. While a device powerful enough to cloak the U.S.S. Enterprise is still in the realm of science fiction, there are obviously potential defense and security applications, and research continues.