Manipulation of Mid-Infrared Emission via Metal Dielectric Metal Approaches

Passive daytime cooling, which transfers the heat from terrestrial objects to outer space without external energy input, has attracted increasing interest.¹ As cooling accounts for 15% of our global energy consumption, it provides a promising pathway to alleviate the global energy demand. With minimizing the adsorption in the solar range (0.3 - 2.5 µm), on the one hand, and maximizing the emissivity (adsorption) in the “sky window” range (8 - 13 µm), on the other hand, materials with designed optical properties could cool down itself below ambient temperature, even under the direct sunlight. Dielectric materials, for instance, silica micro-/nanoparticles, have been widely utilized in passive cooling devices to adjust the optical properties. Remarkable passive cooling performance was theoretically and experimentally demonstrated.² However, tuning of the absorption properties within the sky window range is quite limited with dielectric structures.<br/>In this work, we have investigated two types of metal-dielectric-metal (MDM) approaches to tune the mid-infrared (MIR) emission properties in sky window range, namely ZnS-Au-Si substrate supported Au disc arrays³ and SiO₂-indium tin oxide (ITO) based plasmonic-photonic structure. In the former, we fabricated a 2D periodic array of Au discs on the top of a ZnS-Au-Si substrate via photolithography. We observed that MIR absorptance induced by magnetic polariton resonance of the first order strongly depends on the individual disc diameter. The absorptance peak shifts linearly to longer wavelengths with increasing diameter. In addition, the induced magnetic polariton resonances were observed to be independent of the neighboring disc resonances. Based on that, a mixed resonator with a broadband absorptance that spans the entire sky window range was achieved by constructing multiple discs within one unit cell. In the latter, a non-close packed silica monolayer was self-assembled on the top of Au substrate via polymer-assisted drop-casting technique. By partially covering the SiO₂ sphere with an ITO shell, the MIR absorptance in the sky window range is substantially enhanced, arising from the interaction between the localized surface plasmon (LSP) and the magnetic dipole Mie resonance.<br/>1. Hossain, M. M.; Gu, M., Radiative Cooling: Principles, Progress, and Potentials. <i>Adv. Sci. </i><b>2016,</b> <i>3</i> (7), 1500360.<br/>2. Zhai, Y.; Ma, Y.; David, S. N.; Zhao, D.; Lou, R.; Tan, G.; Yang, R.; Yin, X., Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling. <i>Science </i><b>2017,</b> <i>355</i> (6329), 1062-1066.<br/>3. Pech-May, N. W.; Tobias, L.; Retsch, M., Design of Multimodal Absorption in the Mid-IR: A Metal Dielectric Metal Approach. <i>ACS Appl. Mater. Interfaces </i><b>2021,</b> <i>13</i> (1), 1921-1929.