Nanophotonic designs for efficient propulsion and radiative cooling of the Starshot Lightsail

Breakthrough Starshot is an ambitious project with the goal to design and build a laser-propelled spacecraft that can reach Proxima Centauri b, an exoplanet 4.2 lightyears away from Earth. In order to propel the spacecraft to relativistic speeds (~0.2c), an ultrathin, gram-sized, lightsail must be stably accelerated under MW/cm² laser intensities operating in the near-IR spectral range. Because radiative cooling in space is the only mechanism for nanocraft thermal management, the Starshot Lightsail requires multiband functionality: it must simultaneously exhibit very low absorptivity in the (Doppler-broadened) laser beam spectrum in the near-IR, and high emissivity in the mid-IR for efficient cooling. These engineering challenges present an opportunity for nanophotonic design. Here, we show that optimized nanoscale optical structures could play an important role in the lightsail design due to their ability to achieve desired optical response while maintaining low absorption in the NIR, significant emissivity in the MIR, and a very low mass. Our analysis compares several relevant figures of merit for the interaction between the laser array and the lightsail and points to optimal designs and materials for propulsion and thermal management.