A group of scientists has conducted the first experimental assessments of how miniature lightsails move when influenced by lasers in a laboratory setting.
Traveling across interstellar space with spacecraft powered by ultra-thin sails may seem like a plot from a science fiction story. However, a project launched in 2016 by Stephen Hawking and Yuri Milner, known as the Breakthrough Starshot Initiative, has been investigating this concept. The idea is to utilize lasers to propel small space probes attached to “lightsails,” allowing them to achieve incredibly high speeds to reach our closest star system, Alpha Centauri.
Caltech is at the forefront of the global effort to accomplish this ambitious objective. Harry Atwater, the Otis Booth Leadership Chair of the Division of Engineering and Applied Science and the Howard Hughes Professor of Applied Physics and Materials Science at Caltech, explains, “The lightsail will travel faster than any other spacecraft in history, potentially allowing direct exploration of interstellar distances that are currently only explored through remote observation.”
Atwater and his colleagues at Caltech have created a platform to analyze the ultra-thin membranes that may be used to construct these lightsails in the future. This test platform features a method to measure the force applied by lasers on the sails, which will aid in propelling the spacecraft through space. Their experiments signify an important transition from theoretical designs to tangible observations and measurements of critical concepts and possible materials.
“Developing a membrane suitable for a lightsail presents several challenges. It must endure heat, maintain its shape under pressure, and move stably along the axis of a laser beam,” Atwater states. “Before we can start building such a sail, we need to grasp how materials react to radiation pressure from lasers. Our objective was to discover if we could measure the force on a membrane by simply observing its movements. It turns out we can.”
A study detailing their findings has been published in the journal Nature Photonics. The primary authors of the paper are applied physics postdoctoral scholar Lior Michaeli and applied physics graduate student Ramon Gao (MS ’21), both from Caltech.
The aim is to understand how a freely moving lightsail behaves. Initially, the researchers created a small lightsail that is anchored at the corners to a larger membrane for laboratory testing of materials and propulsion forces.
Utilizing facilities at the Kavli Nanoscience Institute at Caltech, the team employed a technique called electron beam lithography to intricately design a silicon nitride membrane just 50 nanometers thick, creating what resembles a tiny trampoline. This mini trampoline, a square measuring 40 microns on each side, is suspended by silicon nitride springs at its corners. The researchers then illuminated the membrane with visible-wavelength argon laser light, aiming to quantify the radiation pressure on the miniature lightsail by tracking the trampoline’s up-and-down movements.
However, the scenario becomes more intricate when the sail is tethered, according to co-lead author Michaeli. “In this instance, the dynamics grow quite complex.” The sail behaves like a mechanical resonator, vibrating similar to a trampoline when exposed to light. A key obstacle is that these vibrations mainly stem from heat generated by the laser, which can obscure the direct influence of radiation pressure. Michaeli mentions that the team turned this challenge into a beneficial aspect. “We not only mitigated the unintended heating effects but also utilized our insights about the device’s behavior to establish a novel approach for measuring light’s force.”
This new technique allows the device to function as a power meter, measuring both the force and the power of the laser beam.
“While the device signifies a miniature lightsail, a substantial part of our work was developing and implementing a precise method to gauge motion driven by long-range optical forces,” says co-lead author Gao.
To achieve this, the team constructed what’s known as a common-path interferometer. Typically, motion can be detected by the interference of two laser beams: one striking the vibrating sample and the other targeting a fixed point. However, in a common-path interferometer, since both beams traverse nearly the same trajectory, they encounter identical sources of environmental noise, allowing those signals to be filtered out. What remains is a very faint signal from the sample’s motion.
The engineers incorporated the interferometer into the microscope they used to examine the miniature sail and placed the setup inside a specially designed vacuum chamber. This enabled them to detect movements of the sail as slight as picometers (a trillionth of a meter), along with its mechanical stiffness, which indicates how much the springs were compressed when pushed by the laser’s radiation pressure.
Acknowledging that a lightsail in space would not always remain directly aligned with a laser source on Earth, the team adjusted the laser beam’s angle to simulate this condition and measured how the laser pushed the mini sail. They found that the force was less than anticipated, mainly because some of the angled beam contacted the sail’s edge, causing part of the light to scatter in different directions.
Looking ahead, the team aims to utilize nanoscience and metamaterials—materials engineered at a microscopic scale to possess specific properties—to control the side-to-side motion and rotation of a miniature lightsail.
“Our goal is to determine whether we can engineer these nanostructured surfaces to provide a restoring force or torque to a lightsail,” Gao remarks. “If a lightsail were to drift or rotate out of the laser beam, we want it to autonomously return to its initial position.”
The researchers note that they are equipped to measure side-to-side motion and rotation with the platform discussed in the paper. “This is a significant milestone towards detecting optical forces and torques that enable a freely accelerating lightsail to follow the laser beam,” states Gao.
The paper titled “Direct radiation pressure measurements for lightsail membranes” was published on January 30. In addition to Atwater, Michaeli, and Gao, the other Caltech contributors to the paper include senior research scientist Michael D. Kelzenberg (PhD ’10), former postdoctoral scholar Claudio U. Hail, and research professor John E. Sader. Adrien Merkt is also a co-author, having participated in the project as a graduate student at ETH Zürich. The research was funded by the Air Force Office of Scientific Research and the Breakthrough Starshot Initiative.
