  Violating Kirchhoff's law has so far required nonreciprocal materials
patterned in microstructures. In these configurations, the excitation of a
guided or polaritonic mode that lies outside the light cone, often via
gratings, was a requirement. 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.

Optics;Applied Physics