The shape and internal properties of a tiny plastic ‘lightfoil’ bend laser light. The angles of the incoming and outgoing light (red) determine the direction in which the object moves (blue) (Image copyright: New Scientist)

Light has been used to generate aerodynamic-like lift for the first time. The technique, which takes advantage of the fact that light bends, or refracts, when moving from one medium to another, could be used to create solar-sail spacecraft that could steer using light itself.

Photons create pressure when they bounce off objects. Solar sail prototypes are made highly reflective to maximise this push, but the effect does not allow the sails to be easily steered. “It’s well known you can use a light source to push on something, but the steering mechanisms are still up for grabs,” says Grover Swartzlander of the Rochester Institute of Technology in New York.

He says future sails could be manoeuvred if the photons did not just rebound off the material’s surface but passed through it. As they entered and exited the sail, the photons would change direction by an amount dictated by the shape of the material’s surface and its so-called refractive index. The angles of the incoming and outgoing light would control the direction of the sail’s movement (see graphic).

Swartzlander and his colleagues demonstrated the effect in the lab with semi-circular plastic rods, each just a fraction of the size of a human hair.


Asymmetrical shape

They put the rods in a container of water, then shined laser light on them from below. The rods floated due to radiation pressure, as any object of similar mass would. But crucially, they also drifted sideways – a sign that they had been steered by refracted light.

The fact that the rods’ asymmetrical shape affected their movement makes them the optical equivalent of aeroplane wings, or aerofoils, says the team. Wings, whether bird or Boeing, soar in part because air moves faster over their top sides, reducing the pressure above. The relatively high pressure below pushes upwards, providing lift.

Swartzlander says a future solar sail could be fully controlled in 3D with two perpendicular arrays of semi-circular rods.

But Dean Alhorn, lead engineer of NASA’s recently launched NanoSail-D solar sail experiment, says sunlight may be too weak to do this in practice and is exploring ways to control the craft with reflected light alone.

Journal reference: Nature Photonics (DOI: 10.1038/NPHOTON.2010.266)