  Laser sail-based spacecraft -- where a powerful earth-based laser propels a
lightweight outer-space vehicle -- have been recently proposed by the
Breakthrough Starshot Initiative as a means of reaching relativistic speeds for
interstellar spacetravel. The laser intensity at the sail required for this
task is at least 1 GW$\cdot$m$^{-2}$ and, at such high intensities, thermal
management of the sail becomes a significant challenge even when using
materials with low absorption coefficients. Silicon has been proposed as one
leading candidate material for the sail due to its low sub-bandgap absorption
and high index of refraction, which allows for low-mass-density designs.
However, here we show that the temperature-dependent bandgap of silicon
combined with two-photon absorption processes can lead to thermal runaway for
even the most optimistic viable assumptions of the material quality. From our
calculations, we set bounds on the maximum laser intensities that can be used
for a thermally stable, Si-based laser sail.
