New Platform for Lightsail Membrane Characterization

Harry Atwater, the Howard Hughes Professor of Applied Physics and Materials Science at Caltech and the Otis Booth Leadership Chair of the Division of Engineering and Applied Science, and his colleagues have developed a platform for characterizing ultrathin membranes, which could be used in future lightsail technology. Their findings were published in Nature Photonics.

The concept of spacecraft propelled by ultrathin sails for interstellar travel has been explored by the Breakthrough Starshot Initiative, a program established in 2016 by Yuri Milner and Stephen Hawking. The initiative aims to use high-power lasers to accelerate small space probes equipped with lightsails to relativistic speeds, with the goal of reaching Alpha Centauri, the nearest star system.

Caltech is contributing to ongoing research efforts toward achieving this objective.

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.

Harry Atwater, Howard Hughes Professor, Applied Physics and Materials Science, California Institute of Technology

The test platform includes a mechanism to measure the force exerted by lasers on the sails, which are intended to propel spacecraft. The team's experiments represent an initial step in transitioning lightsail concepts and materials from theoretical models to experimental validation.

“There are numerous challenges involved in developing a membrane that could ultimately be used as lightsail. 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,” Atwater added.

The study’s lead authors are Ramon Gao (MS '21), a Caltech graduate student in applied physics, and Lior Michaeli, a postdoctoral scholar in applied physics.

The objective is to characterize the behavior of a freely moving lightsail. As an initial step, the researchers designed a small lightsail tethered to the corners of a larger membrane to study material properties and propulsion forces under controlled laboratory conditions.

Using nanofabrication technology at Caltech's Kavli Nanoscience Institute and electron beam lithography, the team fabricated a 50-nanometer-thick silicon nitride membrane, forming a microscopic trampoline-like structure. Silicon nitride springs suspend the 40-micron-wide and 40-micron-long trampoline at each corner.

The researchers then directed an argon laser with a visible wavelength at the membrane. By measuring the trampoline’s oscillations, they quantified the radiation pressure experienced by the micro lightsail.

However, co-lead author Michaeli noted that tethering the sail alters the physical dynamics of its motion.

In this case, the dynamics become quite complex.

Lior Michaeli, Study Lead Author and Postdoctoral Scholar, California Institute of Technology

When exposed to light, the sail acts as a mechanical resonator, oscillating similarly to a trampoline. A primary challenge is that these vibrations are largely induced by thermal effects from the laser beam, which can obscure the direct influence of radiation pressure. Michaeli noted that the team used this challenge to develop an alternative approach.

Michaeli added, “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 method enables the device to function as a power meter, measuring both the force 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.

Ramon Gao, Study Lead Author and Graduate Student, California Institute of Technology

To achieve this, the researchers developed a common-path interferometer. In conventional interferometry, motion is detected by measuring the interference between two laser beams—one reflecting off the vibrating sample and the other from a stationary reference point.

In a common-path interferometer, however, both beams travel nearly identical paths and experience the same environmental noise sources, such as vibrations from nearby equipment or human activity. As a result, the noise cancels out, isolating the small signal produced by the sample’s movement.

The interferometer was placed inside a custom-designed vacuum chamber and integrated into a microscope used to analyze the miniature sail. Researchers measured the sail’s mechanical stiffness—quantifying how much the supporting springs deformed under laser radiation pressure—and detected displacements as small as picometers (trillionths of a meter).

To account for beam divergence, which causes portions of the laser to miss the sample, the researchers calibrated their measurements against the laser power detected by the device itself. They angled the laser beam to simulate real-world conditions, where a lightsail in space would not always remain perfectly perpendicular to an Earth-based laser source. When measuring the force exerted on the mini sail under these conditions, they observed a lower-than-expected value. The study suggests that when the beam strikes the sail’s edge at an angle, some light scatters in different directions, reducing net radiation pressure.

Future work will focus on controlling the rotation and lateral motion of a microscopic lightsail using nanoscience and metamaterials—engineered surfaces designed to exhibit specific optical and mechanical properties at the nanoscale.

Gao added, “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. 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.”

The platform developed in this study enables precise measurement of rotational and lateral motion.

“This is an important stepping stone toward observing optical forces and torques designed to let a freely accelerating lightsail ride the laser beam,” stated Gao.

Other Caltech contributors to the study include research professor John E. Sader, former postdoctoral fellow Claudio U. Hail, and senior research scientist Michael D. Kelzenberg (Ph.D. '10), along with Atwater, Michaeli, and Gao. Adrien Merkt, who contributed to the project as a Ph.D. student at ETH Zürich, is also a co-author.

The Breakthrough Starshot Initiative and the Air Force Office of Scientific Research supported the research.

Journal Reference:

Michaeli, L., et. al. (2025) Direct radiation pressure measurements for lightsail membranes. Nature Photonics. doi.org/10.1038/s41566-024-01605-w