Scientists call their discovery a 'perfect' invisibility cloak

Functional "cloaking" devices have been around since 2006, but they're far from perfect. All attempts so far have failed to avoid at least some partial light and reflectivity — what has resulted in an unconvincing effect. Part of the problem is finding a way to hide objects in wavelengths longer than the human eye can see, or by hiding objects so small that they cannot be seen. But a demonstration at Duke University using a new device now shows that it may be possible to get true cloaking. As David Smith recently told the BBC, "This to our knowledge is the first cloak that really addresses getting the transformation exactly right to get you that perfect invisibility."

To get it to work, Smith and Nathan Landy utilized what are called "meta-materials" — completely synthetic objects that have properties not typically found in natural ones. And in accordance with the standard invisibility cloak model, these meta-materials can be designed to guide electromagnetic waves around an object, and then have them appear on the other side — what gives the impression of passing light (hence the invisibility effect). And in fact, this entire notion has given rise to a new branch of scientific inquiry, what is called transformation optics.

Scientists call their discovery a 'perfect' invisibility cloak

But making meta-materials with the exact qualities required for convincing invisibility has proven difficult. One issue is that waves are being lost due to reflections at the boundaries of an object — an effect similar to reflections seen on clear glass. Even though glass is transparent, its reflective nature still makes it visible.

So, to address this issue, Landy redesigned the meta-material. The original design featured parallel and intersecting strips of fiberglass etched with copper. Landy's new cloak, on the other hand, retained the row-by-row design, but was given added copper strips to create a more complicated material. The strips are about two-feet square and form a diamond shape, with the center left empty. And it's this diamondoid configuration that has made the difference.

Landy's new microwave cloak is naturally divided into four quadrants, each of which have voids or blind spots at their intersections and corners with each other. Thus, to avoid the reflectivity problem, Landy was able to correct for it by shifting each strip so that is met its mirror image at each interface.

And when the team conducted their tests, the new design worked — they successfully beamed light around a cylinder 7.5 cm in diameter and 1 cm tall. They essentially rendered the object invisible to microwaves.

"We built the cloak, and it worked," said Landy through a release. "It split light into two waves which traveled around an object in the center and re-emerged as the single wave with minimal loss due to reflections."

Scientists call their discovery a 'perfect' invisibility cloak

Now, it's important to note that Landy's invisibility cloak is unidirectional (the illusion only works when viewed from a specific angle), it would be difficult to achieve with visible light (the design principles that make the cloak work in microwaves would be difficult to implement at optical wavelengths), and it only works in two-dimensions. That said, it was an important proof of concept; the researchers successfully reduced the scattering effect of an object ten wavelengths in size. And at the same time, they were able to validate certain assumptions made by transformation optic theory.

Looking to the future, Landy is hoping to see the same technology used to smooth out twists and turns in fiber optic cables (making them seem straighter). The technology could also be used to improve radar by improving microwave performance.

The study can be found at Nature Materials.

Supplemental source: BBC.

Images via Duke.