Multiscale Photonic Emissivity Engineering for Relativistic Lightsail Thermal Regulation

Abstract

The Breakthrough Starshot Initiative aims to send a gram-scale probe to our nearest extrasolar neighbors using a laser-accelerated lightsail traveling at relativistic speeds. Thermal management is a key lightsail design objective because of the intense laser powers required but has generally been considered secondary to accelerative performance. Here, we demonstrate nanophotonic photonic crystal slab reflectors composed of 2H-phase molybdenum disulfide and crystalline silicon nitride, highlight the inverse relationship between the thermal band extinction coefficient and the lightsail’s maximum temperature, and examine the trade-off between minimizing acceleration distance and setting realistic sail thermal limits, ultimately realizing a thermally endurable acceleration minimum distance of 23.3 Gm. We additionally demonstrate multiscale photonic structures featuring thermal-wavelength-scale Mie resonant geometries and characterize their broadband Mie resonance-driven emissivity enhancement and acceleration distance reduction. More broadly, our results highlight new possibilities for simultaneously controlling optical and thermal response over broad wavelength ranges in ultralight nanophotonic structures.

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

“…First experimental investigations into stable beam-riding behavior have focused on materials such as nematic liquid crystals and photopolymers on sapphire . However, for ultrathin lightsails to survive the extreme laser power densities (>1 MW/cm 2 ) for Starshot lightsail propulsion conditions, materials with ultralow absorption are required such as low-loss dielectrics and semiconductors, which can avoid substantial thermal heating, ,, in addition to thermal management strategies on enhancing the emissivity in the thermal wavelength regime. ,, …”

Relativistic Light Sails Need to Billow

“…Light sails with nanometer-scale thicknesses towing gram-scale microchip payloads that are accelerated to relativistic velocities by high-power lasers have the potential to significantly increase humankind’s understanding of the cosmos. − In particular, recent discoveries of exoplanets in the Alpha Centauri system ,− have prompted the Starshot Breakthrough Initiative , to set Proxima Centauri, a star located about 4.2 ly away from Earth, as a target destination for light sail development. Previous studies have examined light sails in terms of their optical properties, − laser-beam-riding stability, ,− and interaction with the interstellar medium − but have largely overlooked their ability to sustain the intense photon pressures required for their acceleration.…”

“…The squares denote the d s – s s design exhibiting the minimum value of L , and the circles denote that with the minimum value of . The values of (k), though small, reflect the low extinction coefficients κ of MoS 2 and Al 2 O 3 near the laser wavelength − and are comparable to absorptivity values used in other light sail models. , Note that the sail designs in the region d s ≈ 1.25 m and s s ≈ 2.5–4 m, while technically viable alternatives, are not robust design choices because small manufacturing imperfections could cause them to overheat or tear.…”

“…Finally, we examine the effect of the sail diameter and radius of curvature on the optical properties of the sail and the corresponding implications for the light sail’s performance. To begin, we stress that the optical properties of the sail film vary greatly with its thickness and can also depend on its photonically engineered structure. − , The film thickness in our calculations depends on the sail’s diameter and spherical radius of curvature, since in Figure we hold the total sailcraft mass m tot constant (see Figure b and Section S2 in the Supporting Information). The d s dependence is the cause of the horizontal variations in optical property values observable in Figure j–l, which show that , , and ε e generally increase with increasing t f (i.e., decreasing d s ).…”

“…We provide three examples, from optical, stability, and mechanical standpoints. First, recent photonic engineering studies of light sails have proposed methods to reduce the film thickness or mass required to achieve adequate optical properties. − , However, even such engineered structured films will experience significant photon pressure and could tear if too thin or, if too thick and massive, will accelerate slowly even though highly reflective. Second, stability calculations have suggested that alternative light sail shapes, such as spheres (i.e., ball shapes) or convex curves (i.e., light sails curved away from the laser beam) may offer advantages in terms of beam-riding stability.…”