How light manages disorder

Lace J. Hernandez Cruz

In the absence of disorder, in a radiation passing through a slab of material, energy flows orderly and is distributed among channels defined by the angle of incidence that remains constant as the light propagates through the slab. If the slab is broken up into random pieces, the progression of energy losses its order and lots of energy is scattered back to where it came from. If the disorder is a great one, almost no energy makes it to the other side of the slab.

Experiments at the University of Twente, leaded by Ivo Vellekoop and Allard Mosk, showed evidence that this scattering may be overcome to allow a large fraction of the incident light to pass through opaque matter, as reported in Physical Review Letters. Their experiment was based on measuring how much light is transmitted through a disordered sample. The modes became mixed up and little light emerged. They then tried to sneak the light through by sending it at the sample from different angles and with different phases. After many years of studies, they found out transmission could be increased in this way by almost a factor of 103, confirming their conclusions.

The experiment was performed on samples of disordered zinc oxide particles with average diameter of 200 nm, which strongly scatter light so that the mean free path is only 0.85 μm. Two sets of samples were used-one 5.7 and the other 11.3 μm thick. Light from a laser was first reflected off of a liquid-crystal display, which could be contoured to shape the reflected wavefront, and then focussed onto the samples. The liquid-crystal display had 3816 independently programmable segments that were adjusted to optimize transmission.

At high levels of disorder a phenomenon known as localization sets in and it is much more difficult to find open channels. This happens because the scattering caused by disorder results in destructive interference of propagating waves, causing the light to be stopped in its tracks. In fact, they found out a relation between channels and thickness of the sample, and it is that the number of open channels falls off exponentially with sample thickness for strong disorder.

The perturbation theory, which has been a useful tool among experiments, seems not to work for strong disorder. This is why the new optical experiments are so important. Localization was initially studied in the context of spin diffusion and electron localization, but the additional complication of electron-electron interaction makes experiments hard to interpret.

At random locations and at random frequencies, resonant traps occur that light finds difficult to enter and difficult to escape from. These form the closed channels that almost always reject the light, preventing it from passing through the system. Sometimes, the resonances could be uniformly spaced across the sample and lined up in frequency so that incident light can use them to step from one resonance to another.

Independent of dimensionality and independent of the strength of the disorder, the open-channel/closed-channel result has been shown to be a universal theorem. For weak disorder, the open channels have a more complex structure: they follow a random path between the two sides of the sample rather than acting as chains of localized resonances. As disorder is increased, they reach a final destination without crossing the sample and those connections that remain take a direct route. Nevertheless, we will need further experiments similar to the ones explained here to have a final decision upon this issue.