Invisibility cloaks might actually make the objects they aim to hide more visible, according to researchers in the US. While existing cloaking concepts might have the potential to render objects invisible to specific electromagnetic frequencies, a recent study has shown that, when integrating over the entire spectrum, the combined scattering of the cloak is always greater than the original uncloaked object. When exposed to short broadband pulses, these cloaks might therefore be turned instead into easier-to-see "beacons". The team proposes two solutions to this problem: a passive approach using thin shells of superconducting material; and an active solution based on metamaterials.

The topic of practical invisibility cloaks – a staple of much fantasy and science fiction – has received a great deal of scientific and media interest of late, especially the possibility of achieving cloaking at visible light frequencies. One promising avenue of enquiry has been with metamaterials, an appropriately designed shell of which can be used to drastically suppress the scattering of light from an object (for a given wavelength), making it almost undetectable. A successful demonstration of this principle, which rendered an object invisible to microwaves, was undertaken in 2006.

Brightly scattered

According to Andrea Alù and his colleague Francesco Monticone of the University of Texas at Austin, most cloaking techniques used today, including popular ones such as transformation cloaks and plasmonic cloaks, are fundamentally limited by causality and passivity to actually scatter more than the uncloaked object, if you integrate over the entire spectrum, instead of looking at just the wavelength being cloaked. "This means that if you excite the cloak with a pulse, you would actually see it more easily than the uncloaked object it is trying to hide," says Alù. The researchers go on to explain that, apart from the scientific significance of solving the scattering problem, it is equally important for a variety of situations – from warfare to commercial uses – where it is essential that a cloaked object at a given frequency does not become a beacon in a range of the other frequencies.

In the new work, the researchers first looked at three different basic types of passive cloaks: a plasmonic cloak; a mantle cloak; and a transformation-optics cloak. The plasmonic cloak showed the most scattering, followed by the mantle cloak with slightly less scattering and the transformation-optics cloak showed the least scattering, but overall they found that all three cloaked objects scattered drastically more than the uncloaked object, over a range of frequencies.

Cloak and dagger

Nonetheless, the team has used its results to propose a number of possible workarounds to this global-scattering issue. The first approach uses passive, and suitably tailored, diamagnetic or superconducting thin shells, providing up to a 25% reduction in the integrated scattering by providing a near-zero permeability for static magnetic fields. A second, active approach would instead use metamaterials with specifically positioned, powered amplifiers. Current cloaking designs have a fundamental constraint on the frequency dispersion of their passive components (described by Foster's reactance theorem) in which the impedance of passive surfaces always grows with frequency, resulting in narrow bandwidths of cloaking and an increased global scattering. By including operational amplifiers in specific positions along the cloaking surface, the team believes it should be possible to break this limit, creating a surface impedance that decreases with frequency, allowing for cloaking over a significantly larger bandwidth.

"I don't think that the paper asks the right question," comments Ulf Leonhardt, a physicist from the University of St Andrews in the UK, who argues that while perfect cloaking may be physically impossible, imperfect invisibility might be perfectly sufficient. In a hypothetical perfect cloak, the speed of light would need to be infinite across all frequencies to create the effect that the light had gone around the cloaked object in the same time that it would have taken to pass through it. In an imperfect cloak, the light would take slightly longer to cross the cloak object than the equivalent amount of empty space. But, Leonhardt proposes, only very sensitive equipment would be able to detect this. "In a scattering analysis, such as the present study," he adds, "the difference between free-space propagation and propagation with the device is considered. If the light takes longer, then this amounts to a big difference that, however, is just an artefact of the analysis. It would tell you that the cloaking device performs rather badly, whereas in reality it works just fine."

The research is published in Physical Review X.