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.

Optical Characterization of Silicon Nitride Metagrating-Based Lightsails for Self-Stabilization

“…While self-stabilizing optical designs implemented on the silicon-on-insulator platform have hitherto been studied, ,− , we identify silicon nitride as a leading material candidate due to its potential for ultralow absorptivity , on the order of 10 –5 to 10 –6 , sufficiently high index contrast, robust mechanical properties, and wafer scalability . With its large band gap pushing the threshold for thermal runaway due to two-photon absorption as reported for silicon to higher temperatures and allowing for laboratory-scale experiments in the visible wavelength regime, its high intrinsic stress has been instrumental in realizations of high-reflectivity and high- Q membrane-based optomechanical resonators. − Moreover, suspended one-dimensional silicon nitride gratings have been demonstrated for applications in sensing, optical filtering, and differentiation. − In fact, one-dimensional subwavelength gratings were shown to result in more efficient lightsail propulsion compared to previously reported optimal photonic geometries . Consequently, our approach is rooted in combining the suspended silicon nitride membrane architecture with optical metagrating designs, the fabrication of which demonstrates natural scalability from microscopic to large-scale demonstrations of dynamical stability (Figure b).…”

Laser Cooling Assisted Thermal Management of Lightsails

“…A critical issue related to the necessary use of high-power laser for lightsail acceleration is the heat generated due to the residual optical absorption, even for a very low-loss dielectric sail, and the resultant heat that has to be dissipated efficiently to avoid material damage. , This is exacerbated by the fact that material absorption in general increases with temperature, causing significant thermal runaway . The challenge is becoming more critical in space where heat cannot be dissipated through conduction or convection.…”

“…17,18 This is exacerbated by the fact that material absorption in general increases with temperature, causing significant thermal runaway. 19 The challenge is becoming more critical in space where heat cannot be dissipated through conduction or convection. Radiative cooling, which is based on thermal radiation, is thus the only approach explored for cooling a lightsail.…”

Multiscale Photonic Emissivity Engineering for Relativistic Lightsail Thermal Regulation

“…Interaction with the interstellar medium at relativistic speeds could also cause heating . Additionally, the sail’s component materials can become more absorptive as their temperature rises, raising concerns about thermal runaway and increasing the probability of sail thermal failure. Our approach seeks to optimize sail design in terms of its acceleration characteristics while ensuring sufficient radiative cooling characteristics to maintain sail integrity.…”