  Dynamic radiative cooling with variable emissive power is experimentally
demonstrated in this study by a wavelength-selective tunable metafilm emitter,
which consists of an opaque aluminum film, a sputtered silicon spacer, and a
thermochromic vanadium dioxide (VO2) layer fabricated by a furnace oxidation
method. The temperature-dependent spectral emittance, experimentally obtained
from spectral reflectance measurements, clearly shows a pronounced emission
peak around 10 um wavelength when the VO2 experiences an insulator-to-metal
phase transition near 65C. The tunable metafilm emitter achieves a significant
total emittance increase from 0.14 at room temperature to 0.6 at 100C.
Theoretical modeling based on thin-film optics indicates that the emission
enhancement at high temperatures is realized by Fabry-Perot cavity resonance
with the metallic VO2 film. Moreover, a calorimetry-based thermal vacuum
experiment was conducted and the enhanced thermal emission of the fabricated
tunable metafilm sample was experimentally demonstrated at temperatures higher
than the phase transition temperature, compared to black, aluminum and doped
silicon samples, whose emittance changes little near room temperatures. The
developed tunable metafilm emitter with variable spectrally-selective emittance
in the mid-infrared holds great promise for both terrestrial and
extraterrestrial dynamic radiative cooling applications.
