Strong nonreciprocal thermal radiation by optical Tamm states in Weyl semimetallic photonic multilayers

The almost-complete violation of Kirchhoff's law possesses various potential applications, such as nonreciprocal thermal emitters, thermal routers, and other logic devices. Recently, Weyl semimetal materials have attracted wide attention. It exhibits a very unusual and extremely large gyroscopic optical response within the mid IR without an external magnetic field, which is due to its unique topologically nontrivial electronic state and inherent time-reversal symmetry breaking. Here, we show a planar multilayer structure composed of Weyl semimetals, which can achieve an almost-complete violation of Kirchhoff's law at an incident angle of 18° without applying any external magnetic field. By tuning the thickness of the ITO layer, the angle of incidence, and the axial vector of the WSM, an almost complete violation of Kirchhoff's law can persist over a wide range of angles (18°–52.3°) and wavelengths (6.6 μm–7.2 μm). Furthermore, through the study of the electric field distribution, it is revealed that this enhanced nonreciprocity is due to the local field-enhancing properties of TPPs. Our work may contribute a practical design scheme for constructing strongly nonreciprocal thermal emitters.