Flying Micro-Lightsails: Optical Levitation and Propulsion of Nanostructured Ultralight Macroscopic Objects*

The Breakthrough Starshot Initiative initiated in 2016 defined an audacious goal of sending a spacecraft beyond to a neighboring star, Proxima Centauri within the next half-century. Its vision for an ultralight spacecraft that can be accelerated by laser radiation pressure from an Earth-based source to ~20% of the speed of light demands the use of materials with extreme properties. Here we examine stringent criteria for the lightsail materials design, thermal management and dynamical stability, and discuss lightsail design and first experimental steps by exploration of small (<1 mm²) microscale lightsails. We explore nanophotonic design of materials, thermal management, and self-stabilizing optical manipulation, levitation and propulsion of lightweight macroscopic (i.e., mm, cm, or even meter-scale) micro-lightsails via radiation pressure. We consider the materials characteristics required to realize robust, thermally stable building blocks, and find that stable trajectories for dynamic motion of macro-objects can be achieved by controlling the anisotropy of light scattering along the object surface. With radiative cooling being the sole mechanism for passive thermal management in vacuum or space, we quantify the stringent requirements on material absorptivity that enable these structures to withstand high laser intensities and prevent excessive heating and mechanical failure. Achievement of stable optical manipulation and propulsion of macroscale (i.e., >> wavelength in size) structures, via radiation pressure appears to be possible by the use of structured optical beams and tailored nanophotonic design. Reaching this goal requires the conception and design of new ultralight photonic structures composed of materials with extreme optical, mechanical and thermal properties.