Widespread Military Applications Seen for Invisibility Technology

(Plasmonic cover cloaking devices)


The Scotsman | March 1, 2005

A CLOAKING device that makes objects invisible is being developed by researchers, bringing the magic of Harry Potter into the world of science fact.

While Harry uses his cloak of invisibility to move about Hogwarts School unseen, electronic engineers at the University of Pennsylvania are working on a real invisibility shield called a "plasmonic cover".

The development, which works by preventing objects from reflecting and scattering light, could have widespread use in the military as it would be more effective than current stealth technology.


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Although no final product has been made, the engineers claim their proposal "does not obviously violate any of the laws of physics".

Other types of invisibility cloaks have been developed before, but these mostly use the "chameleon principle" - colouring objects to match their background, enabling them to blend in and become camouflaged.

One such system involved using light detectors and emitters that project a replica of the image appearing behind an object on to its front surface.

Key points
• American researchers working on 'plasmonic cover' 'cloaking device'
• Invisibility shield would prevent objects reflecting and scattering light
• Device differs from other invisibility research reliant on chameleon principle

Similarly, researchers in Tokyo are developing a camouflage fabric that uses a comparable principle where the background is projected on to light-reflecting beads in the material. Such systems are, however, dependent on the viewer from which the object is being concealed being in the right position.

A report in the journal Nature reveals that the new system is far more ambitious, being both self-contained and reducing visibility from all angles. In this sense it would be more like the shields used by the Romulans in the Star Trek episode Balance of Terror which hid their spaceship at the touch of a button.

Dr John Pendry, a physicist at Imperial College London, said: "The concept is an interesting one. It could find uses in stealth technology and camouflage."

According to Dr Andrea Alu and Dr Nader Engheta, the engineers behind the project, the key is to reduce light scattering.

Objects are visible because light bounces off them; if this can be prevented and if the objects did not reflect any light, they would become invisible. The "plasmonic screen" achieves this by resonating in tune with the illuminating light.

"Plasmons" are created when the electrons on the surface of a metallic material move in rhythm. The developers claim a shell of this material will reduce light-scatter to the extent that an object will become invisible, if the light’s frequency is close to the resonant frequency of these "plasmons". In this way, the scattering from the shell effectively cancels out the scattering from the object.

"The concept is an interesting one. It could find uses in stealth technology and camouflage"

- Dr John Pendry, physicist, Imperial College London

Experiments have shown that spherical or cylindrical objects coated with such shields do produce very little light scattering, which renders them nearly invisible. However, the cloak would have to be delicately tuned to suit each object it hides. Also, a specific shield may only work for one wavelength of light, rendering it invisible in, say, red light, but not in multi-wavelength daylight.

Another problem which has to be solved is that the effect would only work in daylight when the wavelength of the light being scattered is the same size as the object - meaning only tiny things can so far be hidden.