Invisibility Cloaks Can Actually Make Hidden Objects Appear Even Brighter

Near my old neighborhood in Baltimore, there’s currently a debate raging about a new Walmart planned for an old car lot. It’s actually been raging for years now, but the latest fervor has to do with an unpublicized design tweak on the part of the developers from a store weakly attempting to blend in with the dense neighborhood to a full-on suburban sprawl Walmart citadel. Neighbors now faced with the prospect of living around the corner from a brightly lit bargain fortress are upset.

I mention this because it’s the very first thing that comes to mind while considering potential non-military applications of cloaking technology: hiding buildings. This technology, involving the manipulation of light scattered by and around an object, is closer than most people realize. The very first invisibility cloak was created and tested in 2006 by researchers at Duke University. It worked only on light waves in the microwave spectrum (not visible by human eyes), but more or less successfully guided electromagnetic waves around an object without disturbing them, as if there was no object there at all.

There’s a problem with this and other ideas so far suggested for cloaking objects—plasmatic cloaking and so-called mantle cloaks—that arguably makes the advances a bit moot. They only cancel out one narrow band of the electromagnetic spectrum, and if you take all of the other uncloaked wavelengths and add together the amount of light they scatter, you actually get more total visibility.

This is according to a study accepted for publication in the Physical Review Letters led by Prof. Andrea Alù at the University of Texas. The effect is due to the simple fact that all of the different cloaking methods add something to the object being cloaked, some amount of matter that helps disturb the electromagnetic waves moving through a space at your eye (or radar or other sensor).

In effect, the object being cloaked acts as a “beacon,” according to the study. Cloaked by current proposed methods, our Walmart would actually be “lit up” even more than it was before, save for a very narrow band of electromagnetic frequencies.

Physicist Ulf Leonhardt notes in Physics World that for many practical purposes a narrow band of cloaking is enough: “I don't think that the paper asks the right question." In other words, if we want to make the Walmart “invisible” we might only need to cloak the building in the visible spectrum of electromagnetism.

It would, however, remain visible through a variety of other methods that don’t involve the human eye, like radar. It’s really not unlike a conventional illusion, a magic trick. You could liken it to placing a giant screen in front of the store, projecting the trees and railroad tracks behind the store in front of it.

Alù wants something more. “Leonhardt is raising a different issue: he is asking whether we really need to perfectly cloak an object, or if it would just be enough to make it appear invisible to specific observers,” he told me. “To give you an example, imagine covering an object with a flat screen that projects on its front whatever happens behind it. This simple solution makes the object effectively disappear, and it can obviously be done over a very large bandwidth with no limitations. In our theory, we are asking something much more stringent than that: we would like our object to be turned into a transparent object, for all angles of observation.”

The goal then is ultimately a more fully realized invisibility—gone from radar, gone from sight—from any perspective. “If instead you care about making an optical trick that makes an object disappear, then you may have a much easier time than we claim,” Alù said. “People have been realizing for long that all the cloaks had some bandwidth limitations. The beauty of our result is that we can quantify these ultimate limits, and compare different approaches, proposing ways to improve the cloaking technology.”

The paper suggests one such method that may provide broad bandwidth cloaking. It’s a metamaterial cloak covered in amplifiers, essentially covering the cloaked objected in varying levels of electricity. The amplifiers vary the electrical impedance of the cloak in response to different electromagnetic frequencies. So, in effect, the object is cloaked “smartly,” and the scattering of light can be reduced across a wider range of frequencies. More commonly described passive designs don’t have this ability to respond.

For some less stringent applications, Alù said that we’re basically already there with cloaking technology. “Applying cloaks to improve wireless communications, for instance, or making struts, obstacles disappear to radars or cellular antennas, is something that is already doable," he said.

"If you are referring to making large objects disappear at visible wavelengths, probably the techniques that Leonhardt is referring to are more realistic, i.e., by relaxing the requirements on cloaking, it may be possible to realize the visual appearance that an object is not there,” he continued. That may be enough for hiding a Walmart from neighbors but, for now, it remains hardly invisible.

@everydayelk