Scientists at the Vienna University of Technology (TU Wien) showed how the branched flow of light can be made to propagate along a single branch rather than along many branches at the same time. Their approach, which they think could be applied to different types of waves, from scalar to vector and from two to three dimensions, is based on shaping the incoming wavefront. The wave signal is then sent exclusively along a single preselected branch and is hardly noticeable anywhere else. The branched flow of lightwaves occurs when an incoming wave splits into several paths, reaching some places with high intensity, while avoiding others almost completely. To control the branching, the researchers first allowed the wave to branch out in all possible directions. They measured the wave in detail at one of the places where it branched at high intensity. Using these measurements they calculated how a wave would need to be shaped at its source in order to be sent along one selected path, while avoiding other paths. Waves do not always spread uniformly into all directions, but can form a remarkable “branched flow.” At TU Wien (Vienna) a method has now been developed to control this phenomenon. Courtesy of TU Wien. “We used numerical simulations to show how to find a wave that behaves exactly the way we want it to,” professor Stefan Rotter said. This approach could be applied using a variety of different methods, he said. “You can implement it with lightwaves that are adjusted with special mirror systems or with sound waves that you generate with a system of coupled loudspeakers. Sonar waves in the ocean would also be a possible field of application. In any case, the necessary technologies are already available.” The preselected path does not need to be a straight trajectory, researcher Andre Brandstötter said. “Many of the possible paths are curved — the irregularities of the surroundings act like a set of lenses by which the wave is focused and deflected again and again,” he said. The researchers showed that the light flowing along a single branch has a broadband frequency stability, which would allow pulsed signals to be sent through specific paths to transmit information in a targeted manner. The wave signal would arrive exactly where it is supposed to be received while at other locations it would hardly be detected, making eavesdropping more difficult. The branched flow of lightwaves occurs in disordered, irregular environments and can occur with all types of waves and on different length scales, from laser beams that are sent into the surface of a soap bubble to tsunami waves in the ocean, traveling in a branched pattern that depends on the random shape of the ocean seabed. The research was published in Proceedings of the National Academy of Sciences (http://dx.doi.org/10.1073/pnas.1905217116).