Inverting the thermal radiative contrast of vanadium dioxide by metasurfaces based on localized gap-plasmons

Vanadium dioxide (VO2) is a promising phase-change material (PCM) in controlling radiative heat transfer because of the large permittivity contrast between the phases and the moderate metal-insulator transition temperature of 340 K. Widely adopted bare VO2 films on a dielectric substrate permit more radiative heat in the insulating state compared to that in the metallic state. In this paper, we present PCM-insulator-metal metasurfaces that invert the thermal radiative contrast, which means that the radiative heat flux is more promoted in the metallic state. The metasurfaces exhibit similar but broader resonance compared to conventional metal-insulator-metal metamaterials based on localized gap-plasmons when VO2 is in the metallic state. The broad resonance facilitates to maximize the radiative thermal exchange and is explained by the damping of the gap-plasmon mode dominated by the optical loss of VO2. The measured electromagnetic response of the fabricated metasurfaces agrees well with numerical simulations, and it also demonstrates that the resonant wavelength is tuned by the temperature. High emission or absorption contrast at a specific temperature is numerically obtained by geometrical optimization albeit lossy amorphous silicon or alumina is employed as the insulating layer to satisfy the fabrication requirement. We believe that the presented metasurface design contributes to intelligent thermal management systems with flexibility. (C) 2018 Author(s).