  The Starshot Breakthrough Initiative aims to send one-gram microchip probes
to Alpha Centauri within 20 years, using gram-scale lightsails propelled by
laser-based radiation pressure, reaching velocities nearing a fifth of light
speed. This mission requires lightsail materials that challenge the
fundamentals of nanotechnology, requiring innovations in optics, material
science and structural engineering. Unlike the microchip payload, which must be
minimized in every dimension, such lightsails need meter-scale dimensions with
nanoscale thickness and billions of nanoscale holes to enhance reflectivity and
reduce mass. Our study employs neural topology optimization, revealing a novel
pentagonal lattice-based photonic crystal (PhC) reflector. The optimized
designs shorten acceleration times, therefore lowering launch costs
significantly. Crucially, these designs also enable lightsail material
fabrication with orders-of-magnitude reduction in costs. We have fabricated a
60 x 60 mm$^2$, 200nm thick, single-layer reflector perforated with over a
billion nanoscale features; the highest aspect-ratio nanophotonic element to
date. We achieve this with nearly 9,000 times cost reduction per m$^2$.
Starshot lightsails will have several stringent requirements but will
ultimately be driven by costs to build at scale. Here we highlight challenges
and possible solutions in developing lightsail materials - showcasing the
potential of scaling nanophotonics for cost-effective next-generation space
exploration.
