New invisibility cloak combines metamaterials and fancy electronics to be thinner, lighter, more invisible

A researcher at the University of Texas at Austin has devised an invisibility cloak that could work over a broad range of frequencies, including visible light and microwaves. This is a significant upgrade from current invisibility cloaks that only cloak a very specific frequency — say, a few hertz in the microwave band — and, more importantly, actually make cloaked objects more visible to other frequencies. The UT Austin cloak would achieve this goal by being active and electrically powered, rather than dumb and passive like existing invisibility cloaks.

This is what a metamaterial looks like

As you probably know, the last few years have seen a lot of research into invisibility cloaks. These cloaks are mostly based on metamaterials — special, man-made materials that bend radiation in ways that shouldn’t technically be possible, allowing for cloaking devices that bend radiation around an object, hiding it from view. (Check out our featured story, The wonderful world of wonder materials.) The problem with these cloaks is that metamaterials are tuned to a very specific frequency — so, while that specific frequency (say, a thin band of microwaves) passes around the object, every other frequency scatters off the cloak. In a beautiful twist of irony, most invisibility cloaks actually create more scattered light, making the cloaked object stand out more than if it was just standing there uncloaked.

According to Andrea Alù at UT Austin, this is a fundamental issue of passive invisibility cloaks, and the only way to get around it is to use cloaks fashioned out of active, electrically active materials. It might change in the future with more advanced passive metamaterials, but for now active designs are the way forward. Research into active invisibility cloaks is currently being carried out by multiple groups, but none have yet been built.

A graph showing the broadband invisibility of an active cloak. Note how the passive cloak (blue) is actually more visible than a non-cloaked object.

Alù’s proposed design consists of a conventional metamaterial base, but with CMOS negative impedance converters (NICs) placed at the corner of each metamaterial square (top image). A NIC is an interesting electronic component that adds negative resistance to a circuit, injecting energy rather than consuming it. NICs are not widely used as we’re not entirely sure how to use them. Alù seems to propose that by interspersing NICs (which must be powered) with the metamaterial, multiple frequencies can be cloaked. In the image above, you can see a standard metamaterial cloak (blue), vs Alù’s metamaterial-and-NIC cloak (green). Alù’s proposed cloak is invisible over a large range of frequencies, while a standard passive cloak is only invisible for a small range, and more visible than non-cloaked devices in other ranges.