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 type of light bending could lead to advances in medical imaging, electronics cooling, and even nuclear reactor design.

Daniele Faccio Researchers at the University of Glasgow, UK, and their colleagues say they are shocked that this type of light scattering has gone unnoticed. This works on the same principle as clouds, snow, and other white substances that absorb light. When a photon hits the surface of such a material, it is scattered in all directions, with little penetration and reflection in the direction it came from. For example, when sunlight hits a high cumulonimbus cloud, it reflects off the top of the cloud, making that part of the cloud appear brighter and whiter. However, because little light reaches the bottom of the cloud, this area appears gray even though it is made of the same water droplets.

“The light bounces around and tries to penetrate in a sense, and it reflects off all the molecules and defects,” Faccio says. “And in the end, the light doesn’t get in, it just gets reflected. That’s scattering.”

To replicate this process, the team 3D printed the object from an opaque white material, leaving a thin tunnel of transparent resin inside. When light hits a material, it tunnels and scatters, just like light hits snow or clouds. However, rather than scattering randomly in all directions until they are evenly distributed, the photons are directed back into the resin tunnel by the opaque material. The team used this to create a variety of objects that channel light in an organized way.

Although functionally similar to fiber optic cables that transmit light along their length, these 3D printed objects operate on fundamentally different principles. Fiber optic cables control light by infinitely reflecting it inside. When a photon attempts to leave the cable’s plastic or glass inner core, it hits another material with a lower index of refraction and is internally reflected. In this way, light can be carried several kilometers at a time, even around bends.

The researchers say the material increases light transmission by more than two orders of magnitude compared to the same transparent, non-tunneled solid block, and can also direct light around curves. It is much less efficient than fiber optics, making it difficult to achieve long distances, but it is also very easy and cheap.

This light-bending method could provide a new way to perform medical imaging by taking advantage of existing translucent material tunnels, such as tendons or body fluids, within the spinal column. Faccio said the exact same principle works in the direction of heat and neutrons, so it could be applied to a variety of engineering applications, such as cooling systems and nuclear reactors.

“It was not at all clear that this would work. We were shocked,” Faccio said, noting that the phenomenon could have easily been discovered decades or even centuries ago. I believe there is. “It’s not like we’ve created or discovered a really niche, weird equation that has some weird properties.”