Strong nonreciprocal thermal radiation based on topological edge state in one-dimensional photonic crystal with Weyl semimetal

Kirchhoff's law is the theoretical basis for characterizing thermal radiation. The construction of nonreciprocal thermal radiation needs to violate Kirchhoff's law, in which its existing methods usually utilize the magneto-optical effect with an external magnetic field. However, natural materials' relatively weak magneto-optical response in the thermal wavelength range requires a strong or moderate magnetic field. Fortunately, the unique topologically nontrivial electronic state and inherent time-reversal symmetry breaking of Weyl semimetals can exhibit highly unusual and extremely large gyrotropic optical responses in the mid-infrared band without an external magnetic field, which provides a new way to violate Kirchhoff's law. This work theoretically investigates the nonreciprocity of a multilayer structure in which a Weyl semimetal is embedded in two one-dimensional photonic crystals. The results show that the absorptivity and emissivity of the structure almost completely violate Kirchhoff's law over a broad range of angles (7° ∼ 51°) and frequencies (56.4 THz ∼ 65.3 THz) without any external magnetic field. We also discuss the effects of the incident angle, the axial vector of the Weyl semimetal, and the thickness of the Weyl semimetal on the characteristics of nonreciprocal thermal radiation. Furthermore, the electric field distribution reveals that the structurally excited topological edge states significantly enhance the local electric field in the vicinity of the Weyl semimetal, providing positive conditions for the realization of perfect strong nonreciprocal thermal radiation. We hope this work may contribute to the design of strong nonreciprocal thermal emitters.