  The goal of ultrathin lightweight photophoretic flyers, or light-flyers for
short, is to levitate continuously in Earth's upper atmosphere using only
sunlight for propulsive power. We previously reported light-flyers that
levitated by utilizing differences in thermal accommodation coefficient (TAC)
between the top and bottom of a thin film, made possible by coating their lower
surfaces with carbon nanotubes (CNTs). Such designs, though successful, were
limited due to their high thermal emissivity (>0.5), which prevented them from
achieving high temperatures and resulted in their transferring relatively low
amounts of momentum to the surrounding gas. To address this issue, we have
developed light-flyers with undoped germanium layers that selectively absorb
nearly 80% of visible light but are mostly transparent in the thermal infrared,
with an average thermal emissivity of <0.1. Our experiments show that
germanium-coated light-flyers could levitate at up to 43% lower light
irradiances than mylar-CNT disks with identical sizes. In addition, we
simulated our experiments using a combined first-principles-empirical model,
allowing us to predict that our 2-cm-diameter disk-shaped germanium-coated
light-flyers can levitate in the mesosphere (altitudes 68-78 km) under the
natural sunlight (1.36 kW/m2). Similar ultrathin selective-absorber coatings
can also be applied to three-dimensional light-flyers shaped like solar
balloons, allowing them to carry significant payloads and thereby revolutionize
long-term atmospheric exploration of Earth or Mars.
