Silicon Nitride Metagratings for Self-Stabilizing Lightsails*

Accelerating lightsails to relativistic speeds with laser radiation pressure will foster interstellar exploration as envisioned by the Breakthrough Starshot Initiative. Such ultralight spacecrafts must ride the laser beam without external corrective actions, which can be achieved by tailoring their light scattering anisotropy.

We report microscopic lightsail prototypes fabricated in silicon nitride membranes and patterned with photonic metagratings for self-stabilization. From angle-resolved diffraction measurements, we infer the optically induced, angle-dependent forces and torques, which suggest restoring dynamical behavior along one axis. With lightsail development to target the macroscale, ensuring structural rigidity on top of dynamical stability becomes crucial. Our numerical multiphysics study shows that spinning a flexible, subwavelength thick nanostructured lightsail can provide the necessary tensioning forces. A 1-m sized, sub-gram flexible membrane patterned with composite metagratings and spun at 120 Hz is simulated to follow a bounded trajectory during the first five seconds of acceleration even if initially displaced by x = y = 5 cm and tilted by θ = φ = -2°. Our results pave the way toward optical levitation and long-range manipulation of self-stabilizing nanostructured lightsails.