Thermal runaway of silicon-based laser sails

TL;DR

This paper investigates the thermal stability of silicon-based laser sails for interstellar travel, revealing that two-photon absorption can cause thermal runaway at high laser intensities, thus setting limits for safe operation.

Contribution

It demonstrates that silicon's temperature-dependent bandgap and two-photon absorption can lead to thermal runaway, providing bounds on laser intensities for stable silicon-based sails.

Findings

Thermal runaway can occur in silicon sails at high laser intensities.

Two-photon absorption significantly impacts thermal stability.

Bounds on maximum laser intensity for stable silicon sails are established.

Abstract

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.