Cracking the puzzle of white

Clouds, milk, bones – all have one thing in common: they’re white because their constituents are sized and arranged in ways that efficiently reflect light. Now, NCCR Bio-Inspired Materials researchers showed that a specific type of material can diffusely reflect nearly all rays of incoming light – a finding that paves the way for applications in solar energy, telecommunications, and light-based computing.

The work could also help to realize a phenomenon that is known to happen for all kinds of waves, but hasn’t yet been observed experimentally for light. The phenomenon was described in the 1950s by late theoretical physicist and Nobel-prize winner Philip Anderson, who showed how an electron and its quantum wave can get arrested in place in a disordered medium. This so-called ‘Anderson localization’ was then proposed to be applicable also to light waves, and many researchers have tried to study it in disordered optical materials. “Anderson localization is a kind of holy grail in condensed matter and optical physics,” says NCCR Principal Investigator Prof. Frank Scheffold of the University of Fribourg’s Department of Physics.

Scheffold and Dr. Luis Froufe-Pérez, a senior researcher in Scheffold’s group, teamed up with Dr. Jakub Haberko at the AGH University of Science and Technology in Krakow, Poland, to investigate Anderson localization of light in disordered hyperuniform materials. These materials, first proposed about ten years ago by a group of researchers at Princeton University, are computer-conceptualized systems with a very well-defined mathematical basis which are starting to be discovered in nature. Their structure falls between that of highly-organized crystals and that of disordered materials, Scheffold says. “It’s a designer disorder,” he adds.

Using computer simulations, Scheffold and his colleagues found that the nano-architecture of this disordered hyperuniform material can reflect nearly all incoming light diffusely. “If you reach Anderson localization, you have perfect reflection, and if you’re close to it, you have strong reflection,” Scheffold explains. The researchers discovered that under certain conditions, the material reflects more than 99.9% of light. The findings were published in the journal Nature Communications.

“If light cannot penetrate the material, then the material acts like a perfect diffuse reflector,” Scheffold explains. This means that if researchers made a channel inside the material, light could never leave the channel — this would create an optical wire in which light could be ‘channeled’ and ‘guided’ on-demand. Because photons typically move faster than electrons, hyperuniform photonic materials could serve to develop ultrafast and energy efficient optical computers and other devices, nowadays governed by electronics.

The secret behind a perfectly reflective material lies in its complex nano-geometry. The mathematically defined distribution of points can be transformed into a disordered hyperuniform network, for example, by arranging material building blocks so that every junction has four arms that connect with other junctions. This makes every part of the structure look the same on the small scale, even though the material is disordered when zooming out. Scheffold notes that nature has evolved very sophisticated  strategies to reflect light, but to reach Anderson localization, researchers have to go one step further. Confirming and observing this effect experimentally would not only lead to applications in photonics, but it could also explain fundamental physics phenomena of how light interacts with complex structural media.

Reference Haberko, J.; Froufe-Pérez, L. S.; Scheffold, F. Transition from light diffusion to localization in three-dimensional amorphous dielectric networks near the band edge, Nat. Commun., 2020, 11, 4867. 

Text by Giorgia Guglielmi