Nanophotonic Heterostructures for Efficient Propulsion and Radiative Cooling of Relativistic Light Sails

Light sails propelled by radiation pressure from high-power lasers have the potential to achieve relativistic spaceflight. In order to propel a spacecraft to relativistic speeds, an ultrathin, gram-sized light sail will need to be stably accelerated by lasers with ∼MW/cm² intensities operating in the near-infrared spectral range. Such a laser-driven sail requires multiband electromagnetic functionality: it must simultaneously exhibit very low absorptivity in the (Doppler-broadened) laser beam spectrum in the near-infrared and high emissivity in the mid-infrared for efficient radiative cooling. These engineering challenges present an opportunity for nanophotonic design. Here, we show that designed thin-film heterostructures could become multifunctional building-block elements of the light sail, due to their ability to achieve substantial reflectivity while maintaining low absorption in the near-infrared, significant emissivity in the mid-infrared, and a very low mass. For a light sail carrying a payload, we propose a relevant figure of merit - the reflectivity adjusted area density - that can capture the trade-off between sail mass and reflectivity, independent of other quantities such as the incident beam power, phased array size, or the payload mass. Furthermore, we present designs for effective thermal management via radiative cooling and compare propulsion efficiencies for several candidate materials, using a general approach that could apply to a broad range of high-power laser propulsion problems.