The high-level objective of HIPER-OSR was to develop a new class of Optical Solar Reflectors (OSR) characterized by being smart and flexible, whereas:

• Smart means that emittance varies with temperature, from low emittance in the cold state (solar flux low or absent / system in sleep or safe mode) to high emittance in the hot state (solar flux high, system fully operational);
• Flexible means that the proposed OSR consists of a meta-material coating deposited on an Aluminum film.

The proposed coating architecture is shown in the project illustration. In brief, the coating is made of two functional blocks, that is a temperature-variable emitter and a Low Emittance Solar Reflector:

• The variable emitter is a Perfect Metamaterial Absorber PMA, that is it consists of a metal back-reflector, a dielectric spacer, and an array of micron-size squares of thermochromic VO2. At high temperature, the VO2 islands are in the metal phase and interact with the back-reflector generating high emittance. At low temperature, VO2 islands change into a dielectric phase and interactions with the metal backreflector are switched off → emittance is substantially reduced. The operating principle is similar to a classical Fabry-Perot configuration: in the latter the pattern is replaced by a continuous layer of VO2. The main advantage of the metamaterial configuration is reduced solar absorptance α due to the reduced fill factor of VO2 (which is highly absorptive across the entire visible spectrum).
• The solar reflector is added to reduce solar absorptance further. It consists of the superposition of several dielectric filters all made of materials that are transparent both in the VIS and in the thermal IR spectrum. IR transparency is here necessary to enable radiative coupling between the variable emitter and the outer space.

All the layers of the two blocks are deposited by sputtering, while the array is patterned by Electron Beam Lithography EBL.