Using a completely opaque material irradiated from above with a specific wave pattern has the effect, according to the research team, that light waves from the left can now pass through the material without any obstruction, opening up new possibilities for active camouflage. According to the team, the idea could be applied to different kinds of waves and, as a result, should work with sound waves just as well as with light waves.
“Complex materials are opaque, because light waves inside them are scattered multiple times”, says Professor Stefan Rotter (TU Wien). “A light wave can enter and exit the object, but will never pass through the medium on a straight line. Instead, it is scattered into all possible directions.”
Many different attempts have been made to address this problem of 'scattering'. Special materials which are able to guide light waves around an object or objects that can emit light by themselves have been researched. When an electronic display sends out exactly the same light as it absorbs in the back, it can appear to be invisible, at least when looked at in the right angle.
The approach taken by the team at TU Wien is to guide the original light wave through the object, “As if the object was not there at all”, explains Andre Brandstötter, one of the authors of the study. “This sounds strange, but with certain materials and using our special wave technology, it is indeed possible.”
The team at TU Wien are working on optically active materials, which are used for building lasers. To make the laser shine, energy has to be supplied by means of a pump beam. Otherwise, the laser material behaves just like any other material – it absorbs part of the incident light.
“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 Professor Konstantinos Makris from the University of Crete (previously TU Wien). “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.”
If this pattern perfectly corresponds to the inner irregularities of the material which usually scatter the light, then the projection from above can effectively switch off the scattering, and another beam of light travelling through the material from one side can pass without any obstruction, scattering or loss.
“Mathematically, it is not immediately obvious that it is at all possible to find such a pattern”, says Rotter. “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.”
Computer simulations have shown that the method works. Now the idea is to be confirmed in experiments.