Objects around us constantly emit and absorb thermal radiation. For reciprocal systems, the emissivity and absorptivity are restricted to be equal by Kirchhoff's law of thermal radiation \[1\]. This restriction limits the control of thermal radiation and contributes to an intrinsic loss mechanism in photonic energy harvesting systems. Existing approaches to violate Kirchhoff's law typically utilize conventional magneto-optical effects in the presence of an external magnetic field. However, these approaches require either a strong magnetic field (\~3T) \[2\] or narrow-band resonances under a moderate magnetic field (\~0.3T) \[3\], because the non-reciprocity in conventional magneto-optical effects is usually weak in the thermal wavelength range. Here we show that the axion electrodynamics in magnetic Weyl semimetals can be used to construct strongly nonreciprocal thermal emitters \[4\]. Such a thermal emitter can near completely violate Kirchhoff’s law overbroad angular and frequency ranges, without requiring any external magnetic field. The non-reciprocity moreover is strongly temperature tunable, opening new possibilities for active non-reciprocal devices in controlling thermal radiation.

\[1\]       G. Kirchhoff, Annalen Der Physik 185, 275 (1860).   
\[2\]       L. Zhu and S. Fan, Phys. Rev. B 90, 220301 (2014).  
\[3\]       B. Zhao, Y. Shi, J. Wang, Z. Zhao, N. Zhao, and S. Fan, Opt. Lett. 44, 17(2019).  
\[4\]       B. Zhao\*, C. Guo\*, C. A. C. Garcia, P. Narang, and S. Fan, Nano Lett. 20, 1923 (2020).


Nonreciprocal Thermal Radiation From  Magnetic Weyl Semimetals

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Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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