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 just 20 years. 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. In this work, 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.
To address the issues of efficient propulsion and thermal management, we combine material properties of very weak sub-band absorption in semiconductors with phonon-polariton driven emission in the MIR in materials such as silica. By way of nonlinear optimization, we survey a range of canonical nanophotonic structures (including thin-film slabs, multi-layer stacks, and 2D photonic crystal slabs) to reveal a tradeoff between reflectivity, mass, absorptivity, and emissivity. Our analysis compares several relevant figures of merit for the interaction between the laser and the lightsail and points to optimal designs for propulsion and thermal management.