Harry Potter would certainly enjoy the latest development done in the field of metamaterial by Researchers at the U.S. Department of Energy's Ames Laboratory, working with colleagues in Karlsruhe (Germany). They have designed a silver-based, mesh-like material with a negative refractive index for visible light that marks the latest advance in the field of metamaterials, These materials could open the door to a new generation of optical devices such as superlenses able to see details finer then the wavelength of visible light and It may also, ultimately, lead to further breakthroughs in cloaking devices which could hide objects from the human eye. http://www.external.ameslab.gov/final/News/2007rel/metamaterials.htm

This discovery marks a significant step forward from the "invisibility cloak" recently developed by scientists at Duke University and Imperial College in London that operate in the microwave or far infrared regions of the spectrum. The Duke's cloaking device makes a copper disk almost invisible at specific microwave frequencies. http://nanotechweb.org/articles/news/5/10/11?alert=1

Metamaterials are artificially created materials with electromagnetic and optical properties that cannot be found among natural materials. These materials can exhibit negative refraction with quite surprising application implications which are just started to be grasped. The backward-bending characteristic provides scientists the ability to control light. The "fishnet" metamaterial design developed developed by Soukoulis' group at Ames Lab. and fabricated by researchers at the University of Karlsruhe was made by depositing a layer of silver on a glass sheet, covering this with a thin layer of non conducting magnesium fluoride, followed by another silver layer, forming a sandwich 100 nm thick (see picture below). The square holes are roughly 100 nm wide are etched through the sandwich to create a grid, similar to a wire mesh. Scientists have thus fabricated for the first time a negative-index metamaterial with a refractive index of -0.6 at the red end of the visible spectrum for light with a wavelength of 780 nm. Previously, the shortest wavelength at which a negative refractive index had been demonstrated was 1400 nm.

How does it work? Quite simply, the metamaterial structure creates intrinsic electromagnetic field then has the ability of deflecting other electromagnetic fields around it. A photon of light flying towards the metamaterial is caught in the materials electromagnetic field, pulled around and released in a different direction.

Light propagation in an invisibility device, a model of light deviation, showing a "hole of invisibility" in which it would be likely to hide an object (Courtesy of Ulf Leonhardt, St Andrews University, Scotland)

Practically, such an "invisibility cloak" may or may not be possible? According to Allan Greenleaf, professor of mathematics at the university of Rochester http://www.rochester.edu/news/show.php?id=2718, the wearer of the " invisibility cloak" wouldn't be able to see outside as all of the photons we would normally be looking at would have been deflected by the cloak. So we would see a type of mirror in front of us. Calculations of the mathematician also showed that if the wearer of the "cloak" wanted to telephone with its cell phone, it would be likely to break invisibility... unless invisibility is limited only to one particular wavelength, which is, according to Greenleaf's calculation, theoretically possible. Cool!

Rather than cloaking an inactive object, which does not generate a lot of electromagnetic waves, Greenleaf's mathematics allow solutions for cloaking active devices, such as cell phones, computers, and anything else that generate signals. One way to modify the original constructions and obtain invisibility at all wavelengths is to insert a lining material inside the cloaking device. Or, rather than use a single layers of metamaterials, Greenleaf describes another method: "My group suggests that there's another construction of the cloaking device that we call the double coat. In this case, we basically put the metamaterials on the outside of the device, and we make a similar 'matched' construction inside the device and point it inwards." Using Maxwell's Equations, which describe electric and magnetic fields and how they interact with matter, the mathematical models, with either the lined single coating or the double coating, provides strong confirmation of cloaking. It proves full invisibility with sources (i.e. a cell phone transmitting radiation) and sinks (i.e. a cell phone receiving and absorbing radiation) at every frequency. In the future, if physicists and engineers build lined single-coated or double-coated cloaking devices, then regardless of frequency, the metamaterial devices will allow to cloak any object, not only the static one but even those that propagate signals.

Although the metamaterial research is relatively new, the progress has been extremely rapid and promising in this field. In few months the scaling of artificially constructed structures has already been demonstrated across nearly seven orders of magnitude in frequency, from RF to near IR wavelengths. However commercial applications of new generation of optical and electromagnetic cloaking devices (such as "superlense" for imaging system, small and directive antenna, "invisibility" shield for communication, Terahertz devices, automotive radar.. ) are still a long way off. A lot remains to be done; calculation, simulation and especially the fabrication itself. The difficulties in manufacturing materials at such a small scale might limit the possibility to harness light at even smaller wavelengths and to mass produce metamaterial structures over large-area and complex shape devices.