Researchers have created light waves that can penetrate opaque materials.

Source: Allard Mosk/Matthias kuh Mayer

Why is sugar opaque? Because light passing through a piece of sugar is scattered, changed and deflected in a very complicated way. However, a research team from Vienna University and utrecht university, the Netherlands, has now been able to prove that this method is not suitable for a very special light wave: for any specific disordered medium, such as the cube sugar you just put in coffee, you can build a customized light beam, which will not actually be changed by this medium, but will only be attenuated. When the light beam passes through the medium, the light pattern reaches the other side. Its shape is the same as the medium does not exist at all.

The concept of "light scattering invariant mode" can also be used to examine the interior of an object. The research results have been published in the journal Nature Photonics.

Countless possible waveforms

Waves on turbulent water can have countless different shapes-similarly, light waves can be generated in countless different forms. Professor Stefan Rotter of the Institute of Theoretical Physics in Tuveen explained: "When you let light waves pass through a disordered medium, the mode of each light wave will change and deflect in a very specific way."

Stefan Rotter and his team are developing mathematical methods to describe this light scattering effect. Professor Allard Mosk's team in utrecht university has provided professional knowledge for manufacturing and characterizing this complex light field. "As a light scattering medium, we used a layer of zinc oxide, which is an opaque white powder composed of completely randomly arranged nanoparticles," explained Allard Moske, head of the experimental research team.

First of all, you must describe this layer accurately. You irradiate zinc oxide powder with a very special light signal, and then measure how they reach the detector at the back. From this, you can calculate how any other wave is changed by this medium-in particular, you can calculate which wave mode is changed by the zinc oxide layer, as if there is no wave scattering at this layer at all.

"We can prove that there is a very special so-called scattering invariant mode of light grazing, which produces exactly the same wave detector, whether the light wave is only sent through the air or must penetrate the complex zinc oxide layer," Stephen Rogge said. "In the experiment, we saw that zinc oxide didn't actually change the shape of these light waves at all-it just weakened a little overall," Allard Moske explained.

Comparison: no scattered light beam. Source: Allard Mosk/Matthias kuh Mayer

Constellation on photodetector

Although these light modes with constant scattering may be special and rare, theoretically the number of possible light waves is infinite, and people can still find many such light modes. If you combine these light modes with constant scattering in the right way, you will get a constant scattering waveform again.

"In this way, at least within a certain range, you can freely choose the image you want to send through the object without interference," said Jeroen van de Weijer Bosch, a doctoral student. "In the experiment, we chose a constellation as an example: the Big Dipper. In fact, it can be determined that a scattering invariant wave sends the image of the Big Dipper to the detector, regardless of whether the light wave is scattered by the zinc oxide layer or not. For the detector, the beams in both cases look almost the same. "

Internal situation of the unit

This method of finding a light pattern that penetrates a substantially undisturbed object can also be used in the imaging process. "In hospitals, X-rays are used to observe the inside of the human body-their wavelengths are short, so they can penetrate our skin. But the way light waves penetrate an object depends not only on the wavelength, but also on the waveform, "said Matthias Kühmayer, a doctoral student engaged in computer simulation of wave propagation. "If you want to focus the light on certain points of the object, then our method opens up new possibilities. We can prove that using our method, the light distribution inside the zinc oxide layer can also be controlled specifically. " This may be interesting in biological experiments, for example, you want to introduce light at a very specific point to observe the depth of cells.

The paper jointly published by Dutch and Austrian scientists shows how important international cooperation between theory and experiment is to make progress in this research field.

More information: Pritam Pai et al., Invariant Mode of Light Scattering in Complex Media, Natural Photonics (202 1). DOI: 10. 1038/s 4 1566-02 1-00789-。

Journal information: natural photonics