Dual-polarization broadband directional thermal emitter via coupled Fabry-Perot and Berreman modes.

Abstract

Controlling the directivity of broadband thermal radiation has been a fundamental challenge. Broadband directional thermal emission is of great importance for applications such as infrared stealth, radiative cooling, and thermal camouflage. However, most existing studies focus on the directional control of thermal radiation for a single polarization state, and broadband directional emission under dual-polarization conditions remains largely unexplored. In this work, we propose a dual-polarization broadband directional emitter that couples the Berreman mode of epsilon-near-zero (ENZ) materials with the Fabry-Perot (FP) resonance. The structural parameters are optimized by a genetic algorithm. The emitter consists of an MgF₂/SiO/MgF₂/Ta₂O₅ multilayer stack on an Al substrate, achieving large-angle directional emission within the 8-14 μm atmospheric window for both s-polarization and p-polarization. The average unpolarized emissivity in the atmospheric window reaches 0.69 at an incidence angle of 70 °, while it is as low as 0.23 at 0 °, yielding a contrast ratio close to 3:1. Electric field distribution and loss analyses reveal the underlying physical mechanisms: p-polarized emission is dominated by the Berreman mode in the ENZ films, whereas s-polarized emission is governed by the FP cavity resonance formed by the MgF₂ dielectric layer and the ENZ films. By effectively coupling the FP resonance and the Berreman mode, this work provides a new, to the best of our knowledge, strategy for achieving broadband directional thermal emission control under dual polarization.