Thermal Runaway of Silicon‐Based Laser Sails

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 space travel. The laser intensity at the sail required for this task is at least 1 GW m⁻² and, at such high intensities, thermal management of the sail becomes a significant challenge even when using materials with low linear absorption coefficients. Silicon is 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 it is shown that the temperature‐dependent linear absorption of silicon can lead to thermal runaway at temperatures above 400–500 K for even the most optimistic viable assumptions of the material quality. Additionally, above a design‐specific threshold laser intensity, nonlinear two‐photon absorption triggers thermal runaway regardless of initial temperature. Resonator‐based designs, which concentrate the field, exhibit lower threshold intensities than geometries that minimize the electric field such as Bragg reflectors.