Transforming Space with Metamaterials: Invisibility Cloak and beyond

Transforming Space with Metamaterials: Invisibility Cloak and beyond

Abstract

Recent progresses in fabrication techniques allow synthetic sub-wavelength structures to be made with high precision. These “metamaterials” are composed of man-made atoms instead of the real ones, giving rise to unprecedented electromagnetic properties such as the negative refractive index. When light is refracted from air into the material, it bends into the seemingly wrong quadrant. One of the latest approaches in getting such an index will be examined through a chiral metamaterial at Terahertz frequencies. It would introduce new devices (e.g. a perfect lens) into the Terahertz window which is important in a wide range of applications ranging from imaging to security. More interestingly, metamaterials can be tuned to have large ranges of permittivity e and permeability m which can be anisotropic and change from one place to another. This index profile essentially changes the light’s perception of the spatial geometry. Just imagine the space as a rubber sheet. The idea is to deform it arbitrarily and the light will travel along the curved space. The coordinate transformation puts forward a new index profile (anisotropic in general) to be constructed using metamaterials. A folded transformation gives us a negative refractive index as a simple case. Among all of these unlimited possibilities in transformation optics, an invisibility cloak is probably the most exotic example. By creating a void in the coordinate transformation, one can hide an object inside and the object will stay undetectable. While the first experimental demonstration was in microwave at a single frequency, scaling down conventional metamaterials to optical frequencies will cause severe absorption. I will show that by choosing a unique class of coordinate transformations, one obtains a new type of cloak - an invisibility carpet. When the cloak is covered on an object sitting on a ground plane, the object stays undetectable without scattering and the observer perceives the whole system as the original ground plane. The new scheme is deeply related to the topology of the singularity within the coordinate transformation. By crushing the object to a sheet rather than a point or a line, it avoids the usage of metamaterials with extreme indices. With further minimization on the anisotropy with the quasi-conformal map, the cloak can be readily made from all isotropic dielectric materials, compatible with the current silicon technology. Comparing to the existing schemes, the invisibility carpet has a reduced functionality yet a broadband performance with relatively low absorption. It makes cloaking at optical frequencies much easier. .