Near-unity broadband photonic metamaterial absorber for thermoelectric energy harvesting in Space.

Spaceborne power systems must operate reliably for decades with minimal maintenance. Thermoelectric generators (TEGs) are intrinsically suited to long-lived missions, but their output remains constrained by available thermal gradients and the limitations of bulk thermoelectric materials. Here, we introduce a photonic metamaterial (PtMM) coating concept that amplifies the thermal gradient available to a TEG by converting incident AM0 solar irradiance into strongly localised photothermal energy on the TEG hot side. We design, optimise, and numerically characterise two metal-insulator-metal PtMM geometries - nanocross (NC-PtMM) and nanosquare (NS-PtMM) - using standard thin-film materials. The optimised NC-PtMM achieves near-unity peak absorptance (≈99%) and >95% average absorptance across the visible band, with strong field confinement at the resonator/spacer interface that concentrates dissipated power. Coupled electromagnetic-thermal simulations quantify (i) steady/quasi-steady temperature localisation under continuous irradiation and (ii) the intrinsic non-equilibrium photothermal response under ns-scale pulse trains used as a transient probe. The designs are effectively polarisation-insensitive at normal incidence; NS-PtMM is included as a manufacturability-motivated reference geometry, while detailed NS-PtMM optimisation is left for future work. The material stack (Cr, Al2O3, Al/SiO2, and an optically thick Ag ground plane) is compatible with standard microfabrication routes, and we outline a Space-qualification screening matrix (atomic oxygen exposure, radiation/TID, and thermal-vacuum cycling). Overall, the results establish a practical pathway toward metamaterial-augmented thermoelectric harvesting for compact Space platforms (e.g., CubeSats and landers) and motivate related photothermal coating concepts for spacecraft thermal management and hybrid PV-TEG harvesting.