  Dynamic radiative cooling attracts fast-increasing interest due to its
adaptability to changing environment and promises for more energy-savings than
the static counterpart. Here we demonstrate enhanced infrared emission by
thermally switching the excitation of magnetic polariton with microstructured
vanadium dioxide (VO2) metasurfaces fabricated via scalable and etch-free
processes. Temperature-dependent infrared spectroscopy clearly shows that the
spectral emittance of fabricated tunable metasurfaces at wavelengths from 2 to
6 um is significantly enhanced when heated beyond its phase transition
temperature, where the magnetic polariton is excited with metallic VO2. The
tunable emittance spectra are also demonstrated to be insensitive to incidence
and polarization angles such that the VO2 metasurface can be treated as a
diffuse infrared emitter. Numerical optical simulation and analytical
inductance-capacitance model elucidate the suppression or excitation of
magnetic polariton with insulating or metallic VO2 upon phase transition. The
effect of enhanced thermal emission with the tunable VO2 metasurface is
experimentally demonstrated with a thermal vacuum test. For the same heating
power of 0.2 W, the steady-state temperature of the tunable VO2 metasurface
emitter after phase transition is found to be 20degC lower than that of a
reference V2O5 emitter whose static spectral emittance is almost the same as
that of the VO2 metasurface before phase transition. The radiative thermal
conductance for the tunable metasurface emitter is found to be 3.96 W/m2K with
metallic VO2 at higher temperatures and 0.68 W/m2K with insulating VO2 at lower
temperatures, clearly demonstrating almost six-fold enhancement in radiative
heat dissipation.
