Cloaked eye still sees

Hidden sensor could send, receive information without being detected

Cell phone antennas, radio receivers and GPS devices may one day go incognito. In a paper to appear in Physical Review Letters, Nader Engheta and Andrea Alù propose a new cloaking method that cancels out the electromagnetic waves bouncing off an object. The concept may ultimately lead to surreptitious sensors that can collect and send messages without detection.

CLOAK FOR COMMUNICATION Like a rock thrown into a creek, an uncloaked sensor disrupts the uniform energy flow of electromagnetic waves (right). Unperturbed waves are shown in yellow arrows, while large disturbances in energy flow are shown in red. A new proposal for a cloaked sensor (left) would perturb the flow of energy much less, while allowing messages to still reach the sensor. Andrea Alù and Nader Engheta

“This is a fascinating paper addressing a very important challenge,” says physicist Nikolay Zheludev at the University of Southampton in England. “The result could have metrological, environmental and defense applications when the idea is developed as a practical device.”

The new cloak manipulates electromagnetic waves — including light — not by blocking out the waves, but by working with them. Previous cloaks worked by diverting waves around an object. “We are asking the question, ‘Is it possible to put a layer around an object such that when a wave hits the object, the wave scatters less?’ ” says Engheta, of the University of Pennsylvania in Philadelphia.

Separately, the object and the cloak would both be visible. But the cloak would be designed so that when the two are together, the waves scattering off of both objects would add destructively. “If the cloak is designed properly, the effect is reduced, like a cancelling effect,” Engheta says. “It puts in balance the object and the cloak.”

The key is that electromagnetic waves still enter the cloaked area and hit the object — a requirement for an antenna to pick up a signal, for example. This feature is what separates the new work from other cloaking methods that completely isolate objects from the environment. “Some cloaks divert waves around an object. If you have a sensor in there, it won’t be able to measure the field,” says Engheta.

The cloak could also be designed to not interfere with certain types of outgoing waves (other than the ones being dampened), allowing the sensor to still send unperturbed messages.

“If you want to communicate, this is the cloak for you,” says John Pendry, a theoretical physicist at Imperial College London. In 2006, Pendry and his colleagues created the type of cloak that directs microwaves around an object. But he notes that isolating the object in this way would effectively rule out the option of sending or receiving signals.

A material with this new selective shielding ability “would be of great practical use,” Pendry says. Wrapping this type of cloak around devices perched on top of military vehicles could minimize telltale scattering while still allowing the sending and receiving of crucial messages.

Researchers have a long way to go before these cloaks become a reality. Each cloak has to be fine-tuned to suit the object it’s cloaking, and will probably work for only a small range of electromagnetic waves, Engheta says.

But the blueprint for the cloaks could work for any kind of electromagnetic waves, including light waves, radio waves and microwaves. “The concept is to scatter waves,” he says. “Whether you deal in microwaves or optics, the concept is similar.”

Laura Sanders

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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