light wave Image
A light wave is irradiated in a disorderly, irregular environment and is divided into several branched paths. Bottom: By deliberately shaping the wave at the bullet (left), the wave only moves on a single preselected lane instead of ramifying.

In free space, the light wave of a laser beam spreads on an exactly straight line. Under certain circumstances, however, a much more complicated behavior occurs. If the movement of the wave is influenced by a disorderly, irregular environment, a strange phenomenon can occur: The wave splits into several paths, it branches out in a complicated way, some places reach them with high intensity, others almost not at all.

Such "wave ramification" was first observed in 2001. Now, at the Vienna University of Technology, a method has been developed to make targeted use of this effect. The core idea of this new method is to send a wave signal only along a single selected branch, making the wave hardly noticeable anywhere else.

"Originally, this effect was discovered by examining electrons that move as quantum waves through tiny microstructures," explains Prof. Stefan Rotter from the Institute for Theoretical Physics of the Vienna University of Technology. "Such structures that send the electrons are never perfect, there are always some irregularities and, amazingly, they cause the electron wave to split and ramify, which is why the term 'Branched Flow' has become established for this phenomenon."

It soon became clear that this wave phenomenon not only occurs in quantum physics, but is basically possible with all kinds of waves on very different scales. For example, if you let laser rays enter the surface of a soap bubble, they split into several sub-beams like tsunami waves in the ocean; the latter do not spread regularly across the ocean, but in a complicated, ramified pattern depending on the random shape of the ocean floor. Thus, it can happen that a distant island is hit very hard by a tsunami, while the neighboring island is only reached by much weaker wave fronts.

"We wanted to know now whether these waves can be influenced in such a way that they no longer propagate along a branched network of paths in very different directions, but remain on a single, previously selected lane," explains Andre Brandstötter (TU Vienna). "And as it turns out, it is actually possible to target individual branches."

It takes two steps to do that. First, you let the wave ramify as usual on all lanes. In one of the places that are reached with high intensity, the wave will now measure accurately. With the method, which was developed at the Vienna University of Technology, it can then be calculated from how the wave has to be formed when shooting, so that in the second step it only moves along the selected path and avoids all other paths.

"We used numerical simulations to show how to find a wave that behaves exactly in the way you want it, and if you know that result, you can use different methods," says Stefan Rotter. "You can do it with lightwaves matched with special mirroring systems, or with soundwaves created with a system of paired loudspeakers, and sonar waves in the ocean are a potential application, and the technology is there for you."

All these waves could be sent on a journey along a selected train using the new method. From the ramified network of paths on which the wave previously moved, only a single path remains. "This path does not even have to be straight," explains Andre Brandstötter. "Many of the possible paths are curved - the irregularities of the environment act like several lenses, from which the wave is repeatedly focused and distracted."

Even pulsed signals can be transmitted on these special paths, so that you can selectively transmit information about them. Thus, a wave signal is guaranteed to arrive where it is to be received, in other places it can hardly be detected and thus not be heard.

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