Metamaterial-based smart and flexible Optical Solar Reflectors

Abstract In the frame of projects funded by the European Commission and the ESA, we developed a new type of Optical Solar Reflector (OSR) that combines the flexibility and easy handling of Second Surface Mirrors with the temperature-variable emittance necessary to ensure both effective radiative cooling in the hot phase and reduced heat losses in the cold phase. The new smart OSR consists of a metamaterial coating deposited on Kapton film. The coating is made of two functional blocks, namely a variable emitter topped by a dielectric solar reflector. The variable emitter is a Metamaterial Perfect Absorber designed for strong and broadband plasmonic resonance absorption in the thermal IR. It consists of a metal back-reflector, a dielectric spacer, and an array of doped VO 2 thermochromic micro-antennas that are switched-off when the temperature drops below the metal-to-insulator transition point. The solar reflector is a wideband dielectric filter made of materials that are transparent across the entire spectrum from the VIS to the thermal IR. All the layers of the two blocks are deposited by standard vacuum techniques, while the array is patterned by Nanoimprint Lithography, a technique that is often performed at the wafer level but allows for up-scaling via roll-to-roll or roll-to-plate production setups. The paper reports on the characterization and testing of samples of size up to 100 mm x 100 mm, at the Beginning of Life and after thermal, humidity, irradiation and handling tests.