Ultrathin High Quality‐Factor Planar Absorbers/Emitters Based on Uniaxial/Biaxial Anisotropic van der Waals Polar Crystals

Abstract Tailoring the absorption/emission through nanostructures from broadband to narrowband has attracted increasing attention due to the merits of high efficiency and compactness. However, most of the reported narrowband emitters require either complex fabrication process or thick coating of the suitable materials (~mm). In this paper, by exciting Berreman mode through a combination of Au mirrors and ultra‐thin layers of low‐loss uniaxial/biaxial anisotropic 2D van der Waals polar crystal hBN/α‐MoO₃, a narrowband absorption/emission peak is realized in the mid‐infrared region. The measured quality‐factor of Berreman mode for hBN/Au and α‐MoO₃/Au structure can reach up to 134 and 164, respectively. The deep subwavelength thickness (~nm) and bilayer architecture simplify the fabrication process. Furthermore, taking advantage of the in‐plane birefringence originating from α‐MoO₃, the polarization of the absorbed/emitted radiation can be controlled through this planar configuration. Such ultrathin narrowband absorbers/emitters provide new opportunities for the advancements in thermal sources, infrared sensors, and thermophotovoltaic power generation systems.