Photonic crystals may be just what Breakthrough Starshot is looking for

Lightsail technology is a fascinating concept and a major game-changer in rocket propulsion. It may not be as big and impressive as the Saturn V, the Space Shuttle, or the new Starship rocket, but when it comes to interstellar travel, light sails may be the answer. What better material could be used to construct a sail? This makes me want to say it again and again: photonic crystals. It sounds like an episode from Star Wars, but a new paper puts their feasibility to the test.

The concept of light sails is actually very simple. Instead of fabric sails attached to a ship using the power of wind, light sails use the power of light to propel a spacecraft across the universe. They rely on the pressure of photons emitted by powerful light sources such as lasers to generate low-level thrust.

The idea is that photons of light carry momentum and exert a force when they reflect from a surface. Deploying large, ultra-thin sails made of reflective material, such as Mylar commonly used in amateur astronomy for observing the sun, the spacecraft can be propelled, slowly accelerating, but eventually reaching astonishing speeds.

While light sails may not be able to get a spacecraft off the ground and into orbit, once deployed in space, long periods of slow acceleration are a very efficient way to travel through space. This isn’t just science fiction, the Starshot Project has become a leader in the field, aiming to develop a spacecraft that could send humans to Proxima Centauri (4.3 light-years away) within our lifetime.

Recently published in arXiv A preprint server from a team led by Jin Chang explores the possibility of a new material for light sails called nanofabricated photonic crystals. These crystals are optical nanostructures (between microscopic and molecular scales) whose refractive index changes periodically. These occur naturally in the animal kingdom, such as the reflective nature of cats and dogs’ eyes.

The team showed how silicon nitride photonic crystals with thin silicon films can achieve high levels of reflectivity in the 1300 nm to 1500 nm wavelength range required for light sails. The potential for sails is huge, with manufacturing technology capable of scaling up to several meters, which may well lay the foundation for eventually, if not slowly, sending humans across the galaxy.