The Voyager 1 spacecraft, launched in 1977, recently left our solar system after covering a distance of only 0.5% of the distance to Alpha Centauri, our star neighbor. On our present technology, travel to this star system would be impossible in fewer than 10,000 years. But imagine the following: interstellar travel produced in twenty years. This ambitious goal is the aim of the Breakthrough Starshot Initiative, to send gram mass probes to Alpha Centauri on 20% of the speed of light in laser powered lightsails.
The idea of lightsails thin reflective material pushed by powerful lasers has been at the center of this pursuit. But the engineering hurdle of creating sails that are light but robust, and can travel at relativistic speeds, has stood in the way for decades. Developments in nanotechnology and computer simulation have now started to break down these hurdles, bringing us closer to making this dream a reality.
Amongst reaching this milestone is the production of pentagonal photonic crystal mirrors, a novel material that combines the newest advancement in nanophotonics and neural topology optimization. Scientists created a 60 × 60 mm², 200 nm thick silicon nitride (SiN) membrane containing more than a billion nanoscale holes. It achieves a record-breaking 9000-fold cost savings per square meter in production, making lightsails not only possible but economically viable to manufacture on an industrial scale.
The secret to this success is the pentagonal lattice structure of the photonic crystal. In contrast with the square or hexagonal structure of conventional designs, the pentagonal lattice can achieve maximum reflectivity within a wide wavelength range that can encompass the Doppler red shift as the sail accelerates. This enables efficient propulsion at minimum mass for the sail a factor of prime significance in acquiring the required acceleration. The neural topology optimization method used in the research allows non-obvious solutions to be discovered, at the expense of performance and manufacturability, in a way that is impossible for other methods.
Lead researcher Dr. Richard A. Norte emphasizes the importance of this approach: “Neural topology optimization introduces a neural network before a physics solver, shifting the optimization problem to finding the weights and biases of the neural network that minimize the objective function. This method is particularly advantageous for lightsail design.” The approach is especially valuable for design in lightsailing. The outcome is a material with the highest aspect ratio to date for a nanophotonic device that can survive the harsh conditions of interstellar travel.
The actual fabrication process is a marvel of modern engineering. Utilizing i-line photolithography a cheap and scalable method the researchers cut the writing time of a 10 m² sail from 15 years to one day. This change of focus from electron beam lithography to optical lithography is a paradigm shift in nanofabrication, which allows for large-scale suspended photonic crystals to be produced at record speeds and prices.
Although pentagonal photonic crystal mirrors are an innovative breakthrough, there are issues. Lightsails must be able to resist structural stress from intense laser radiation, not suffer thermal fracturing, and move steadily down the laser beam axis. Researchers are studying nanostructured surfaces and metamaterials that may create restoring forces or torques to permit stabilizing of flight position and orientation.
Social and scientific implications of the technology are stunning. With a virtual reduction in cost and fineness of interstellar drive units, lightsails potentially could make travel to deep space affordable for all. Leading researcher Harry Atwater summarizes the revolutionizing possibility: “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.”
This is consistent with the overall goals of the Starshot Initiative, which uses breakthroughs in materials science and directed energy technology to explore beyond current boundaries in space travel. The shotgun method of sending fleets of lightsails makes success more likely even if one sail fails since others will arrive at their destination.
In the future, multi objective optimization techniques can again optimize lightsail designs with respect to variables like thermal stability, strength, and photonic efficiency. As the discipline evolves, the union of scientific and economic considerations will be the decider of whether or not interstellar travel is feasible.
The invention of pentagonal photonic crystal mirrors is a milestone towards achieving interstellar travel. Through the union of nanotechnology’s current advances and new computational techniques, scientists have not only overcome centuries-old obstacles but also paved the way for further exploration. The vision of going to other star systems is no longer a dream but now reality, with this new frontier on the horizon.
