Tailor-made light beams pass through solid objects

Scientists have manipulated light beams to pass through opaque objects such as sugar to project an image beyond the obstacle.

When light penetrates a medium such as a piece of sugar it is scattered, altered and deflected, meaning it is not transparent. Now a research team from TU Wien and Utrecht University has been able to show that tailor-made light beams can be constructed that will not be altered by a medium, instead passing through to show an unaltered light pattern on the other side.

Professor Stefan Rotter, from the Institute of Theoretical Physics at TU Wien, said: "Each of these light wave patterns is changed and deflected in a very specific way when you send it through a disordered medium."

Together with his team, Rotter is developing mathematical methods to describe such light scattering effects. The expertise to produce and characterise such complex light fields was contributed by a team around Professor Allard Mosk at Utrecht University. "As a light-scattering medium, we used a layer of zinc oxide – an opaque, white powder of completely randomly arranged nanoparticles," explained Mosk, the head of the experimental research group.

In a press release the research team said: “First, you have to characterise this layer precisely. You shine very specific light signals through the zinc oxide powder and measure how they arrive at the detector behind it. From this, you can then conclude how any other wave is changed by this medium – in particular, you can calculate specifically which wave pattern is changed by this zinc oxide layer exactly as if wave scattering was entirely absent in this layer.”

Mosk added: "As we were able to show, there is a very special class of light waves – the so-called scattering-invariant light modes, which produce exactly the same wave pattern at the detector, regardless of whether the light wave was only sent through air or whether it had to penetrate the complicated zinc oxide layer," says Stefan Rotter. "In the experiment, we see that the zinc oxide actually does not change the shape of these light waves at all – they just get a little weaker overall," explains Allard Mosk.

The teams say that the scattering-invariant light modes are special and rare and that theoretically there are an unlimited number of possible light waves. However, they add that many can still be found and if combined correctly, a scattering-invariant waveform can be created again.

Jeroen Bosch, who worked on the experiment as a Ph.D. student, said: "In this way, at least within certain limits, you are quite free to choose which image you want to send through the object without interference.

"For the experiment we chose a constellation as an example: The Big Dipper. And indeed, it was possible to determine a scattering-invariant wave that sends an image of the Big Dipper to the detector, regardless of whether the light wave is scattered by the zinc oxide layer or not. To the detector, the light beam looks almost the same in both cases."

Image via TU WIEN, ©Allard Mosk/Matthias Kühmayer

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