Bridging the Fabry–Perot cavity and asymmetric Berreman mode for long-wave infrared nonreciprocal thermal emitters

The long-wave infrared band (8–14 µm) is essential for several applications, such as infrared detection, radiative cooling, and near-field heat transfer. However, according to Kirchhoff’s law, the intrinsic balance between thermal absorption and emission limits the further improvement of photon energy conversion and thermal management. Thus, breaking Kirchhoff’s balance and achieving nonreciprocal thermal radiation in the long-wave infrared band are necessary. Most existing designs for nonreciprocal thermal emitters rely on grating or photonic crystal structures to achieve nonreciprocal thermal radiation at narrow peaks, which are relatively complex and typically realize bands larger than 14 µm. Here, a sandwich structure consisting of an epsilon-near-zero (ENZ) magneto-optical layer (MOL), a dielectric layer (DL), and a metal layer is proposed to achieve a strong nonreciprocal effect in the long-wave infrared band, which is mainly attributed to the strengthening of the asymmetric Berreman mode by the Fabry–Perot cavity. In addition, the impact of the incident angle, DL thickness, and DL refractive index on the nonreciprocal thermal radiation has been investigated. Moreover, by replacing the ENZ MOL with the gradient ENZ MOL, the existence of the DL can further improve the nonreciprocity of the broadband nonreciprocal thermal radiation. The proposed work promotes the development and application of nonreciprocal energy devices.