Metasurface Optical Solar Reflectors Using AZO Transparent Conducting Oxides for Radiative Cooling of Spacecraft

Optical solar reflectors are devices that combine high reflection for visible wavelengths with a strong emissivity in the infrared. Compared to the conventional rigid quartz tiles used on spacecraft since the 1960s, thin-film solutions can offer a significant advantage in weight, assembly, and launch costs. Here, we present a metasurface-based approach using an Al-doped ZnO (AZO) transparent conducting oxide as infrared plasmonic material. The AZO is patterned into a metasurface to achieve broad plasmonic resonances with an enhanced absorption of electromagnetic radiation in the thermal infrared. In the visible range, the transparent conducting oxide provides low losses for solar radiation, while intrinsic absorption losses in the ultraviolet range are effectively suppressed using a multilayer reflecting coating. The addition of high-emissivity layers to the stack eventually results in comparable emissivity values to the thin plasmonic device, thus defining a window of opportunity for plasmonic absorption as a design strategy for ultrathin devices. The optimized experimental structure achieves solar absorptance (α) of 0.16 and thermal emissivity (ε) of 0.79. Our first prototype demonstrator paves the way for further improvement and large-area fabrication of metasurface solar reflectors and ultimately their application in space missions.