Scattering Confirms Wideband Invisibility Cloak Using Fractal Metamaterials

Researchers from Boston-area Fractal Antenna Systems, Inc., report additional measurements that confirm its claims of a working ‘invisibility cloak’.

In March, 2009, the firm’s research group disclosed the first invention of the invisibility cloak. It had unprecedented ability to work ‘wideband’ and render an object invisible to microwaves. The wideband aspect also demonstrated a path for making invisibility cloaks in the full spectrum of visible light. A previous invisibility cloak effort by Duke University-based researchers had shown some degree of cloaking , but over a narrow frequency band. That cloaking also rendered the object partially detectable/visible by the presence of shadows.

The firm’s unprecendented invisibility cloak uses layers of state of the art metamaterial, made from self repeated designs called fractals. The layers surround the object to be cloaked like an onion skin. The microwaves slip stream around the object and its cloak layers.

The new measurements by the firm’s researchers concentrate on the scattering, which details how much of the impinging waves bounce off the sides, the front, and the back of the invisibility cloak. Previous reports showed how the area in back of the cloak acts in a ‘see around’ fashion, as expected. The new measurements show additionally what happens to the waves when viewed from the sides and towards the front. A true invisibility cloak must ‘scatter’ these waves in a minimal way, as if no obstacle was there at all. The new measurements confirm this minimal scattering behavior, which is essential for true invisibility.

Using copper layers in place of the invisibility cloak, an experimental ‘control’ demonstrated the effect was not a remnant of some other effects: the control showed substantial shadowing from the back and the sides, which amounts to significant scattering.

Notes inventor Nathan Cohen: “The scattering profile matches an empty-space one with good to high fidelity. It’s as if there is nothing there. In particular we see no shadows. Substantial shadowing was present with the control. It is a striking difference and confirmation.”

The wideband invisibility cloaking is enabled with the firm’s proprietary fractal metamaterial technology. The firm now uses the technology in several of its products, unrelated to cloaking. Cohen cautioned that the invisibility cloak is a technology demonstrator and not a practical device. “You can’t hide a starship or a battleship in it,” he remarked, “but we now have to ask where such cloaks can be useful rather than merely intellectually amusing.”

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