Invisibility cloaks could slim down with active approach

Light-blocking antennas could hide objects of any shape and size

LIGHT BLOCKER  An invisibility cloak made of small antennas surrounds an aluminum cylinder. The thin cloak hides the cylinder from microwaves but could be adapted to block visible light.

M. Selvanayagam and G. Eleftheriades/Physical Review X  

A new light-canceling technique could help scientists make thin invisibility cloaks that block a large range of wavelengths.

The new technique is “a smart engineering trick” that is “quite different from the way cloaking has been approached so far,” says Andrea Alù, a physicist at the University of Texas at Austin.

Nearly all invisibility cloaks use light-manipulating compounds called metamaterials to bend light around an object, rendering the object transparent. Such technology has become increasingly sophisticated since it was introduced in 2006. But most metamaterial cloaks are still impractically bulky or limited to narrow wavelength ranges and physical configurations.

So University of Toronto electrical engineers George Eleftheriades and Michael Selvanayagam tried what they call an active approach to cloaking. They surrounded an aluminum cylinder roughly 11 centimeters in diameter and 4 centimeters tall with small loop antennas that broadcast microwave radiation. The scientists adjusted the currents running through the antennas to emit microwaves with just the right size and timing to cancel out radiation that scattered off the cylinder. The result, the researchers report November 12 in Physical Review X, was a cylinder invisible to microwave radiation. By changing the antennas’ currents, the researchers could alter the apparent size of the cylinder or make it appear to move.

Eleftheriades says the antennas could be adapted to block visible light just as they do with microwave frequencies. In addition, he hopes to add sensors that measure the scattered light and automatically set the currents in the antennas to broadcast canceling radiation.

Alù thinks the new method could disguise larger objects than has been possible with most of the previous cloaking approaches. But he notes that because tuning the antennas takes time, the technique would work best for hiding stationary objects. 

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