A new invisibility cloak developed at the Karlsruhe Institute of Technology (KIT) is reportedly able to hide any object that can fit inside a one-inch diameter cylinder. The cloaking device is among the first to rely on common materials like polymers and acrylic paint, and could be used for a practical demonstration of cloaking technology.
Although building a practical invisibility cloak is proving an extremely challenging feat, the principle behind it is quite simple: to make an object invisible, scientists must find a way to let in light rays through one side of the cloak, smoothly guide them around the object to be hidden, and then restore their original direction to show no sign that they were diverted in the first place.
In order to achieve this the first invisibility cloak, like the vast majority of the ones that followed, relied on metamaterials, special man-made composites that can refract light in very unique ways. Unfortunately, most of the cloaks built so far can only hide tiny objects and work on small portions of the visible spectrum – for instance, making objects invisible to red light but not violet light. Ultimately, the cloaks we've seen so far don't work too well in the real world.
Researchers Robert Schittny and colleagues at KIT now say they have managed to build an invisibility cloak that makes use of common materials like polymers, metal and acrylic paint. Although this is not the first cloak built from common materials (we've even seen 3D-printed ones), their cloak can reportedly hide macroscopic objects from sight and, crucially, work well enough that it could be used for practical demonstrations.
The problem of bending light around the object was solved not through refraction, as was the case with metamaterials, but through scattering instead. The cloak is made of a block of polydimethylsiloxane, a polymer used for contact lenses and shampoos, with added nanoparticles of titanium dioxide to help it scatter light more effectively. Inside the block is a hollow metal cylinder coated with reflective acrylic paint.
For the cloak to be truly undetectable, light rays must not only come out of the cloak in exactly the same direction they came in, but also travel through the cloak in a curved path (around the hidden object) in the same time it would take them to travel straight through the cloak. Scattering achieves this by first slowing down the speed at which light propagates inside the cloak, to create some margin to work with, and then by speeding it up to make up for the longer path that light needs to travel.
The cloak has been designed in such a way that the average time it takes light to travel through the polymer block will be consistent with the time it takes to travel through the cloak, rendering the object inside the cloak invisible from sight.
While the early applications of invisibility cloaks are likely to be of a military nature, the aim for Schittny and colleagues is more peaceful, as they hope their cloak will be used in classrooms and labs around the world to excite and educate students about physics.
"It is a macroscopic cloak that you can look at with your bare eyes and hold in your hands," says Schittny. "With a reasonably strong flashlight in a not too bright room, it is very easy to demonstrate the cloaking. That means no fancy lab equipment, no microscopes, no post-processing of measurement data. The effect is just there for everyone to see."
The technology was presented last week at this year's Conference on Lasers and Electro-Optics (CLEO) in San José, California.
Source: Cleo 2015
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