Ultra-broadband all-dielectric metamaterial thermal emitter for passive radiative cooling

Passive radiative cooling, which pumps heat to outer space via thermal radiation, has been a promising energy free technology to maintain the earth surface temperature. Nighttime radiative cooling technology is quite mature, while daytime radiative cooling still poses many challenges due to the requirement of minimization of incident solar absorption and maximization of the mid-infrared emissivity in the atmospheric transparency windows. However, the mid-infrared emissivity efficiency of natural materials is usually poor, providing a low cooling efficiency and the realization of a high performance daytime radiative cooler is still quite challenge. In this work, we design and numerically investigate a three dimensional (3D) all-dielectric pyramidal multilayer metamaterial (PMM), which not only avoids the problem of high absorptivity loss of metal materials to solar, but also provide extremely high infrared absorptivity due to the attenuation effect of moth-eye structure and the electromagnetic resonant absorption in the metamaterial, achieving the purpose of both extremely low solar spectrum absorption and strong infrared emissivity within the atmospheric windows under the direct sunlight. Eventually, our designed cooler presents the potential to achieve a net radiative cooling power exceeding 156 W/m² at ambient temperature of 300 K under direct solar irradiation, leading to a temperature reduction of 42.4°C. At nighttime, the net cooling power is more than 199 W/m² at ambient temperature, resulting in a temperature reduction of 58.5°C. Even considering the non-radiative heat exchange conditions, this metamaterial cooler can still cool down 9.6°C at the daytime and 12.3°C at the nighttime respectively. Therefore, this work further promotes the development of all-dielectric metamaterial based passive radiative coolers and is of great significance for energy conservation.