Ultra-Broadband Infrared Metamaterial Absorber for Passive Radiative Cooling

Infrared metamaterial absorber (MMA) based on metal-insulator-metal (MIM) configuration with flexible design, perfect and selective absorption, has attracted much attention recently for passive radiative cooling applications. To cool objects passively, broadband infrared absorption (i.e. 8–14 µm) is desirable to emit thermal energy through atmosphere window. We present a novel MMA composed of multilayer MIM resonators periodically arranged on a PbTe/MgF_2 bilayer substrate. Verified by the rigorous coupled-wave analysis method, the proposed MMA shows a relative bandwidth of about 45% (from 8.3 to 13.1 µm with the absorption intensity over 0.8). The broadband absorption performs stably over a wide incident angle range (below 50^\circ) and predicts 12 K cooling below ambient temperature at nighttime. Compared with the previous passive radiative coolers, our design gets rid of the continuous metal substrate and provides an almost ideal transparency window (close to 100%) for millimeter waves over 1 mm. The structure is expected to have potential applications in thermal control of integrated devices, where millimeter wave signal compatibility is also required.