  Highly ambitious initiatives aspire to propel a miniature spacecraft to a
neighboring star within a human generation, leveraging the radiation pressure
of lasers for propulsion. One of the main challenges to achieving this enormous
feat is to build a meter-scale, ultra-low mass lightsail with broadband
reflectivity. In this work, we present the design and fabrication of such a
lightsail composed of two distinct dielectric layers and patterned with a
photonic crystal structure covering a 4" wafer. We overcome the crucial
challenge of achieving broad band reflection of >70% spanning over the full
Doppler-shifted laser wavelength range during spacecraft acceleration, in
combination with low total mass in the range of a few grams when scaled to
meter size. Furthermore, we find new paths to reliably fabricate these
sub-wavelength structures over macroscopic areas and then systematically
characterize their optical performance, confirming their suitability for future
lightsail applications. Our innovative device design and precise
nanofabrication approaches represent a significant leap toward interstellar
exploration.
