Invisibility cloaks are the stuff of wizards and alien starships. But on 27 March, 2009 Fractal Antenna Systems, Inc. publicly demonstrated, in reality beyond the fantasy, the first true 'see thru' invisibility cloak. Demonstrated with a prototype device using microwaves instead of visible light, the cloak's wide bandwidth dramatically expands the capability of the new science of metamaterials and renders another key example of the importance and benefits of fractals in electronics and sensors technology.
Led by physicist Nathan Cohen, the firm's team wanted to solve a problem: how to make metamaterials practical by expanding their bandwidth capabilities. Metamaterials are exotic, crazy quilt-like collections of tiny resonators. Without wide bandwidths, however, metamaterials have very limited practical use. Previous efforts, such as with cloaks and lenses, only demonstrated useful metamaterial behavior in the narrowest frequency or color bands,“like playing a song on a piano with only one key,” noted Cohen. “We're playing music with the full keyboard.”
Using fractals, geometric shapes that are built up from multiple scaling of a simple pattern, the team showed that metamaterials can now be enabled over a far greater bandwidth. “We are the world's experts in fractals,” notes Cohen, who himself invented fractal resonators and showed a basic property of widebandedness with fractals in the famous Maxwell's equation. “So it was natural for us to use our patented, proprietary technology to build a metamaterial cloak, one that works far beyond the tiny bandwidths already reported.”
The firm's cloak is a simple looking device packed with fractal technology. Built from belts of circuit boards festooned with fractal resonators, these belts slip the microwaves around the cloaked object so the object is effectively invisible and “see thru”. The observer sees the original image or signal, without it being blocked by the cloaked object. Using no power, the fractal cloak replicates the original signal (that is, the signal before blocking) with great fidelity from 500 MHz to almost 1500 MHz. Engineers refer to this as a “3:1” bandwidth.
“While we had to show this by working at microwaves, physicists especially are excited by this result, in part because it means there is now a path and a recipe to reaping the benefits at visible light, which is a 2:1 bandwidth. So nature has shown us, through the use of fractals, that you have the science to cloak at the full spectrum of colors at visible light,” said Cohen.
The much celebrated previous efforts on metamaterial cloaks by a Duke University-based international team had failed to demonstrate 'see thru' cloaking beyond a narrow band, and in a recent update, had abandoned the 'see thru' approach altogether, using a 'mirror cloak' instead. “I guess it's cloaking, in the sense that it 'conceals', but in my opinion, fans of Harry Potter might not accept it. People equate 'cloak' with 'see-thru'. Using a mirror-like camouflage is a form of cloaking, but I believe it's hard to see why that's new or important. In contrast we have the real deal,” said Cohen. “To use an appropriate pun, our results are transparent.”
Cohen notes that the obstacles to making wideband visible light cloaks with metamaterials are daunting. “You won't see it soon. The technology to make the stuff doesn't exist. Maybe if you want to cloak a pin-head sized object, but not to do a 'see thru' one of Harry. I would be very skeptical of scientists who predict this is happening in a few weeks or months.”
Despite the obstacles, Cohen and his team will be leveraging the results of the cloak study to make products of wideband metamaterial for devices in addition to cloaks. “It's an important new tool in our toolbox, a natural extension of our vast experience in fractal antennas and related technologies. There lies the excitement for us. We now can say metamaterials won't have a disappearing act in the history of new technologies.”