PERSPECTIVE https://doi.org/10.1038/s41563-018-0075-8 1 Department of Applied Physics and Materials Science, California Institute of Technology, Pasadena, California, USA. 2 Department of Physics, California Institute of Technology, Pasadena, California, USA. *e-mail: haa@caltech.edu T he Starshot Breakthrough Initiative has challenged a broad and interdisciplinary community of scientists and engineers to design an ultralight spacecraft or ‘nanocraft’ that can reach Proxima Centauri b — an exoplanet within the habitable zone of Proxima Centauri and 4.2 light years away from Earth — in approxi- mately 20 years 1,2 . Such a spacecraft, represented pictorially in Fig. 1, would consist of two components: a lightsail propelled by laser radiation pressure, and a payload or ‘StarChip’ that contains the electronics and sensors necessary to gather data and transmit it back to Earth 1 . The proposed concept is inspired by the existing body of work on solar sails 3,4 , most notably the IKAROS (Interplanetary Kite-craft Accelerated by Radiation of the Sun) mission launched in 2010 5 . The IKAROS spacecraft uses sunlight as the source of radia- tion pressure, its sail consisting of a thin (7.5 μm) polyimide film with a subwavelength (80 nm) aluminium reflective coating that provides ~80% specular reflectivity. Equipped with thin-film solar cells and reflectivity-control devices (RCD) for attitude control, it can reach a maximum velocity of 400 m s –1 (ref. 6 ). By contrast, the Starshot Breakthrough Initiative aims to launch a nanocraft that reaches a relativistic speed of ~60 000 km s –1 (20% the speed of light) using radiation pressure from a high-powered phased array of lasers on Earth (~10 GW m –2 of net laser intensity). Though the methodology of the IKAROS project can provide useful guidelines for light-based propulsion of miniature spacecrafts, the targets of the Starshot mission — in particular, the need to achieve a velocity five orders of magnitude greater — demand a strikingly different approach. The Starshot effort envisions propulsion using a laser sys- tem capable of continuous wave power generation at the 50–70 GW level for an impulse of approximately 1,000-second duration 2,7 . This laser system is likely to be an optically dense phased array of indi- vidual laser elements such as kW-scale solid-state diode laser ampli- fiers that are phase locked when fed by a common seed laser 8–10 . In order to reach relativistic speeds, the Starshot lightsail should have an area of ~10 m 2 and be kept to a mass of under ~1 gram, which translates into an equivalent thickness of approximately 100 atomic layers. The design of the lightsail will therefore need to push the boundaries of materials science, photonic design and structural engineering to enable high performance with minimal mass. In this Perspective, we identify key design criteria and funda- mental material challenges for the Starshot lightsail. Specifically, we discuss materials with extreme optical, mechanical and thermal prop- erties required for the design of the lightsail. For such a laser-driven nanocraft, we reveal a balance between the high reflectivity of the sail, required for efficient photon momentum transfer; large band- width, accounting for the Doppler shift; and the low mass necessary for the spacecraft to accelerate to near-relativistic speeds. We show that nanophotonic structures may be well-suited to meeting such requirements. Such structures may include two-dimensional pho- tonic crystal slabs 11 , where periodic nanostructures in a thin dielec- tric slab open up a photonic bandgap; metasurfaces, where arrays of resonant elements can be collectively excited to modify their reflection profile 12 ; or 1D photonic crystals, where alternating layers of high and low refractive index can result in a spectral band of high reflectivity 11 . In each design, the combination of material prop- erties and nanostructure will be crucial for minimizing mass while maximizing photon momentum transfer. With radiative cooling being the sole mechanism for passive thermal management in space, we quantify stringent requirements on material absorptivity that enable the lightsail to withstand high laser intensity and prevent excessive heating and mechanical fail- ure. Materials selected to form the lightsail must have extremely low optical losses (absorptivity <10 –5 ) in the near-infrared (IR) at ele- vated temperatures, placing strong constraints on the design space and requiring the consideration of fundamental limits to absorp- tion imposed by different absorption mechanisms in materials. To better understand and characterize the limiting absorption mecha- nisms, we propose a renewed effort for ultrasensitive measurements of the optical properties of candidate materials, including the use of techniques such as photothermal deflection spectroscopy and photoacoustic spectroscopy. In addition, the extreme constraints on the mass of the nanocraft necessitates the use of materials in ultra- thin-film form. Consequently, we discuss several approaches for synthesis, fabrication, assembly and handling of materials in such ultrathin, but macroscopic, structures. Lastly, we discuss the requirements for nanocraft stability during acceleration phase. We show that together with photonic and ther- mal considerations, important factors such as the sail shape, beam profile and mechanical properties must be considered. Importantly, we argue that a successful design of the lightsail will require syner- getic engineering: simultaneous optimization and consideration of all of the parameters described above. In this Perspective, we address the above issues and offer a posi- tive outlook on the challenges and opportunities for designing the Starshot lightsail based on these constraints, and suggest pathways Materials challenges for the Starshot lightsail Harry A. Atwater  1 *, Artur R. Davoyan 1 , Ognjen Ilic 1 , Deep Jariwala 1 , Michelle C. Sherrott  1 , Cora M. Went 2 , William S. Whitney 2 and Joeson Wong  1 The Starshot Breakthrough Initiative established in 2016 sets an audacious goal of sending a spacecraft beyond our Solar System to a neighbouring star 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 design and discuss fundamental materials challenges. We pre- dict that major research advances in photonic design and materials science will enable us to define the pathways needed to realize laser-driven lightsails. © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. NATURE MATERIALS | www.nature.com/naturematerials