Defect-mode and Fabry-Perot resonance induced multi-band nonreciprocal thermal radiation

According to Kirchhoff’s radiation law, the spectral-directional absorptivity (α) and spectral-directional emissivity (e) of an object are widely believed to be identical, which places a fundamental limit on photonic energy conversion and management. The introduction of Weyl semimetals and magneto-optical (MO) materials into photonic crystals makes it possible to violate Kirchhoff’s law, but most existing work only report the unequal absorptivity and emissivity spectra in a single band, which cannot meet the requirements of most practical applications. Here, we introduce a defect layer into the structure composed of one-dimensional (1D) magnetophotonic crystal and a metal layer, which realizes dual-band nonreciprocal thermal radiation under a 3-T magnetic field with an incident angle of 60°. The realization of dual-band nonreciprocal radiation is mainly due to the Fabry-Perot (FP) resonance occurring in the defect layer and the excitation of Tamm plasmon, which is proved by calculating the magnetic field distribution. In addition, the effects of incident angle and structural parameters on nonreciprocity are also studied. What is more, the number of nonreciprocal bands could be further increased by tuning the defect layer thickness. When the defect layer thickness increases to 18.2 µm, tri-band nonreciprocal thermal radiation is realized due to the enhanced number of defect modes in the photonic band gap and the FP resonance occurring in the defect layer. Finally, the effect of defect location on nonreciprocity is also discussed. The present work provides a new way for the design of multi-band or even broad-band nonreciprocal thermal emitters.