Violation of Kirchhoff’s Law of Thermal Radiation with Space–Time Modulated Grating

We present an infrared thermal emitter that exhibits unequal emissivity and absorptivity for a given frequency and angle of incidence, violating Kirchhoff’s law of thermal radiation. We exploit spatio-temporal modulation of the refractive index of a grating to drive photonic transitions between guided resonance modes. We show via simulations that the modulation results in directional-dependent reflection. Using a generalized law of thermal radiation valid in the presence of mode conversion, we directly calculate absorptivity and emissivity for our structures. We show that a strong contrast between absorptivity and emissivity can be obtained close to a wavelength of 3.6 μm for realistic modulation frequencies of 10s of GHz. Furthermore, we show that for strong modulation, the system exhibits directional Rabi splitting, giving rise to a stronger contrast between emissivity and absorptivity. The results thus predict the feasibility of nonreciprocal infrared thermal emission without the use of magneto-optical materials.