PORTLAND, Ore. Chinese researchers, fearful that a U.S. invisibility cloak could hide objects from view while also blinding anyone inside, have devised what they call an anti-cloaking layer that solves the latter problem.
In work performed at the Shanghai Jiao Tong University, the theoretical anti-cloak would fit around an object but inside the invisibility cloak, permitting anyone inside to see out by merely pressing the anti-cloak against the invisibility cloak.
However, David Schurig, co-inventor of the invisibility cloak at North Carolina State University, said the anti-cloak layer is unnecessary since invisibility cloaks work only at select wavelengths.
"It is true that the cloak will not only hide you from observers, but will also blind you at those wavelengths since [wavelengths] cannot penetrate the cloak to enable you to see out. But the solution here is to just use other wavelengths of light to see out," said Schurig. "For instance, if you had a cloak that worked at visible wavelengths, then you could just use, say, infrared wavelengths, to see out."
Schurig developed the invisibility cloak in 2006 as a post-doctoral fellow at Duke University. Since then, U.S. researchers have demonstrated a working cloak that shields a five-inch-square area from microwave wavelengths.
A separate group at Duke, led by professor Steven Cummer, subsequently demonstrated a working acoustic cloak that could render submarines invisible to sonar. In addition, researchers working at the Energy Department's Ames Laboratory have demonstrated the world's first cloak for visible wavelengths.
All the cloaking devices, however, only shield objects from observation using specific bands of electromagnetic radiation. No one has yet proposed an approach that would cloak all wavelengths.
The Chinese proposal is theoretical and has yet to be implemented in a working device. It was, however, simulated for a perfect electrical conductor at a wavelength of 150 millimeters (2 GHz). The anti-cloaking layer was simulated using anisotropic optical materials with a negative index of refraction.
An invisibility cloak works by surrounding an object with anisotropic metamaterials with a variable index of refraction by virtue of their permittivity and permeability being less than that of free space. By arranging the metamaterial so that their index of refraction diverts waves upward, around and then down again, the shielded wavelengths are diverted around the object so that it is invisible to observers.
Metamaterials were first hypothesized in 1968 by Russian theorist Victor Veselago, who theorized that a wave could interact with metamaterials in a way fundamentally different from natural materials, which all have positive permeability and permittivity.
All natural materials bend electromagnetic radiation in the same predictable direction: away from a line perpendicular to their surface (away from "normal"). On the other hand, metamaterials substitute periodic mechanical structures that can force a wave to travel a path where it bends toward normal, thereby causing them to be diverted around objects.
Engineered metamaterials are composites that substitute macroscopic objects for atoms in a giant crystalline-like lattice, thereby enabling the pitch of the passive-component arrays to set the affected wavelengths.
For microwave frequencies, simple arrays of R-C-L (resistor-capacitor-inductor) circuits mounted on dielectric materials and positioned in free space can bend microwaves down any specified path.
The Chinese anti-cloaking layer works by using an anisotropic metamaterial that is impedance-matched to the refractive index of the invisibility cloak. By pressing the anti-cloak against the invisibility cloak, some light could be guided inside to allow observers inside to peek out.
The Chinese research was performed in cooperation with researchers at the Hong Kong University of Science and Technology. The Chinese research will be posted online in the Sept. 15 issue of Optics Express.