  Kirchhoff's law of thermal radiation imposes a constraint on photon-based
energy harvesting processes since part of the incident energy flux is
inevitably emitted back to the source. By breaking the reciprocity of the
system, it is possible to overcome this restriction and improve the efficiency
of energy harvesting. Here, we design and analyze a semitransparent emitter
that fully absorbs normally incident energy from a given direction with zero
backward and unity forward emissivity. The nearly ideal performance with
wavelength-scale thickness is achieved due to the magneto-optical effect and
the guided-mode resonance engineered in the emitter structure. We derive the
general requirements for the nonreciprocal emitter using the temporal coupled
mode theory and the symmetry considerations. Finally, we provide a realistic
emitter design based on a photonic crystal slab consisting of a magnetic Weyl
semimetal and silicon.

Optics;Applied Physics