Bi-layer photonic random meta-composite for cryogenic thermal control by ultra-broadband scattering matched reflectance.

Cryogenic thermal control coatings represent a significant advancement over existing coatings limited to high reflectance in the 0.2-2.5 μm solar spectrum range, being able to reflect 96.6% of solar irradiance and to reach about 145 K equilibrium temperature, offering transformative potential for deep-space exploration, remote sensing, and other cryogenic applications. To achieve the cryogenic temperature further lower than 100 K in space, the peculiar optical property to reflect 99.9% of solar irradiance becomes necessary, which can be realized by an ultra-broadband reflectance from 0.2 to 8 μm. Here, we propose a bi-layer meta-composite comprising two distinct photonic random media, where rationally designed scatterer sizes selectively target short- and long-wavelength solar irradiance. By matching the scattering peak regimes, the meta-composite achieves a weighted solar reflectance of 97.3% over an ultrabroad 0.2-8 μm region. In the home-built deep-space simulator, the bi-layer meta-composite maintains an equilibrium temperature of 145 K, superior to existing coatings designed for room-temperature thermal control. Ground simulated irradiation tests further demonstrate exceptional optical stability under charged particles and atomic oxygen irradiations, with degradation of less than 1.5%. Moreover, the meta-composite retains optical properties comparable to existing coatings even after long-term ultraviolet irradiation. This work not only highlights the viability of photonic random meta-composites for cryogenic thermal control but also introduces a scattering regime matching strategy to broaden the spectral selectivity of disordered photonic systems.