Cloud-inspired material can bend light around corners

Scientists have discovered a technique whereby light can be bent around corners, inspired by the way clouds scatter sunlight. This type of light-bending could lead to advances in medical imaging, electronics cooling and even nuclear reactor design.

Daniele Faccio at the University of Glasgow, UK, and his colleagues say they are shocked this type of light scattering wasn’t noticed before. 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 at all and getting reflected out the way they came. For instance, when sunlight hits a tall cumulonimbus cloud, it bounces 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 grey – despite being made up of the same water droplets.

“The light bounces around and sort of tries to get in, and it’s bouncing off all the molecules and the defects,” says Faccio. “And eventually what happens 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 within. When light is shone at the material, it travels into these tunnels and is scattered – just as light is on snow or clouds. However, instead of scattering randomly in every direction until they are evenly dispersed, the photons are directed to return to the resin tunnel by the opaque material. The team put this to use, creating a range of objects that steer light in an organised way.

These 3D-printed objects are functionally similar to fibre optic cables, which route light along their length, but they operate on fundamentally different principles. Fibre optic cables steer light by infinitely reflecting internally. When photons attempt to leave a cable’s inner core of plastic or glass, they hit another material with a lower refractive index and are reflected back inside. In this way, light can be carried for kilometres at a time, even around bends.

The researchers say their material boosts light transmission by more than two orders of magnitude compared with solid blocks without the same clear tunnels, and also allows it to be directed around curves. This is much less efficient than fibre optic, and will therefore struggle to achieve the great distances that it does, but it is also very simple and cheap.

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

“It wasn’t 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 some really niche, weird equation with some weird properties.”

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