Scientists from Tuskegee University, exploring conceptual light sails for interstellar travel, have demonstrated a photonic light-crystal sail design that is more efficient than earlier designs.
Based on the concept of directed energy propulsion, light sails propelled by massive lasers could accelerate unmanned probes to as fast as 20% the speed of light, enabling missions to nearby stars in a few decades rather than the hundreds of thousands of years conventional chemical rockets would need to reach Earth’s nearest stellar neighbor, Proxima Centauri.
The research team behind the proposed photonic crystal light sail suggests that their design could enable near-term interplanetary missions and potentially lead to longer-term interstellar missions to other star systems.
Light Sails Traveling at 20% Light Speed Could Reach Proxima Centauri in 20 Years
To reach space, humans rely exclusively on chemical rockets. To explore beyond Earth’s orbit, those rockets must carry additional fuel. However, adding fuel adds weight, necessitating even more fuel. Engineers have quantified this trade-off with a mathematical tool called the rocket equation.
While some emerging alternatives, such as electric propulsion, enable satellites to maneuver in orbit or even explore deep space, their low speeds also limit the distances they can travel within a human lifetime. The Debrief has covered some sci-fi-sounding alternatives, such as the Wind Rider plasma magnets and warp drives, but those options are either too slow or too theoretical to serve as viable interstellar propulsion systems.
More recently, researchers have explored the concept of light sails. Similar to the more well-known concept of solar sails that “sail” on the pressure of the solar wind, light sails use the energy from a light source to sail at increasingly faster speeds. This design removes the need for onboard propellant.
Some estimates, such as the proposed Breakthrough Starshot initiative, suggest that current technology could design a light-sail-equipped microprobe driven by a powerful laser capable of reaching up to 20% of the speed of light. At that speed, such a probe could reach Proxima Centauri in a little over 20 years.
Light sail by Masumi Shibata, courtesy of Breakthrough Initiatives
While a seemingly practical alternative to chemical propulsion, the practical application of light sails has been limited by engineering challenges. For example, current designs propose metal-coated polymer films. These materials offer a favorable combination of energy reflectivity and strength.
However, these designs also absorb some of the directed energy and convert it into heat. Efforts to capture this wasted heat by increasing reflectivity involve adding materials, thereby increasing weight. As a result, designers of light sails have encountered a tradeoff similar to the rocket equation.
How Photonic Light Crystal Sails Increase Reflectivity and Propulsion
According to a statement announcing the proposed photonic light crystal sail design, the sail’s structure consists of nanoscale patterns from three dielectric components. The first layer is composed of germanium pillars, the second of air holes, and the final layer of a polymer matrix.
Where conventional light sails are made of two material photonic structures, the three-layer dielectric material combination of high-index germanium pillars, low-index air voids, and the polymer host form a wavelength-selective photonic bandgap structure that the research team described as “optimized for propulsion-specific reflectivity.”
Nanoscale features of a laser-driven light sail showing germanium pillars and air holes embedded in a poly(methyl methacrylate) matrix, designed to achieve high wavelength-selective reflectivity. Image Credit: Dimitrov and Harris.
“This configuration establishes a narrow photonic band gap centered at the propulsion wavelength, resulting in high reflectivity within that spectral window while remaining largely transparent outside the designed band,” they explain.
The researchers attributed the exceptional reflectivity of their light sail design to nanoscale patterns in the dielectric materials that control light propagation. They also noted that the ability to arrange materials with different ‘refractive indices,” they were able to create a photonic gap, defined as “a range of wavelengths that cannot pass through the structure and are instead reflected.”
“By designing a narrow photonic band gap aligned with the propulsion laser frequency, the proposed sail can stay mostly transparent to ambient solar radiation while maintaining high reflectivity in the specific operating band,” explained study author Dimitar Dimitrov, an assistant professor at Tuskegee University.
Experiments Confirm Improved Sail Material Performance
To test the concept, the Tuskegee team designed a photonic crystal structure using plane-wave expansion and finite-difference time-domain simulations. After running several simulations, the team achieved approximately 90% reflectivity at a wavelength of 1.2 micrometers.
After the successful simulations, the team fabricated real-world ‘proof-of-concept’ material membranes, such as those used in light sails. Due to the delicate nature of the finished product, the team used electron-beam lithography and vacuum deposition.
“The membranes were fabricated using a sequential nanolithography and material infill process involving patterned polymer templating, selective germanium deposition, lift-off processing, and secondary electron-beam structuring,” they explained.
According to the team, this multi-step fabrication approach allowed them to create three-dielectric photonic crystal architectures “at the sub-200-nanometer scale.” The final versions of the fabricated structures contained 200-millimeter-wide germanium pillars and 400-nanometer-diameter air holes embedded in a 200-nanometer-thick polymer layer.
The team was able to confirm this level of precision engineering and nanoscale patterning with an electron microscope. Dimitrov said demonstrating the feasibility of constructing these precise, multi-dielectric crystal nanostructures was a “key continuation: of the team’s work.
“The results show that these can be engineered to combine low mass, strong wavelength selectivity, and scalable fabrication potential,” the researcher explained.
Devices for Laser-Driven propulsion Enabling Future Interplanetary Exploration
To see if light sails made with their process would maintain reflectivity in simulated spaceflight conditions, the researchers modeled a one-square-meter sail and illuminated it with a 100-kW laser. As hoped, these tests showed that their design could generate continuous thrust. These results also suggested that a light sail made with the three-dielectric material could accelerate a probe to “speeds of several hundred meters per second within one hour under idealized conditions.”
While this speed is far below what would be required for an interstellar mission, the researchers said it is also robust and reflective enough to enable light sails designed for interplanetary missions within our solar system to take a fraction of the time of current rocket-propelled missions. They also concede that further research will be needed before a photonic light crystal sail is deployed in space, while noting that their work “demonstrates a possible pathway from theoretical design to fabrication.”
“Despite current limitations, our research could serve as a foundation for the design and fabrication of multi-dielectric photonic crystal sails,” Dimitrov explained. “It may provide a pathway to experimentally validated, scalable, lightweight devices for laser-driven propulsion, enabling future interplanetary exploration with minimal onboard mass,”
The study “Design and manufacture of a photonic crystal light sail” was published in the Journal of Nanophotonics.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.