Maximal violation of Kirchhoff's law in planar heterostructures

Violating Kirchhoff's law is generally achieved using patterned nonreciprocal materials, where a guided or polaritonic mode that lies outside the light cone, often via gratings, is excited. Here, we describe how nonreciprocity manifests itself in pattern-free heterostructures. We demonstrate that a resonant mode in a dielectric spacer separating a nonreciprocal film from a back reflector suffices to maximally violate Kirchhoff's law, and identify the minimal dielectric requirements for such functionality, which are satisfied by currently available materials.