Light Caught In Invisible Chains

In cooperation with the Technion, the Israeli institute for technology, physicists from the University of Rostock succeeded in providing the first experimental evidence of a new type of physical effect that prevents light waves from propagating spatially. Until now, it was assumed that this effect was too weak to actually block light: the discovery made by the team of scientists shows that structures that are almost invisible to light can also have a dramatic effect on the propagation of light waves. The research results were recently published in the renowned journal Science Advances.

In 1958, Phil Anderson surprised the international research community by showing mathematically that an electrical conductor such as copper can suddenly behave like an electrical insulator such as glass once the atomic lattice order is disturbed enough. Such a “disorder” can suddenly hold (“localize”) the otherwise freely moving electrons in place – and thus prevent any current flow. This phenomenon was given the name "Anderson localization" and could only be explained with the help of modern quantum physics, in the context of which electrons are considered not only as particles but also as waves at the same time. In the meantime, this effect, for the prediction of which Phil Anderson received a share of the Nobel Prize in Physics in 1977,

Professor Alexander Szameit from the University of Rostock and Professor Mordechai Segev from the Technion deal with the properties of light and its interaction with matter in their work. Only recently, Professor Segev's team made an amazing discovery: light waves could also be stopped by a new type of disorder that is practically invisible to the waves. This type of disorder goes well beyond Phil Anderson's 1958 consideration, as it strongly favors certain spatially periodic distributions. "Previously it was thought that only those waves can be influenced (and therefore show an Anderson localization) whose spatial structures match the spatial distribution of the disorder," explains Sebastian Weidemann, a doctoral student at the Rostock Institute of Physics in Szameit's group. "Other waves, on the other hand, spread almost undisturbed," adds Dr. Mark Kremer, also from the group around Professor Szameit.

However, the Israeli team from the Technion recently predicted in a theoretical paper that the propagation of waves could also be strongly influenced by an "invisible disorder". "By light waves interacting with the almost invisible disorder several times in a row, an unexpectedly strong effect can arise that even forces such light waves to (Anderson) localize," explains doctoral student Alex Dikopoltsev from the Technion team.

In close cooperation, the physicists from Rostock and Israel have now designed and carried out an experiment that demonstrates this effect for the first time. "We could clearly see that light waves are limited to small spatial areas even if the disorder is supposed to be practically invisible to them," says Weidemann. For their experiment, the scientists created the disordered structures artificially in the laboratory: "To do this, we connected kilometers of optical glass fibers in such a way that the propagation of light in these fibers imitates the movement of electrons in disordered materials," explains Weidemann, who, together with Mark Kremer, developed the conducted an experiment at the Rostock Institute for Physics.

The discoveries are an important step in basic research into wave propagation in disordered systems and potentially form the basis for further technical applications in which disorder can selectively suppress currents - whether for light, sound or electrons.

The research, the results of which have now been published in the journal Science Advances , was funded by the German Research Foundation (DFG), the European Union and the Alfried Krupp von Bohlen und Halbach Foundation.