'Invisibility cloak' slows down light - ZDNet UK

Researchers at the University of St Andrews have come up with a cloaking technology design that could help make objects appear invisible against a multicoloured background.

The design, revealed on Tuesday, is for a lentil-shaped chamber covered in electromagnetic materials called metamaterials. These manipulate the speed and paths of light falling on the chamber. Objects placed inside the chamber would not be visible to an observer, said researcher Janos Perczel.

"The 'cloak' will be made from metamaterials made from very small electrical circuits," Perczel told ZDNet UK. "We're proposing a method of slowing light down that can be engineered into the electrical circuits."

Some previous optical invisibility research has focused on theoretically speeding light up so it bends over an object and hides it, in the same way that water flows over a stone in a river. To do this, the light would have to apparently reach 'super-luminal' speeds, or speeds faster than the speed of light in space — c — as it has to take a longer path around an object than it would if the object was not there. Although light cannot actually exceed c, aspects of it — its phase — can appear to do so, and this phase velocity can be infinitely fast within metamaterials.

However, this technique can be used to mask only a specific light frequency, meaning that only one part of the spectrum — essentially, one colour — would be rendered invisible, and an object camouflaged using the technology would be only be invisible as long as it did not move; once in motion, there would be distortion.

We've proven that invisibility can be achieved without super-luminal speeds.

– Janos Perczel

"Such a cloak would only work for one specific frequency in the electromagnetic spectrum," said Perczel. "We've proven that invisibility can be achieved without super-luminal speeds."

Slowing light down would still have the effect of bending light around an object, but would escape the limitations of speeding light up, according to Perczel. This idea works by engineering a material so that light loops-the-loop when it goes through it, and then putting the object to be hidden in the centre of the loop. To work, all light, whether or not it goes across the area with the object, has to go through the same length of loop. The researchers' insight is in designing the device so that this geometry holds true. Once done, this is true for all frequencies of light.

The idea to slow light down came from Perczel's professor, Ulf Leonhardt, and researcher Tomas Tyc of Masaryk University in the Czech Republic. The team needed to come up with a design for a device to slow the light that also built on previous investigations into cloaking. They decided to combine the device and the cloaking early on in their work, coming up with the idea of a chamber. Perczel then worked out how this could be achieved.

The researchers' findings were published on Tuesday in the August edition of the New Journal of Physics.

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