Researchers at the Universitat Autonoma de Barcelona have recently designed a peculiar material — called a dc metamaterial — that has the property of making objects wrapped in it undetectable to magnetic and very low-frequency electromagnetic fields. The breakthrough brings the dream of "invisibility cloaking" closer to reality and could have important repercussions in both the military and medical fields.

Once physically built, the material will make objects "invisible" by annulling the magnetic field residing inside it, but without altering the exterior field. Objects will therefore become impossible to detect through light at these very low frequencies.

Metamaterials and the quest for invisibility

In order to obtain invisibility, scientists need to manipulate the trajectory of light through the electromagnetic properties of the medium in question. Until no more than a decade ago, scientists thought this to be impossible to achieve; however, recent discoveries have revealed that such manipulation can be possible thanks to the unique properties of a class of exotic materials known as metamaterials.

Metamaterials are built by combining two or more materials at the macroscopic level rather than with a chemical process. The one property that makes them particularly attractive is their negative refraction index.

When electromagnetic waves such as light are directed at a transparent object, part of the beam is reflected by the object, while the remaining part propagates within it. The direction of the beam that enters is modified by an angle that depends on the object's refraction index. While the vast majority of objects found in nature have a positive refraction index, its value for metamaterials is negative, which allows to bend light and other electromagnetic waves in a very different way, making invisibility possible.

Dc metamaterials and their applications

The team's theoretical work brought to the design of a metamaterial consisting in an irregular network of superconductors, which give them specific magnetic properties that can create invisible areas in the magnetic field and in very low frequency electromagnetic fields.

"The theoretical work provides the details for constructing a real dc metamaterial and represents another step towards invisibility," Àlvar Sánchez, who led the research team, explained. "Now comes a very important stage: building a prototype in the laboratory and applying this device to improving magnetic field detection technology."

Invisibility in visible light has not yet been achieved with experiments, but scientists are working with other types of light such as microwaves, low frequency electromagnetic fields such as radio or television waves, and even with the Earth's magnetic field.

Possible applications — apart from the often cited military field, where this technology would be used to prevent detection of ships or submarines — would be in the medical field, where this technology would vastly improve our magnetoencephalographic and magnetocardiographic technologies, in which there is the need to shield out all external magnetic fields.