Scientists Move Closer to Creating an Invisibility Cloak

As far as the microwaves were concerned, the 7-inch-long tube did not exist — is true invisibility that far away?

The microwave field around the objects without (left) and with the cloaking material (right).
The microwave field around the objects without (left) and with the cloaking material (right). Image from "Experimental Verification of three-dimensional plasmonic cloaking in free space"

For years, science-fiction and fantasy authors have dreamed up magical objects—like Harry Potter’s invisibility cloak or Bilbo Baggins’ ring—that would render people and things invisible. Last week, a team of scientists at the University of Texas at Austin announced that they have gone one step further toward that goal. Using a method known as “plasmonic cloaking,” they have obscured a three-dimensional object in free space.

The object, a cylindrical tube about 7 inches long, was “invisible” to microwaves, rather than visible light—so it’s not like you could walk into the experimental apparatus and not see the object. But the achievement is nonetheless quite stunning. Understanding the principles of cloaking an object from microwaves could theoretically lead to actual invisibility soon enough. The study, published in late January in the New Journal of Physics, goes beyond previous experiments in which two-dimensional objects were hidden from various wavelengths of light.

How did the scientists do it? Under normal conditions, we see objects when visible light bounces off them and into our eyes. But the unique “plasmonic metamaterials” from which the cloak was made do something different: they scatter light in a variety of directions. ”When the scattered fields from the cloak and the object interfere, they cancel each other out and the overall effect is transparency and invisibility at all angles of observation,” said Professor Andrea Alu, co-author of the study.

To test the cloaking material, the research team covered the cylindrical tube with it and subjected the setup to a burst of microwave radiation. Because of the plasmonic material’s scattering effect, the resulting mapping of microwaves did not reveal the object. Other experiments revealed that the shape of the object did not affect the material’s effectiveness, and the team believes that it is theoretically possible to cloak multiple objects at once.

The next step, of course, is creating a cloaking material capable of obscuring not only microwaves, but visible light waves—an invisibility cloak we might be able to wear in everyday life. Alu, though, says that using plasmonic materials to hide larger objects (like, say, a human body) is still a ways away:

In principle, this technique could be used to cloak light; in fact, some plasmonic materials are naturally available at optical frequencies. However, the size of the objects that can be efficiently cloaked with this method scales with the wavelength of operation, so when applied to optical frequencies we may be able to efficiently stop the scattering of micrometre-sized objects.

In other words, if we’re trying to hide something from human eyes using this method, it’d have to be tiny—a micrometre is one-thousandth of a millimeter. Still, even this could be useful:

Cloaking small objects may be exciting for a variety of applications. For instance, we are currently investigating the application of these concepts to cloak a microscope tip at optical frequencies. This may greatly benefit biomedical and optical near-field measurements.

In 2008, a Berkeley team developed an ultra-thin material with the potential to someday render objects invisible, and earlier this year, a group of Cornell scientists funded by DARPA was able to hide an actual event 40 picoseconds long (that’s 40 trillionths of a second) by tweaking the rate of light’s flow.

Invisibility cloaks may still be years away, but it seems we’ve entered the Age of Invisibility.

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