Invisibility cloaks and how to use them

The "invisibility cloaks" being made in labs today can hide objects when viewed from a wide range of directions and in visible light – both considered implausible developments when the first working invisibility cloak was demonstrated just four years ago. But the technology that makes objects vanish looks set to be more useful for the safety of offshore structures and for unlocking cosmological secrets than for would-be Harry Potter impersonators.

In 2006, John Pendry's team at Imperial College London made the news with a design for a cloak that could steer light around an object to render it invisible. Within months a team led by David Smith of Duke University in Durham, North Carolina, had built such a device using exotic "metamaterials" – materials with unusual electromagnetic properties that are not found in nature.

But that first cloak could only hide two-dimensional objects viewed from specific directions – and only if they were "viewed" using one particular microwave frequency. Producing a cloak to hide objects from visible light, which has a wavelength several orders of magnitude smaller than microwaves – let alone cloaking objects when viewed from any direction – seemed a more remote possibility.

Just four years later that's no longer the case. "While full cloaking has not been achieved, it shows promises in the right direction," says Ulf Leonhardt at the University of St Andrews, UK.
Carpet trick

Last year, physicists at the University of California, Berkeley, and Cornell University in Ithaca, New York, independently built optical-frequency cloaks. These were so-called carpet cloaks, made from silicon, which were placed over the object to be cloaked. The object created a bump in the carpet, but the carpet appears flat when light arriving from a specific direction reflects off the surface.

For now, such technology can cloak only objects with a surface area of a few square micrometres and a few hundred nanometres deep. But "in principle, you can make the [cloaked] object larger and larger", says Thomas Zentgraf, a member of the Berkeley team.

Another limitation of the technology – that it works for specific viewing angles only – is already being overcome. Earlier this year, Tolga Ergin of the Karlsruhe Institute of Technology in Germany and colleagues demonstrated a version of the technology that could hide an object from view from a wider range of directions, bringing 3D cloaking a step closer. They arranged photonic crystals in a woodpile-like stack, filling the gaps between the crystals with varying amounts of a polymer to control the refractive index of the metamaterial. This changed the refractive index to a differing degree across the metamaterial, allowing it to mask a bump in a gold foil over a wide viewing angle of about 60 degrees.

"We are optimistic that we can do this [for any viewing angle] in a few years," says Zentgraf.
Waves and event horizons

But even 2D cloaking technology could have real-world uses. Stefan Enoch at the Fresnel Institute in Marseille, France, and colleagues have shown that metamaterials could guide waves around offshore structures, protecting them from storms or tsunamis.

Meanwhile, metamaterials could also shed light on black holes. In 2008, Leonhardt and his team showed how to mimic an event horizon in the lab.

If the medium through which an electromagnetic wave is propagating is moving as fast as the wave itself, the wave is effectively trapped and cannot escape the medium. This has the same effect as a black hole's event horizon, the point of no return for light: an observer outside an event horizon could see nothing inside, as no light can escape the black hole's gravity to cross the horizon to the universe outside.

To mimic this, Leonhardt's team fired laser pulses into a specially fabricated optical fibre. The pulses were designed to modify the fibre's optical properties, so as the laser pulse travelled along the fibre, the change in the fibre's properties moved along it at the same speed. It was as if a virtual fibre was moving at the speed of light, effectively trapping the light

A black hole emits so-called Hawking radiation, and theory says that Leonhardt's team laboratory analogue should do so too, albeit at levels too small to be easily detected yet. Even Harry Potter's cloak wouldn't be capable of that.
References: The two optical frequency cloak papers: University of California, Berkeley study, Cornell University study; Ergin's 3D cloaking study: Science, DOI: 10.1126/science.1186351; Leonhardt's black hole study: Science, DOI: 10.1126/science.1153625