Researchers from the University of Rostock and the Technion - Israel Institute of Technology demonstrated a mechanism that prevented lightwaves from spreading freely. The underlying physical effect was previously considered too weak to fully arrest wave expansion. The researchers observed that such light localization was nevertheless possible, demonstrating the sensitivity of wave propagation across a wide range of spatial length scales. In 1958, physicist Phil Anderson predicted that an electrical conductor, such as copper, could abruptly turn into an insulator, such as glass, when the atomic crystal order was sufficiently changed. The presence of this so-called disorder could stop the motion of the otherwise freely moving electrons, preventing any substantial electrical currents through the material. The phenomenon is called Anderson localization, and it can be explained by quantum mechanics, in which electrons are treated as particles as well as waves. The effect also applies to classical settings, in which disorder can suppress the propagation of sound waves and light beams. In the recent work, the researchers discovered that lightwaves may even show that Anderson localization is induced if the disorder is practically invisible to them. This type of disorder exclusively contains spatially periodic distributions with certain wavelengths. “Naively, one would expect that only those waves whose spatial distributions somehow match the length scales of the disorder can be affected by it and potentially experience Anderson localization,” said Sebastian Weidemann, a doctoral student at the University of Rostock Institute for Physics. “Other waves should essentially propagate as if there were no disorder at all,” researcher Mark Kremer said. In contrast, the recent theoretical work from the Technion team suggested that the propagation of waves could be dramatically affected even by such “invisible" disorder. “When lightwaves can interact multiple times with the invisible disorder, a surprisingly strong effect can build up and arrest all light propagation,” doctoral student Alex Dikopoltsev said. Laser light ensnared in an invisible trap. Light propagates in coupled optical fibers. Even though the disorder (not shown), should not affect the lightwaves, the propagation into neighboring optical fibers strongly suppressed Anderson localization, such that the light remained contained within a few optical fibers. The structure of the fiber mesh allows light to emulate the motion of electrons in disordered materials. Courtesy of the A. Szameit/University of Rostock. The collaborating research groups constructed artificial disordered materials from kilometers of optical fiber, which they arranged so that the optical networks emulated the spatial spreading of electrons in disordered materials. This enabled the researchers to make direct observations of the practically invisible structures ensnaring lightwaves. According to the researchers, the demonstration and discovery could pave the way toward a new generation of synthetic materials that harness disorder to selectively suppress currents — whether light, sound, or electrons. The Deutsche Forschungsgemeinschaft, the European Research Council, and the Alfried Krupp von Bohlen und Halbach Foundation supported the work. The research was published in Science Advances (www.science.org/doi/10.1126/sciadv.abn7769).