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Engineers develop 'less bulky' invisibility cloak

08 July 2015

Researchers at UC San Diego have developed a new type of invisibility cloaking device that overcomes a particular limitation of existing designs: bulkiness.

The cloak is a thin Teflon sheet (light blue) embedded with many small, cylindrical ceramic particles (dark blue). Image courtesy of Li-Yi Hsu/UC San Diego

Electrical engineers at the University of California (UC), San Diego have designed a cloaking device that is both thin and does not alter the brightness of light around a hidden object. The technology behind the cloak is expected to have applications beyond 'invisibility', including concentrating solar energy and increasing signal speed in optical communications.

"Invisibility may seem like magic at first, but its underlying concepts are familiar to everyone," says UC San Diego's Professor Boubacar Kanté. "All it requires is a clever manipulation of our perception. Full invisibility still seems beyond reach today, but it might become a reality in the near future thanks to recent progress in cloaking devices."

As their name implies, cloaks are devices that cover objects to make them appear invisible. The idea behind cloaking is to change the scattering of electromagnetic waves (light or radar, for example) off an object to make it less detectable to these wave frequencies. However, one of the drawbacks of cloaking devices is that they are typically bulky.

"Previous cloaking studies needed many layers of materials to hide an object, the cloak ended up being much thicker than the size of the object being covered," says UC San Diego PhD student, Li-Yi Hsu. "In this study, we show that we can use a thin single-layer sheet for cloaking."

The cloak also overcomes another fundamental drawback of existing cloaking devices: being 'lossy'. Cloaks that are lossy reflect light at a lower intensity than what hits their surface.

"Imagine if you saw a sharp drop in brightness around the hidden object, it would be an obvious telltale,2 says Kanté. "This is what happens when you use a lossy cloaking device. What we have achieved in this study is a 'lossless' cloak. It won't lose any intensity of the light that it reflects."

Many cloaks are lossy because they are made with metal particles, which absorb light. The researchers report that one of the keys to their cloak's design is the use of dielectrics, which, unlike metals, do not absorb light. This cloak includes two dielectrics, a proprietary ceramic and Teflon, which are structurally tailored on a very fine scale to change the way light waves reflect off the cloak.

In their experiments, the researchers specifically designed a 'carpet' cloak, which works by cloaking an object sitting on top of a flat surface. The cloak makes the whole system - object and surface - appear flat by mimicking the reflection of light from the flat surface. Any object reflects light differently from a flat surface, but when the object is covered by the cloak, light from different points is reflected 'out of sync', effectively cancelling the overall distortion of light caused by the object's shape.

"This cloaking device basically fools the observer into thinking that there's a flat surface," says Kanté.

The cloak was modelled using CAD software as a thin matrix of Teflon in which many small cylindrical ceramic particles were embedded, each with a different height depending on its position on the cloak.

"By changing the height of each dielectric particle, we were able to control the reflection of light at each point on the cloak," says Hsu. "Our computer simulations show how our cloaking device would behave in reality. We were able to demonstrate that a thin cloak designed with cylinder-shaped dielectric particles can help us significantly reduce the object's shadow."

"Doing whatever we want with light waves is really exciting," says Kanté. "Using this technology, we can do more than make things invisible. We can change the way light waves are being reflected at will and ultimately focus a large area of sunlight onto a solar power tower, like what a solar concentrator does. We also expect this technology to have applications in optics, interior design and art."

An article describing this work is published in the journal, Progress In Electromagnetics Research




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