The notion of traversing interstellar space with spacecraft driven by ultrathin sails might seem like a concept from science fiction literature. However, in reality, a project initiated in 2016 by Stephen Hawking and Yuri Milner called the Breakthrough Starshot Initiative has been investigating this concept. The plan involves utilizing lasers to propel tiny space probes linked to “lightsails” in order to achieve high velocities and ultimately reach our closest stellar system, Alpha Centauri.
Caltech is spearheading the global effort working towards realizing this ambitious objective. “The lightsail will exceed the speed of any prior spacecraft, potentially enabling direct exploration of interstellar distances that are currently attainable only through remote observation,” states 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.
Currently, Atwater and his team at Caltech have crafted a platform to characterize the ultrathin membranes that may one day facilitate the creation of these lightsails. Their testing platform incorporates a mechanism for gauging the force that lasers apply on the sails, which will be utilized to propel the spacecraft at high speeds through space. The experiments conducted by the team represent the initial phase in transitioning from theoretical designs and proposals of lightsails to real-time observations and assessments of essential concepts and viable materials.
“There are many difficulties associated with creating a membrane that could ultimately serve as a lightsail. It must endure heat, maintain its shape under pressure, and travel stably along the line of a laser beam,” Atwater points out. “However, before we embark on constructing such a sail, we need to comprehend how these materials react to radiation pressure from lasers. Our goal was to ascertain if we could gauge the force acting on a membrane simply through observing its movements. As it turns out, we can.”
A publication detailing the research is featured in the journal Nature Photonics. The principal authors of this publication are postdoctoral researcher in applied physics Lior Michaeli and graduate student in applied physics Ramon Gao (MS ’21), both affiliated with Caltech.
The objective is to characterize the dynamics of a freely moving lightsail. Nevertheless, as an initial step to begin examining the materials and propelling forces in the laboratory, the team developed a small-scale lightsail that is anchored at the corners within a larger membrane.
The researchers employed equipment located in the Kavli Nanoscience Institute at Caltech, utilizing a method known as electron beam lithography to meticulously pattern a silicon nitride membrane that is just 50 nanometers thick, creating a structure reminiscent of a microscopic trampoline. This mini trampoline, measuring a square of 40 microns by 40 microns, is suspended by silicon nitride springs at each corner. Subsequently, the team illuminated the membrane with argon laser light at a visible wavelength, aiming to assess the radiation pressure experienced by the miniature lightsail by recording the trampoline’s movements as it oscillated.
However, the analysis from a physics standpoint alters when the sail is tethered, according to co-lead author Michaeli. “In this scenario, the dynamics become rather intricate.” The sail functions as a mechanical resonator, oscillating like a trampoline when illuminated by light. A significant challenge arises because these oscillations are primarily induced by heat from the laser beam, which can obscure the direct impact of radiation pressure. Michaeli explains how the team transformed this challenge into an advantage. “We not only mitigated the undesired heating effects but also leveraged our findings on the device’s behavior to design a novel technique for measuring the force of light.”
The innovative approach allows the device to additionally function as a power meter for assessing both the force and power of the laser beam.
“The device signifies a small lightsail, but a significant portion of our efforts involved devising and implementing a strategy to accurately measure the motion caused by long-range optical forces,” states co-lead author Gao.
To achieve this, the team constructed what is referred to as a common-path interferometer. Generally, motion can be observed through the interference of two laser beams, one of which strikes the vibrating target while the other stays fixed in position. However, in a common-path interferometer, since both beams have traversed nearly the same route, they are subjected to the same sources of environmental interference, such as nearby equipment operation or even conversations, allowing those signals to be canceled out. What remains is merely the subtle signal generated from the sample’s motion.
The engineers incorporated the interferometer into the microscope used to examine the miniature sail and housed the apparatus within a specially designed vacuum chamber. This enabled them to record movements of the sail as minuscule as picometers (trillionths of a meter) and assess its mechanical rigidity—that is, the extent to which the springs deformed when the sail was pushed by the radiation pressure of the laser.
Understanding that a lightsail in space would not consistently maintain a perpendicular stance to a laser source on Earth, the researchers next inclined the laser beam to simulate this scenario and once again measured the force with which the laser propelled the mini sail. Critically, the team considered the spreading of the laser beam at an angle, which could cause it to miss some areas of the sample by calibrating their findings to the laser power recorded by the device itself. Nevertheless, the force under these conditions was less than anticipated. In their paper, the researchers speculate that some of the beam, when angled, collides with the sail’s edge, resulting in a portion of the light being scattered in different directions.
Looking ahead, the team aspires to utilize nanoscience and metamaterials—substances meticulously designed at a micro level to possess favorable characteristics—to assist in regulating the lateral motion and rotation of a miniature lightsail.
“The ultimate aim would be to determine if we can leverage these nanostructured surfaces to, for instance, provide a restoring force or torque to a lightsail,” says Gao. “If a lightsail were to move or rotate away from the laser beam, we would want it to autonomously return to its original position.”
The researchers assert that they can measure side-to-side motion and rotation using the platform highlighted in the paper. “This represents a significant advancement toward observing optical forces and torques designed to enable a freely accelerating lightsail to ride the laser beam,” asserts Gao.
The article, “Direct radiation pressure measurements for lightsail membranes,” was published on January 30. In addition to Atwater, Michaeli, and Gao, other Caltech contributors to the paper include senior research scientist Michael D. Kelzenberg (PhD ’10), former postdoctoral researcher Claudio U. Hail, and research professor John E. Sader. Adrien Merkt is also a co-author of the paper who engaged in the project as a graduate student at ETH Zürich. This research was funded by the Air Force Office of Scientific Research and the Breakthrough Starshot Initiative.