Science Takes the Fiction Out of the Invisibility Cloak
Taking a page from movies and comic books, researchers at Duke University have developed what they call an “invisibility cloak,” a rudimentary device that hides objects by bending electromagnetic waves so that they flow around the object like water around a rock.
Because none of the waves are reflected back at the observer, the object seems invisible.
Their device, reported today in the online version of the journal Science, works only with microwave radiation -- not visible light waves -- and only in two dimensions. And it does not yet provide complete invisibility; it produces a small shadow that can be detected.
But it represents a proof of principle for a theory first published only five months ago.
“It’s a very good achievement,” physicist Ulf Leonhardt of the University of St. Andrews in Britain told Science. “It’s surprising that it’s as simple as it is and that it works so well.”
Leonhardt was not involved in the research but has developed a similar theory.
Team leader David R. Smith, an electrical engineer at Duke, said the device’s imperfections were a result of their haste to demonstrate their ideas.
“We did this work very quickly ... and that led to a cloak that is not optimal,” he said. “We know how to make a much better one.”
The current device cloaks a copper cylinder about 0.4 inch tall. The team is now building a version that will cloak an object the size of a toaster in all three dimensions.
The technique is different from stealth technology, which uses unusual shapes and radar-absorbing materials to reduce reflections back to the source. That technology can reduce the radar image of a plane to a size similar to that of a bird.
The key to the device is a new family of materials called “metamaterials.” In essence, they are metal patterns laid onto the surface of materials that are commonly used in making circuit boards, such as ceramics and fiber composites.
If the right metals are used in the right configuration, incoming radiation interacts with electrons in the metals and is bent around the object.
A key consideration is that the metal shapes must be significantly smaller than the wavelength of the radiation. For this reason, Smith and postdoctoral fellow David Schurig used microwaves, which have a long wavelength.
Bending visible light waves would require much smaller metal patterns, which would probably have to be produced by nanofabrication techniques.
And because a different set of materials is required for each wavelength of light, producing a device that could block visible light could be extremely challenging, Smith said. Practical applications might require that the user be painted all one color.
Despite the limitations, experts envision a variety of uses. It could be used in communications, for example, to bend a wireless transmission around a building. It could also be used for focusing solar energy onto solar cells, and to protect sensitive devices from electromagnetic radiation.
The military could also find a variety of uses for the technology, which is why the research has been sponsored in large part by them.
But cloaking a Romulan spaceship, a tank or even a human would create a serious limitation.
Because all of the incoming light is bent around the cloaked object, those inside would be left literally in the dark, and would not be able to see where they were going. Any device used to provide vision for the occupants would itself be visible, Smith said.