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[DIV class=headline]Experts create invisibility cloak [/DIV][!--Smvb--]
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[TD vAlign=bottom][!--Smvb--]By Paul Rincon
Science reporter, BBC News [!--Emvb--][/TD][/TR][/TBODY][/TABLE]
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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. [DIV class=bo]
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We've opened the door into the secret garden [!--Emva--][!--Smva--]Prof John Pendry, Imperial College London [!--Emva--][/TD][/TR][/TBODY][/TABLE]
[DIV class=bo]The cloak consists of 10 fibreglass rings covered with copper elements and is a "metamaterial" - an artificial composite that can be engineered to produce a desired change in the direction of electromagnetic waves. This is done by tinkering with the physical structure of the material, not by altering its chemistry. In this case, the precise variations in the shape of copper elements patterned on to the ring surfaces determines their properties. Like light waves, microwaves bounce off objects, making them visible and creating a shadow. But at microwave frequencies the detection has to be made by instruments rather than the naked eye.
New chapter The metamaterial cloak channelled the microwaves around the object like water in a river flows around a smooth rock. When water flows around a rock, the water recombines on the opposite side. Someone looking at the water downstream would never guess it had passed around an obstruction.
[DIV class=bo]"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." The authors admit the cloaking they demonstrated was not perfect. But they are confident they can make it much better. 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 - 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. John Pendry said a metamaterial cloak could be manufactured to wrap around an 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." Cloaking differs from stealth technology, which does not make an aircraft invisible but reduces the cross-section available to radar, making it hard to track. 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.
[img height=131 alt="Laboratory at Duke University Image: Duke University" hspace=0 src="vny!://newsimg.bbc.co.uk/media/images/42219000/jpg/_42219052_cloak3_duke_203.jpg" width=203 border=0] [DIV class=cap]Scientists were able to watch waves bending around the cloak
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[DIV class=cap] [img height=152 alt="Invisibility cloak Image: Duke University" hspace=0 src="vny!://newsimg.bbc.co.uk/media/images/42215000/jpg/_42215112_cloak2_duke_203.jpg" width=203 border=0] [DIV class=cap]The cloak is constructed from advanced "metamaterials"[/DIV][/DIV][/DIV][/DIV][/DIV]
