Infrared emissivity of Al-doped ZnO-coated SiO2 rod hyperbolic metamaterials with cuboidal patterns for radiative cooling

The development of radiative cooling materials through colloidal particle assembly methods represents a promising advancement in thermal management through spectral control. In this work, a cuboidpatterned array of SiO₂ nanorods is proposed to form hyperbolic metamaterials with slow-light waveguiding behavior. The SiO₂ nanorods are vertically assembled into three-dimensional arrays and coated with aluminum-doped zinc oxide (AZO) on their inner surfaces via atomic layer deposition, resulting in effective indefinite dielectric properties—i.e., hyperbolic dispersion—in the infrared regime. Due to the bulk plasmonic resonance of AZO in the infrared range, determined by its doping level, a relative group velocity vg/c as low as one-thousandth can be achieved within the 3–10 μm wavelength range, offering a novel route to enhance infrared emissivity. Furthermore, the fabricated structure exhibits an average emissivity of 84.7% in the 8–13 μm atmospheric window and a low average absorptivity of only 14.6% in the full solar spectrum. Both the experimental results and the electromagnetic simulations confirm the structural potential for daytime radiative cooling.