New technique to detect invisibility cloaks

Invisibility cloaks have become one of the most prominent advances among the wide range of applications in the field of metamaterials. Until now, most of the effort in the science of cloaking has been devoted to achieving practically achievable cloak designs and improving the effectiveness of these devices. However, little attention has been paid to the opposite side of the technology: the development of more effective techniques for the detection of these cloaking devices.

And this is where a new study focuses, which will rethink the design of these layers, since it will significantly facilitate their detection. And all thanks to diffraction tomography.

A team made up of, among others, Carlos García Meca, Research Director of the DAS Photonics company, and Francisco J. Díaz-Fernández, from the Nanophotonics Technology Center (NTC) at the Polytechnic University of Valencia in Spain, promises to revolutionize the design of the so-called invisibility cloaks. With his proposal, these layers will no longer be so invisible. And the key is apparently simple: apply diffraction tomography techniques to detect these devices.

On the applications of the proposal that emerged from the NTC laboratories, the Valencian research team highlights that it would cover multiple fields. “One of them would be the ability to ensure the proper use of the electromagnetic spectrum to allow the detection of the presence of hostile agents, despite the use of countermeasures by them to avoid it. Consider, for example, a soldier who uses an invisibility cloak to hide a small observation post and go unnoticed by standard detection methods. The technique that we have developed would make it possible to identify him”, explains Carlos García Meca.

Much more information thanks to tomography

Until now, the evaluation of the benefits of invisibility cloaks is based exclusively on adding all the energy that the cloak re-emits when it is illuminated. “What is done is to estimate all that energy re-emitted in all possible directions, so that the resulting value determines the performance of the layer. If the number is very low, the layer is considered very good; if it re-emits a lot, it doesn’t make the object you want to hide invisible,” explains Fran Díaz, a researcher at the UPV’s Center for Nanophotonic Technology.

The proposal presented in their study by the NTC and DAS Photonics team allows for much more information about the layers. The tomography reveals the optical properties of the analyzed area. Using different angles of illumination, this technique intelligently processes previously unknown information (phase and distribution of the scattered field), thus obtaining a map of the refractive index of the object.

“In this way, we achieve a drastic improvement in the detection sensitivity, and we can even obtain images of the invisibility cloaks, with their size and shape. All this makes the layers more easily detectable. For this reason, with this technique, the invisibility cloaks are no longer so invisible”, emphasizes Fran Díaz.

According to the Valencian researchers, this method could also be extended to detect acoustic invisibility cloaks.

Research team members. (Photo: Vicente Lara Saez / AC UPV)

More applications in electronic warfare and biomedical imaging

Since diffraction tomography represents a much more demanding test for evaluating the performance of invisibility cloaks, this new detection technique could lay the foundations for a new design and characterization standard for this type of device.

Furthermore, this new finding opens up a range of potential benefits in a variety of fields, from fundamental advances in the science of cloaking, to technological applications in the realm of signals intelligence and electronic warfare, and even in the field of biomedical image. In this sense, the study carried out by researchers from DAS Photonics and the UPV suggests that invisibility cloaks based on the so-called “dispersion cancellation” could be applied to improve the resolution of tomographic techniques when seeking to obtain images of groups of small particles.

The study is titled “Imaging Cloaked Objects: Diffraction Tomography of Realistic Invisibility Devices”. And it has been published in the academic journal Laser & Photonics Reviews, where it has also been selected for the internal cover of the magazine. (Source: UPV)