Broadband absorption of infrared dielectric resonators for passive radiative cooling

Infrared resonator metamaterials, exhibiting spectral selective absorption of light, have recently been of great interest for passive radiative cooling. In this approach, coolers radiate power passively, with emissivity equals to its absorptivity. However, high-efficient energy dissipation requires radiation windows of broad bandwidth, i.e. 8–14 μm, that challenges the narrow-band feature of resonators. Here, we numerically realize the broadband absorption (above 80% in 8–11 μm) for dielectric resonators over a wide incident angle range (0°–60°), predicting 12 °C cooling below the ambient temperature at nighttime. The electromagnetically resonating eigenmodes of the dielectric resonators are perturbated by losses to reshape the dispersion relation in radiation windows, which are implemented by two methods: inserting metal components and coating lossy dielectrics. Retrieval constitutive parameters, as well as S parameters, map the reshaping process for broadband considerations. Additionally, ideal transparency (nearly 100%) above the wavelength of 0.5 mm is achieved. These crucial features offer an effective solution to the microwave signals shielding problem generally encountered in previous radiative coolers, regarding applied scenes such as buildings and cars. (paper)