Cloud-inspired material can bend light around corners

Scientists have discovered a technique that can bend light around corners, inspired by the way clouds scatter sunlight. This kind of light bending could lead to advances in medical imaging, electronic cooling and even nuclear reactor design.

Daniele Faccio at the University of Glasgow, UK, and his colleagues say they are shocked that this type of light scattering was not noticed earlier. It works on the same basis as clouds, snow and other white materials that absorb light: once photons hit the surface of such a material, they are scattered in all directions, barely penetrating and reflected back the way they came. For example, when sunlight strikes a tall cumulonimbus cloud, it reflects off the top, making this part of the cloud appear bright white. But so little light reaches the bottom of the cloud that this part appears gray – despite being made up of the same water droplets.

“The light bounces around trying to get in, so to speak, and it bounces off all the molecules and defects,” says Faccio. “And what ends up happening is it just gets reflected back because it can’t get in. This is this scattering.”

To replicate this process, the team 3D printed objects from opaque white material, while leaving thin tunnels of clear resin inside. When light shines on the material, it travels into these tunnels and is scattered – just like light strikes snow or clouds. However, instead of spreading randomly in each direction until they are evenly dispersed, the photons are sent through the opaque material to return to the resin tunnel. The team took advantage of this and created a series of objects that direct light in an organized way.

These 3D printed objects are functionally similar to fiber optic cables, which conduct light along their length, but they work on fundamentally different principles. Fiber optic cables direct the light by internally reflecting infinitely. When photons try to leave the plastic or glass inner core of a cable, they hit another material with a lower refractive index and are reflected back in. In this way, light can be transported for miles, even around bends.

The researchers say their material increases light transmission by more than two orders of magnitude compared to solid blocks without the same bright tunnels, and also allows it to be focused around curves. This is much less efficient than fiber optic and will therefore struggle to reach the long distances that it does, but it is also very simple and cheap.

This method of light bending could use existing tunnels of translucent material, such as tendons and fluid in the spine, to provide new ways to perform medical imaging. Faccio says the exact same principle also works to direct heat and neutrons, and therefore could also find application in a range of engineering applications such as cooling systems and nuclear reactors.

“It was not obvious that this would work at all. We were shocked,” says Faccio, who believes the phenomenon could easily have been discovered decades or even centuries ago. “It’s not like we’ve created or found a very niche, weird equation with some weird properties.”