Fig. 1. Design of hybrid polar dielectric metasurface thermal emitter. (a) The proposed hybrid metasurface with the unit consisting of alternating Si₃N₄ and SiO₂ polar dielectric materials. P_atm is the downward atmosphere thermal emission and P_rad is the thermal emission of the metasurfaces. (b) The normalized scattering cross-section of the optimized 3-layer (SiO₂/Si₃N₄/SiO₂) Mie resonator with the electrical field distributions at various resonance wavelengths and the extinction coefficients of Si₃N₄ (red) and SiO₂ (green). (c) Schematic of the multilayer perception neural network for inverse design with input, hidden and output layers. (d) Emissivity spectrum of the inversely optimized 3-layer selective thermal emitter, compared with the ideal spectrum. (e) Emissivity, selectivity, and emission angle of this work, in comparison with state-of-the-art results.

Fig. 2. The fabricated 2-layer metasurface and its emissivity. (a) A photography of the fabricated 4 inches metasurface thermal emitter. (b) Top-view and tilted SEM images of an array of the hybrid SiO₂/Si₃N₄ dielectric resonators (scale bar: 10 μm and 2 μm for the left and right two figures, respectively). (c) Energy-dispersive X-ray spectroscopy elemental mappings of the metasurfaces (scale bar: 5 μm). (d) The measured infrared emissivity of the metasurfaces. (e) Multipolar decomposition of the 2-layer dielectric resonator. ED: electrical dipole; MD: magnetic dipole; EQ: electric quadrupole; MQ: magnetic quadrupole. (f) Electric and magnetic field profiles at the two resonance wavelengths 8.9 μm (top) and 10.5 μm (bottom).

Fig. 3. Angle-resolved thermal emissivity. (a, b) Net cooling power P_cooling of the ideal selective thermal emitter with various thermal emission angles in the cases of heat exchange coefficient h = 0 W/m²/K (a) and 2 W/m²/K (b). T_a and T_r are ambient and radiative cooler temperature, respectively. (c) The steady temperature of the radiative cooler as a function of the emission angle for various heat exchange coefficients h. (d–f) The angle-resolved emissivity (unpolarized, s polarized, and p polarized) of the metasurface thermal emitter.

Fig. 4. (a) Schematic of the apparatus and the metasurface radiative cooler. (b) Photo of the apparatus on the test rooftop in Shanghai, China. (c, d) The solar irradiation and environment (wind speed and relative humidity) during the rooftop test. (e) Steady-state temperature of the metasurface radiative cooler and the ambient temperature. (f) The net cooling power of the radiative cooler using the experimental spectra data (solar absorbance: q=5.4%; heat exchange coefficient: h=4.04 W/m²/K).

Fig. 5. (a) Schematic diagram of the urban heat island effect. The ~N W/m² represents the net growth rate of the anthropogenic heat. (b) The calculated deployment area to reduce the temperature by our metasurface radiative cooler with emission angle of 80° and 60°, respectively. (c–f) The calculated area for eliminating the heat island effect and achieving a comfortable body temperature during summertime for China’s four big cities: Beijing, Shanghai, Chongqing, and Tianjin.