Epsilon-near-zero thin film enhanced nonreciprocal thermal radiation in Weyl semimetal-based heterostructures

Violating Kirchhoff's radiation law plays a significant role in enhancing photonic energy conversion efficiency and controlling radiative heat transfer at the nanoscale. Here, we consider a heterostructure consisting of a phononic layer and a Weyl semimetal (WSM) interlayer sitting on a silver (Ag) substrate. We study the interaction of electromagnetic waves with the design and demonstrate that the epsilon-near-zero (ENZ) mode supported in ultra-thin aluminum nitride (AlN) can greatly improve the nonreciprocal response of the WSM layer. An attenuated total reflection (ATR) geometry in the classic Otto configuration is also considered. Our results demonstrate that the phononic thin layer can boost the difference between the emissivity and absorptivity in a broad range of wavelength as compared to an uncoated device. The nonreciprocity at resonance wavelength can reach near-unity with a small incident angle. By engineering the structural parameters, we reveal a surprising result of breaking Kirchhoff's law without requiring any surface patterning and external magnetic field, which identifies unique fundamental and technological prospects of WSMs for engineering thermal radiation with various requirements.