Direct Observation of Kirchhoff Thermal Radiation Law Violation

Thermal emission, the process through which all objects with finite temperature radiate electromagnetic energy, has generally been thought to obey reciprocity whereby absorbed and emitted radiation from a body are equal for a given wavelength and angular channel. We report direct experimental measurement of an inequality between the thermal emissivity and absorptivity for a photonic structure that supports a transverse magnetic guided-mode resonance (GMR) coupled to a magneto-optic material. This inequality occurs under the application of an in-plane magnetic field that changes the permittivity of the magneto-optic InAs to a non-diagonal tensor. The non-diagonal permittivity tensor results in an antisymmetric angular relationship where the magnetic tuning of stronger thermal emission for a given angle of incidence correlates to stronger absorption in the opposite channel. We relate this to the Onsager-Casimir relations for a nonreciprocal absorber. Using an angle-resolved thermal emission spectroscopy (ARTES) setup, we observe that the Kirchhoff-violating behavior is most pronounced where the GMR and the InAs’ Brewster mode spectrally overlap (12.8 – 12.5 um) for an angular range of 60 - 80°. The tuning of larger emissivity saturates at the blackbody limit; the lower bounds of the emissivity tuning is limited by the low-emissivity limit of the InAs. We also discuss the conical diffraction mounting of the sample in an external magnetic field and its implications for future Kirchhoff-violating emitters.