Scientists demonstrate that transport of light in random nanowire mats is strongly correlated and governed by mesoscopic interference contributions

Physicists from the University of Southampton have demonstrated very strong, collective multiple scattering and mesoscopic (between macroscopic objects and the microscopic, atomic world) light transport in three-dimensional nanowire mats, which is of considerable importance for applications in next-generation light-harvesting and optoelectronics devices.

For many years, researchers have been excited by the possibility to stop light in strongly scattering media by self-interference of many light paths, in an effect known as Anderson localisation. The concept of wave localisation was first proposed by physicist Philip Anderson in 1958, who received a Nobel Prize for his idea in 1977.

While initially most localisation research was done on electrons, it was soon realised that the predicted effects are universal for all types of waves, including acoustic and electromagnetic (light) waves. The direct observation of these kinds of interference effects for light in the optical regime requires very strong scattering materials, and has so far been elusive.

Now, researchers at the University of Southampton have demonstrated very pronounced interference correlations in the way that light is transmitted through a strongly scattering layer. The layers were fabricated at the University of Eindhoven in the Netherlands, using mats of semiconductor nanowires, which are one of the strongest three-dimensional scattering materials for light. Such nanowire mats are of considerable technological interest for applications in solar cells and next-generation LED lighting, as has been investigated for many years by the Eindhoven group in close collaboration with Philips Research laboratories.

In the study, which is published in Nature Photonics, the Southampton researchers showed that, because of the interferences, light transmission takes place through a minimum of around three open transmission modes, which is a record low value for light in a three-dimensional medium. Dr Otto Muskens, from the Quantum Light and Matter Group at the University of Southampton and co-author of the paper, says: “By using statistical methods originally developed for microwave waveguides, we were able to demonstrate that transport of light through nanowire mats is strongly correlated and governed by mesoscopic interference contributions.

“Rather than guiding of light through individual nanowires, as might have been intuitively expected, these modes represent the collective light scattering by the dense array of wires. The presence of strongly correlated transport shows that traditional light diffusion models are no longer valid when describing photon transport and emission in strongly scattering nanowire mats, which is of significant importance for the optimisation of light management in nanowire devices for harvesting and emission.”

The present results do not yet show Anderson localisation, however the researchers believe that this landmark is within reach and might be reached by further optimisation of the nanowire mats. By tuning the geometry and arrangement of the nanowires, next-generation devices may be built that exploit disorder in new and unexpected ways.