Nonreciprocal thermal photonics

Thermal nanophotonics has undergone a revolution in recent decades, benefiting from in-depth studies in nano-optics and breakthroughs in nanotechnologies. The majority of thermal devices are constrained by Kirchhoff's law of thermal radiation, whereby spectral directional absorptivity and emissivity are identical according to Lorentz reciprocity. This restriction introduces an intrinsic loss in a plethora of energy technologies and prevents us from controlling emission and absorption independently. Recently, there has been considerable progress in realizing nonreciprocal radiation to overcome these limitations. Here we summarize the fundamental physics of nonreciprocal radiation, lay out different approaches to generalize Kirchhoff's law, and highlight several promising nonreciprocal thermal applications. This comprehensive overview of nonreciprocal thermal photonics may reveal new physics, unprecedented nonreciprocal effects and broader potential applications. This Review discusses the physics of nonreciprocal radiation and Kirchhoff's law generalization in the context of nanophotonics-enabled nonreciprocal thermal applications.