Tiny ‘metajets’ could use light to steer sails for interstellar travel

Interstellar travel propelled by light just got one step closer. Light sails, which are huge sheets pushed along by light that bounces off of them, may be the best way to travel enormous distances through space, and now we may have a way to steer them.

“We knew already that any light or laser can impart momentum transfer, but now we can control the direction as well,” says Kaushik Kudtarkar at Texas A&M University. He and his colleagues created a tiny device called a metajet that uses refraction of light, not just reflection, to move in more than one direction at once.

The device is a material called a metasurface, an extremely thin sheet textured to manipulate light. In this case, the researchers flipped that on its head, using the light to manipulate the metasurface. A series of tiny pillars on the material steers the light that hits it, with the size and pattern of the pillars controlling the strength and direction of the momentum that the light imparts on the entire device as it moves through. The whole thing is about 0.01 millimetres across.

To test it, the researchers dropped the silicon device in water and shone a laser on it, watching it with a microscope to track its motion. They found that the metajet both levitated and moved horizontally, reaching a maximum speed of about 0.07 millimetres per second.

“Now that we know about the forces on this device, you can change the metasurface design and then you can steer it in any way you want,” says Kudtarkar. There are metasurfaces that change their shapes over time, and such a material could be used on light sails to steer them through space, he says.

“For space, you can expand it, but you can keep it the same size and use it for biomedical applications as well: these devices could literally push drugs to a specific location,” says Kudtarkar. Lasers can already be used to do this, but their heat can damage the molecules, and with metajets, the drugs would not be directly exposed to the heat and light of a laser beam.

The researchers are now looking to make their device work with different wavelengths of light, especially the broad spectrum of sunlight to make them more compatible with the type of light sail that would be used for space travel. “It’s all a bit sci-fi,” says Kudtarkar.