Cloak of Invisibility Advancing Towards Reality

Purdue University (West Lafayette, IN) researchers are working to perfect a "cloak of invisibility" made from nano metamaterials. An apparatus and method of cloaking are described in U.S. Patent Application 20080165442. The application is based on research sponsored by the U.S. Army Research Office under 50342-PH-MUR. Photonic metamaterials are engineered nano-composite structures with optimized responses to electromagnetic fields at optical frequencies that lead to super- and hyper-lens and invisibility cloaks.

The cloak may be designed to decrease scattering (particularly "backscattering") from an object contained within, while at the same time reducing the shadow ("forward scattering"), so that the combination of the cloak and the object contained within it have a resemblance to free space, according to inventors Wenshan Cai, Vladimir M.; Shalaev, Uday K. Chettiar and Alexander V. Kildishev.

The materials used for the cloak may have properties where, generally the permeability and permittivity tensors are anisotropic and where the magnitudes of the permeability and permittivity are less than one, so that the phase velocity of the electromagnetic energy being bent around the cloaking region is greater than that of the group velocity.

Materials having such properties have not been discovered as natural substances, but may be produced as artificial, man-made materials, where the permittivity and permeability are less than unity, and may be negative. Metamaterials, an extension of the concept of artificial dielectrics, were first designed in the 1940s for microwave frequencies. They typically consist of periodic geometric structures of a guest material embedded in a host material. Analogous to the circumstance where homogeneous dielectrics owe their properties to the nanometer-scale structure of atoms, metamaterials derive their properties from the sub-wavelength structure of its component materials. At wavelengths much longer than the unit-cell size, the structure can be assigned parameters that may be used describe homogeneous dielectrics, such as electric permittivity and refractive index.

An object to be cloaked is disposed such that the cloaking apparatus is between the object and an observer. The appearance of the object is altered and, in the limit, the object cannot be observed, and the background appears unobstructed. The cloak is formed of a metamaterial where the properties of the metamaterial are varied as a function of distance from the cloak interfaces, and the permittivity is less than unity. The metamaterial may be fabricated as a composite material having a dielectric component and inclusions of particles of sub-wavelength size, so as to have a permeability substantially equal to unity.

A brief video of how the cloak interacts with light can be found at Purdue University as well as more details on the research.