Rendering things invisible sounds like it belongs in the realm of Harry Potter, but it is technically possible. Researchers from TU Wien in Austria have developed a new process that allows light waves to pass right through an opaque material by projecting a matching wave pattern onto it, actively camouflaging the target from view. The technique could one day be used as a kind of invisibility cloak, and it might work just as well on sound waves.

Objects appear visible to us thanks to our retinas picking up some of the light waves bouncing off of things. Many previous attempts at cloaking have tried to counter that principle by bending the path of light waves around objects, but it only works on small scales and often creates other visual artefacts, like a shimmer effect.

"We did not want to reroute the light waves, nor did we want to restore them with additional displays," says Andre Brandstötter, co-author of the study. "Our goal was to guide the original light wave through the object, as if the object was not there at all. This sounds strange, but with certain materials and using our special wave technology, it is indeed possible."

Under ordinary conditions, the material that the team is working with will absorb light and appear opaque. But this material is optically active, meaning that when energized it will shine, and precisely controlling how it shines can allow incidental light to pass right through it.

"The crucial point is to pump energy into the material in a spatially tailored way such that light is amplified in exactly the right places, while allowing for absorption at other parts of the material," says Konstantinos Makris, co-author of the study. "To achieve this, a beam with exactly the right pattern has to be projected onto the material from above – like from a standard video projector, except with much higher resolution."

Light scatters from an object due to an intricate pattern of irregularities in the material, so to allow light to pass through the projected pattern needs to perfectly correspond to that of the material and effectively switch off the scattering.

"Mathematically, it is not immediately obvious that it is at all possible to find such a pattern," says Stefan Rotter, co-author of the study. "Every object we want to make transparent has to be irradiated with its own specific pattern – depending on the microscopic details of the scattering process inside. The method we developed now allows us to calculate the right pattern for any arbitrary scattering medium."

So far, the TU Wien team has only tested the technique in computer simulations, but the next step is to try it out in practical experiments. In theory, the technology should work for acoustic cloaking as well, so the researchers say they might test it out on sound waves before moving onto light waves.

The research was published in the journal Light: Sciences and Applications.

Source: TU Wien

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