Scientists from the California Institute of Technology described how work was progressing on the creation of a vehicle with a space sail, which should go on a journey to a neighboring star.

Fantastic idea
The idea of traveling through interstellar space using spacecraft propelled by ultra-thin sails may seem like something out of a sci-fi novel. But in fact, a program founded in 2016 by Stephen Hawking and Yuri Milner, known as the Breakthrough Starshot Initiative, is exploring the idea.
The concept is to use lasers to drive miniature space probes attached to “light sails” to reach high speeds and finally our nearest star system, Alpha Centauri. The California Institute of Technology is leading a global community working toward this bold goal.
“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 Harry Atwater, the Otis Booth Chair in Engineering and Applied Science and the Howard Hughes Professor of Applied Physics and Materials Science at the California Institute of Technology (Caltech).
Now Atwater and his colleagues at the California Institute of Technology have developed a platform to characterize ultrathin membranes that could one day be used to make these light sails. Their test platform includes a way to measure the force that lasers apply to the sails, which will be used to send the spacecraft into space. The team’s experiments are the first step in moving from theoretical proposals and light sail design to actual observations and measurements of key concepts and potential materials.
Experiments with membranes
The purpose of the paper is to characterize the behavior of a freely moving light sail. But as a first step to begin exploring materials and driving forces in the lab, the team created a miniature light sail tied at the corners to a larger membrane.
The researchers used equipment from the Kavli Institute for Nanoscience at the California Institute of Technology and a technique called electron-beam lithography to carefully fabricate a silicon nitride membrane just 50 nanometers thick, creating something resembling a microscopic trampoline.
The mini trampoline, a 40 micron wide square, is suspended at the corners with silicon nitride springs. The team then hit the membrane with visible wavelength argon laser light. The goal was to measure the radiation pressure to which the miniature light sail was subjected by measuring the movements of the trampoline as it moved up and down.
But the physics picture changes when the sail is tethered, says study co-author Michaeli: “In this case, the dynamics become quite complex.”
Mini sail
The sail acts as a mechanical resonator, vibrating like a trampoline when light hits it. A key challenge is that these oscillations are mainly due to heat from the laser beam, which can mask the direct effect of radiation pressure. Michaeli said the team turned the challenge into an advantage, noting: “We not only avoided the unwanted heating effects but also used what we learned about the device’s behavior to create a new way to measure light’s force.”
The new method allows the device to additionally act as a power meter, measuring both the strength and power of the laser beam.
“The device represents a small lightsail, but a big part of our work was devising and realizing a scheme to precisely measure motion induced by long-range optical forces,” said study co-author Gao.
Common-path interferometer
For this purpose, the team built a so-called co-pass interferometer. Overall, motion can be detected using the interference of two laser beams when one of them hits the vibrating sample while the other tracks its stationary position.
However, in a co-pass interferometer, since both beams have traveled almost the same path, they encounter the same sources of environmental noise, such as equipment operating nearby or even people talking, and these signals are eliminated. Only a very small signal from the sample motion remains.
The engineers integrated the interferometer into the microscope they used to study the miniature sail and placed the device in a specially made vacuum chamber. Afterward, they were able to measure the sail’s movements to the nearest picometer (trillionths of a meter), as well as its mechanical stiffness — that is, how much the springs deformed when the sail was pushed under the pressure of the laser light.
Beaming at an angle
Because the researchers know that a light sail in space does not always remain perpendicular to the laser source on Earth, they changed the angle of the laser beam to mimic this, and again measured the force with which the laser pushed the mini-sail.
Crucially, the researchers took into account that the laser beam propagated at an angle and therefore missed the sample in some places, calibrating their results to the laser power measured by the device itself. However, the strength under these conditions was less than expected. In the paper, the researchers hypothesize that part of the angled beam hits the edge of the sail, causing some of the light to be scattered and sent in other directions.
In the future, the team hopes to use nanoscience and metamaterials – materials carefully developed at such a tiny scale to have the desired properties — to help control the side-to-side motion and rotation of the miniature light sail.
According to phys.org