Thanks to its wave-like properties, light can produce complex interference patterns when it interacts with materials – rather like the water ripples on the surface of a lake. For many years now, scientists have been trying to produce materials that interact so strongly with light that they modify its flow. Examples of such materials are photonic crystals, which are periodic structures that act as “cages” for light.

For disordered structures, random light scattering and interference can produce a new effect called localization in which a light wave becomes “stuck” in closed paths inside the material, bouncing back and forth in complex looping paths called “modes”. Photons cannot easily escape but instead travel around in circles inside the medium.

This effect (which works for all types of waves, be they acoustic or electromagnetic) was predicted in1958 by a scientist called Philip Anderson. He received the 1977 Nobel Prize in Physics for his discovery. If it could be made to occur inside a device that works by absorbing photons (a solar cell, for example), then the cell could be more efficient at converting light into electricity. In LEDs the opposite effect would be seen: photons would build up in the device and lasing would be seen in certain regions as the photons started interacting with each other – something that would reduce the threshold for light emission, explains team leader Otto Muskens.

Critical threshold

Actually observing Anderson localization in real experiments is no easy task, however, and no-one has ever done so. Indeed, researchers have only very recently started to see localization effects in the laboratory for acoustic and matter waves. “For light waves in 3D materials, we can only analyse the light that ‘leaks out’ of a material,” said Muskens, “but in our new experiments we have clearly seen some of the predicted interference patterns near a so-called critical threshold for Anderson localization. Such observations are exciting in themselves because they show that we are very close to seeing the real Anderson localization regime.”

The Southampton team studied a mat of GaP semiconductor nanowires, fabricated with the help of colleagues at the University of Eindhoven and Philips Research Laboratories in Germany. GaP is one of the strongest 3D light-scattering materials known and the researchers were able to convert a coherent laser beam into random light by passing the beam through the nanowires structure.

Strongly correlated transport

“By using methods from statistical optics, we were able to show that the laser light becomes ‘grainy’ after passing through the GaP mat,” Muskens told nanotechweb.org. “We collected several thousands of different snapshots of these grainy light patterns leaking out of different places on the sample, something that allowed us to compare our results with theoretical predictions.”

It turns out that the light transported through the nanowires does not become completely random but still remains strongly correlated, interfering inside the nanostructure thanks to so-called mesoscopic interference. Such strongly correlated transport invalidates conventional light diffusion models for describing photon transport and emission in certain nanostructures, like these light-scattering nanowire mats, says Muskens.

Collective versus single nanowire effects

Mesoscopic simply means on a length scale between that of the individual nanowires and the macroscopic scale of the sample, he explains. “In practice, mesoscopic effects become important on a scale of 10–100 times the diameter of the nanowires, but it is important for us to be able to distinguish collective effects from the effects caused by single nanowires,” he said. “Anderson himself once said that ‘more is different’ (the title of one his papers), implying that when you increase the complexity and scale of a system, new physics will emerge. It is not just the simple sum of the individual building blocks (nanowires in our case) that is important, but how the blocks are arranged. A good example of this principle is high-temperature superconductivity, but in photonics an emergent phenomenon is wave localization.”

Because arrays of nanowires are increasingly being used in technologies such as light-emitting diodes and solar cells, such mesoscopic effects might be exploited to improve these applications, he adds. “It is particularly tempting to think about how increased light trapping might enhance nanowire photovoltaics. For example, the effect of trapped light travelling in circles could be used to increase light absorption in thin-film devices. We might even be able to improve LED performance by coupling to the light looping modes, but much more research is still needed before we can say this with any certainty.”

The current work is published in Nature Photonics.