ABSTRACT
The history of invisibility probably begins with James Clerk Maxwell, who was aware that no magnetic field would be produced outside a toroidal magnet and such a toroid would be magnetically invisible from the outside. The methodology was applied to find electromagnetic cloaks, shells of anisotropic material capable of rendering any object within their interior cavities invisible to detection from outside the cloaks. The perfect cloak ensures that for any incident field, the electromagnetic-scattered field vanishes in the free space external to the cloaking shell, and the total field vanishes inside the free-space cavity of the shell. For bodies with no interior free-space cavities, the formulation is used to derive nonscattering spherical and cylindrical concentrators that magnify the incident fields near their centers. The chapter shows that causality—energy conditions, which the diagonal elements of the relative permittivity—permeability dyadic must obey, imply that incident fields with a finite bandwidth cannot be perfectly cloaked.
