Modeling Dispersive Media in Finite-Difference Time-Domain Method for Radiative Cooling Applications

Radiative cooling offers a new strategy to address building cooling problems in an environmentally friendly way, compared with traditional cooling methods that require high-power consumption. The goal of radiative cooling is to maximize absorption at the mid-infrared (MIR) spectrum (8– $13~\mu m$ ). Thus, it is important to characterize the dispersive relationship of the radiative cooling materials (TiO₂, Si₃N₄, SiC, SiO₂) in the MIR regime. In this work, we use the multi-pole Lorentz dispersion equation to model frequency-dependent dielectric constants of radiative cooling materials, which requires particle swarm optimization to solve multidimension optimization problems. This modeling can be directly applicable to the auxiliary differential equation (ADE)-based finite-difference time-domain (FDTD) method for simulating emissivity of radiative cooling materials. Further, we limit the number of Lorentz poles to five, which minimizes the computational burden in ADE–FDTD. Then, we validate our FDTD results against an analytic prediction calculated by the transfer matrix method. Finally, we demonstrate a highly efficient radiative cooling structure based on a simple SiO₂ grating. The proposed material modeling can accelerate the nano/microstructural design of the radiative cooling strategies.