Experts test cloaking technology

By Paul Rincon
Science reporter, BBC News

The cloak is constructed from advanced "metamaterials"

A US-British team of scientists has successfully tested a cloak of invisibility in the laboratory.

The device mostly hid a small copper cylinder from microwaves in tests at Duke University, North Carolina.

It works by deflecting the microwaves around the object and restoring them on the other side, as if they had passed through empty space.

But making an object vanish before a person's eyes is still the stuff of science fiction - for now.

We've opened the door into the secret garden Prof John Pendry, Imperial College London

The cloak consists of 10 fibreglass rings covered with copper elements and is classed as a "metamaterial" - an artificial composite that can be engineered to produce a desired change in the direction of electromagnetic waves.

Like visible light waves, microwaves bounce off objects, making them apparent and creating a shadow. But at microwave frequencies, the detection has to be made by instruments rather than the naked eye.

New chapter

Water behaves differently. When water in a river flows around a smooth rock, the water closes up on the opposite side. Someone looking at the water downstream would never guess it had passed around an obstacle.

The metamaterial cloak channelled the microwaves around the object like water flows around the rock.

Scientists were able to watch waves bending around the cloak

"These metamaterials have opened a new chapter in electromagnetism. We've opened the door into the secret garden," co-author Professor John Pendry, from Imperial College London, told BBC News.

In the experiment, the scientists first measured microwaves travelling across a plane of view with no obstacles. Then they placed a copper cylinder in the same plane and measured the disturbance, or scattering, in the microwaves.

Next, the researchers placed the invisibility cloak over the copper cylinder. The cloak did not completely iron out the disturbance, but it greatly reduced the microwaves being blocked or deflected.

Hidden from view

"This cloak guides electromagnetic waves around a central region so that any object at all can be placed in that region and will not disturb the electromagnetic fields," explained co-author Dr David Schurig from Duke University.

"There is reduced reflection from the object, and there is also reduced shadow."

In principle, the same theoretical blueprint could be used to cloak objects from visible light. But this would require much more intricate and tiny metamaterial structures, which scientists have yet to devise.

"As an application, it's not clear that you're going to get the invisibility that everyone thinks about - as in Harry Potter's cloak, or the Star Trek cloaking device," said Professor David R Smith of Duke.

Broad applications

John Pendry commented: "There's a rule about the internal structure of the metamaterial: it has to be smaller than the wavelength of radiation. So for radar waves that's 3cm. You can easily engineer something a few millimetres across.

"You go up to optical radiation - visible light - and the wavelength is less than a micron. So your microstructure has to be a few tens of nanometres across. and we're only just learning how to do nanotechnology... maybe in five or 10 years' time you could do this, but not today."

The researchers say that if an object can be hidden from microwaves, it can be hidden from radar - a possibility that will ensure interest from the military.

Professor Pendry said a metamaterial cloak could be manufactured to wrap around a fighter plane or tank. But, he said: "You mustn't demand that the cloak be too thin. Despite the hype around Harry Potter, this isn't anything that flaps around in the breeze; it's more like a shed."

It could be used to hide objects from terahertz waves, a regime that is being exploited for numerous sensing technologies, or even from mobile phone frequencies and magnetic fields.

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