Multi-scale photonic emissivity engineering for relativistic lightsail thermal regulation

Abstract: The Breakthrough Starshot Initiative aims to send a gram-scale probe to Proxima Centuri B 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 the acceleration distance and setting realistic sail thermal limits, ultimately realizing a thermally endurable acceleration minimum distance of 16.3~Gm. We additionally demonstrate multi-scale photonic structures featuring thermal-wavelength-scale Mie resonant geometries, and characterize their broadband Mie resonance-driven emissivity enhancement and acceleration distance reduction. Our results highlight new possibilities in simultaneously controlling optical and thermal response over broad wavelength ranges in ultralight nanophotonic structures.

• The paper presents a novel nanophotonic approach that optimizes emissivity to maintain thermal regulation in relativistic lightsail designs.
• It employs MoS2 and Si3N4-based photonic crystal slabs to achieve a thermally endurable acceleration minimum distance of 16.3 Gm under extreme laser power.
• Multi-scale Mie resonant structures boost hemispherical exitance by 2.75× at 1000 K, indicating scalable solutions for interstellar propulsion.

Multi-scale Photonic Emissivity Engineering for Relativistic Lightsail Thermal Regulation

The paper presents a significant contribution to the field of nanophotonics by addressing a practical challenge related to the Breakthrough Starshot Initiative: thermal regulation of a laser-accelerated lightsail designed to reach relativistic speeds. This endeavor seeks to send a gram-scale probe to Proxima Centauri B, leveraging the momentum transfer from photons. A core concern in lightsail design lies in managing the thermal stress imposed by intense laser powers required for acceleration while maintaining the desired acceleration performance.

A primary innovation reported in the study is the development of nanophotonic photonic crystal slab reflectors composed of molybdenum disulfide (MoS\textsubscript{2}) and silicon nitride (Si\textsubscript{3}N\textsubscript{4}). These materials highlight the inverse relationship between the thermal band extinction coefficient and the lightsail's maximum temperature, underpinning the need to balance reflective and emissive properties. The study reports a thermally endurable acceleration minimum distance of 16.3 giga-meters, showing the possibility of optimizing photonic structures to enhance emissivity across broad wavelength ranges.

The research further introduces multi-scale photonic structures incorporating Mie resonant geometries that enhance broadband emissivity. This innovation underscores the potential for controlled optical and thermal response in lightweight structures, which holds implications for both theoretical nanophotonic design and practical interstellar vehicle engineering.

Key Numerical Results and Findings

• Acceleration Distance: The study reports an optimal acceleration distance merited design of 10.6 Gm, which translates to 15.2 Gm when accounting for a mechanical support structure.
• Thermal Endurance: The introduction of a thermally endurable acceleration minimum (TEAM) distance emphasizes the significance of incorporating realistic thermal constraints into design metrics. The TEAM distance under specified thermal limits is reported to be 16.3 Gm, adjusting to 21.3 Gm with the support structure.
• Emissivity Enhancement: Through a multiscale approach, the sails demonstrate a $2.75 \times$ increase in hemispherical exitance at 1000 K compared to conventional designs, which is attributed to enhanced Mie resonance.

Theoretical and Practical Implications

The work exemplifies the role of nanophotonics in enabling ultralight photonic devices capable of managing disparate optical interactions across wide spectral bands. By showcasing the benefit of layered photonic structures, it introduces methodologies applicable not only to interstellar propulsion but also to broader contexts requiring precise thermal management under extreme conditions.

Practically, the findings suggest pathways for improving the feasibility of deploying lightsails within the challenging constraints of interstellar travel. The successful employment of materials like MoS\textsubscript{2} and Si\textsubscript{3}N\textsubscript{4} due to their favorable optical properties provides a scalable model for future materials integration in such designs.

Future Directions

Future research will likely explore improving absorption characteristics and exploring new material combinations to enhance efficiency. There is also a strong need for investigating the dynamics of mechanical stability and thermal resilience under real-world acceleration conditions. Further computational and experimental work, possibly involving topological optimization methods, could yield designs with even shorter accelerative distances.

This paper contributes a meaningful advancement to space propulsion technologies and signals a burgeoning interest in nanophotonic applications beyond conventional paradigms, fostering further innovations in the optical regulation of engineered materials.