Novel ‘scattering-invariant’ light mode probes opaque materials

15 Apr 2021

Researchers at Utrecht and TU Vienna develop new form of light waves that travel through opaque materials.

Researchers at Utrecht University, The Netherlands, and at TU Wien (Vienna, Austria) have created a special type of lightwaves that can penetrate opaque materials and pass through – as if the material were not obstructing or interrupting them.

The scientists say that, for any specific disordered medium, the tailor-made light beams can be constructed that are practically not changed by the medium, but only attenuated.

This concept of “scattering-invariant modes of light” can also be used to specifically examine the interior of objects, say the scientists. The results have been published in Nature Photonics.

Multiple waveforms

“Each of these light wave patterns is changed and deflected in a specific way when you transmit it through a disordered medium,” said Prof. Stefan Rotter from the Institute of Theoretical Physics at TU Wien.

Together with his team, Prof. Rotter is developing mathematical methods to describe such light scattering effects. The expertise to produce and characterise such complex light fields was contributed by the team around Prof. 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,” said Prof. Mosk, the head of the experimental research group.

“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 the zinc oxide layer as if wave scattering was entirely absent in this layer. The zinc oxide actually does not change the shape of these light waves at all – they just get a little weaker overall,” said Prof. Mosk.

Prof. Rotter added, “As we have shown, there is a 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.”

Light detector

As unusual as these scattering-invariant light modes may be, with the theoretically unlimited number of possible light waves, it is possible to identify many of them. Combining several of the scattering-invariant light modes in the correct way leads to a scattering-invariant waveform once 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 (The Plough). 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.”

This method of identifying light patterns that penetrate an object largely undisturbed could also be used for imaging procedures, say the scientists. “In hospitals, X-rays are used to look inside the body – they have a shorter wavelength and can therefore penetrate our skin. But the way a light wave penetrates an object depends not only on the wavelength, but also on the waveform,” said Matthias Kühmayer, who works as a Ph.D. student on computer simulations of wave propagation.

“Our method opens up completely new possibilities. This could be interesting for biological experiments, for example, where you want to introduce light at very specific points in order to look deep inside cells,” said Bosch.