Evrim Yazgin
Cosmos science journalist
Ultrathin lightsails using lasers to propel probes to the nearest star systems at high speeds are a step closer.
The idea of lightsails is something out of science fiction. As early as the 1950s and ‘60s, science fiction writers John Campbell, Arthur C. Clarke and others were toying with the idea of “solar sails” for spacecraft propulsion.
A program started in 2016 by theoretical physicist Stephen Hawking and scientist Yuri Milner is trying to bring that science fiction dream into reality.
The program, called Breakthrough Starshot Initiative, is based at the California Institute of Technology (Caltech). Project researchers report in a paper published in Nature Photonics that they have developed a platform to test the materials that could one day form lightsails.
“The lightsail will travel faster than any previous spacecraft, with potential to eventually open interstellar distances to direct spacecraft exploration that are now only accessible by remote observation,” explains Starshot Lightsail research director and senior author Harry Atwater.
Atwater’s team have engineered a test to measure the force that lasers exert on ultrathin membranes of different materials.
“There are numerous challenges involved in developing a membrane that could ultimately be used as lightsail,” Atwater says. “It needs to withstand heat, hold its shape under pressure, and ride stably along the axis of a laser beam.
“But before we can begin building such a sail, we need to understand how the materials respond to radiation pressure from lasers. We wanted to know if we could determine the force being exerted on a membrane just by measuring its movements. It turns out we can.”
In principle, lightsails act as mechanical resonators. They vibrate when hit by light. The issue is that these vibrations are mainly driven by heat from the laser bean, masking the direct effect of radiation pressure.
The team turned this into an advantage and tested for both the force and power of a laser beam on a mini lightsail.
“We not only avoided the unwanted heating effects but also used what we learned about the device’s behaviour to create a new way to measure light’s force,” says first author Lior Michaeli.
A lightsail in space won’t always be perpendicular to a laser source on Earth. So, the team angled the laser beam and measured its force on the mini sail. They were also able to account for the laser beam spreading out at an angle.
They found the force under these conditions was lower than expected. The team suggests that this is because light is scattering off the edges of the lightsail. They believe the use of nanomaterials and metamaterials could help control the sideways and rotation of a miniature lightsail.
“The goal then would be to see if we can use these nanostructured surfaces to—for example—impart a restoring force or torque to a lightsail,” says co-author Ramon Gao. “If a lightsail were to move or rotate out of the laser beam, we would like it to move or rotate back on its own.”
“This is an important stepping stone toward observing optical forces and torques designed to let a freely accelerating lightsail ride the laser beam,” Gao adds.
