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

Abstract

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 * To whom correspondence should be addressed † Electronics ‡ Physics ¶ CREO § NILT ZnO (AZO) transparent conducting oxide as infrared plasmonic material. The AZO is patterned into a metasurface to achieve broad plasmonic resonances with 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.

Nanophotonic engineering of far-field thermal emitters

“…This approach can also be made dynamic; a recently proposed structure based on VO2 exhibited low emissivity at temperatures below a target temperature, and high emissivity otherwise, mitigating temperature fluctuations 127 . Applications of radiative cooling can range from cooling of dwellings in hot temperatures 122,126 to enhancing vapor condensation 128 to the thermoregulation of spacecraft 129,130 . Temperature of different thermal emitters throughout on a clear day in California, including a photonic radiative cooler based on a thin-film multilayer that achieves cooling below the ambient temperature.…”

Designing metamaterials with quantum annealing and factorization machines

“…A metamaterial, which only emits or absorbs the thermal radiation within the transparency window of the atmosphere (8-13 μm), is preferable for radiative sky cooling [49,53]. Various material structures have been proposed to match the spectrum of the atmospheric window, including planar multilayer structures [2,48], patterned meta-surface structures [47,50,51,54], and polymers doped with nanoparticles [45,52,55]. Most of these structures do not have a high emittance over the whole span of the atmospheric window.…”

“…Another application that has recently attracted much attention, in response to concerns of global warming and energy crises, is radiative sky cooling that utilizes the untapped 3 K cold space as a heat sink. Previous designs on radiative cooling [2,[45][46][47][48][49][50][51][52][53][54][55] have focuses on simultaneously blocking solar energy (0.4-4 μm) while maximizing thermal radiation loss (>4 μm) to the surroundings. These designs have been experimentally demonstrated to be successful in dry and clear weather.…”

Heavily Doped Semiconductor Metamaterials for Mid‐Infrared Multispectral Perfect Absorption and Thermal Emission

“…This is beneficial for the conventional metamaterial applications where a well‐defined absorption band is required, for example, for biochemical sensing applications or tailored thermal emission. TCOs are, due to their high losses and consequently their larger resonances, more adapted for broadband absorption and find their application, for example, as optical solar reflectors for thermal control of spacecraft and satellites, where they benefit moreover from their resistance to harsh environments …”