Directly Violating the Kirchhoff Thermal Law

The Kirchhoff thermal radiation law provides an inherent constraint on the ability to harness thermal radiation, requiring that the spectral emissivity and absorptivity are equal and exhibit identical angular distributions for a given polarization. This equality is built on the fundamental assumption that all materials obey Lorentz reciprocity. One class of materials that does not satisfy Lorentz reciprocity, and thus can violate the Kirchhoff thermal law, is magneto-optically-active materials. In this work, we experimentally demonstrate a direct inequality between the absorptivity and emissivity in the mid-infrared of a guided-mode-resonant (GMR) structure with a magneto-optical InAs underlayer that breaks time-reversal symmetry, and thus reciprocity, in a moderate external magnetic field. We demonstrate the angular and spectral distribution changes of both the emissivity and the absorptivity in magnetic fields. Furthermore, we use the symmetry of the GMR structure to tune the Kirchhoff violating emission’s dispersion and magnetic field dependence. We conclude with an outlook on the dynamic tunability of such structures and their practical implications.