Despite being tightly confined in a micro­scopic volume, a new kind of disorder allows optical signals to suddenly show up at far away regions. Such abrupt transport had previously been considered impossible, and challenges the current under­standing of light waves. 

In 1958, Phil Anderson astonished the scientific community by predicting that an electrical conductor will suddenly lose its conduc­tivity and turn into an insulator, as soon as its atomic lattice is perturbed beyond a critical level: In the jargon of physicists, disorder can bring the free motion of electrons to a halt and block any electrical current from flowing through a previously conduc­tive material. This Anderson locali­ation lies outside the scope of classical physics, and only a quantum-mechanical treatment of electrons as both particles and waves can explain the metal-insulator transition resulting from it.

Today we know that this effect, for which Phil Anderson won a share of the Nobel Prize in Physics 1977, applies in general: Disorder can likewise suppress the propagation of sound waves or even light beams. Recently, Alexander Szameit and his graduate students Sebastian Weidemann and Mark Kremer made a surprising disco­very: Every realistic physical system inevitably exchanges energy with its environ­ment, and, as soon this energy exchange becomes disordered, (light) waves can also become localized. This new class of disorder transcends the mechanism that Phil Anderson considered in 1958, since his calcu­lations were based on the assumption that no inter­actions take place with the environment.

Szameit explains: “In our experiments, we could clearly observe how light becomes focused into tiny regions in space, as soon as the energy exchange in the environment becomes randomized.” At first glance, these results appeared to be merely a generali­zation of the well-known suppression of transport. However, much to their surprise, the researchers soon discovered quite the opposite: “At first, we did not believe our eyes when we saw how the brightest light spot seemed to suddenly jump to an entirely different regions in space, again and again, even though conven­tional light propa­gation should have been suppressed entirely arrested by the disorder.” 

Responsible for this hitherto unknown behaviour of light waves is the complex energy exchange with the environment. Szameit recalls: “Any remaining doubts vanished when were able to prove that this effect can shuffle around light signals between specific points in 5-kilometre-long optical fiber.” These pio­neering results are a concep­tional break­through for funda­mental science, and the universal nature of the under­lying mechanism may inform new techniques to shape not only the flow of light, but of acoustic or particle waves as well. (Source: U. Rostock)

Reference: S. Weidemann et al.: Coexistence of dynamical delocalization and spectral localization through stochastic dissipation, Nat. Phot., online 17. Juni 2021; DOI: 10.1038/s41566-021-00823-w

Link: Experimental Solid-State Optics, Institute for Physics, University of Rostock, Rostock, Germany