Breakdown of the Kirchhoff’s law of thermal radiation by a spatiotemporally modulated nonreciprocal metasurface

Kirchhoff’s law of thermal radiation, which dictates that the emissivity of a surface equals its absorptivity under thermal equilibrium, fundamentally limits the efficiency of photonic systems by enforcing reciprocal energy exchange between source and detector. Breaking this reciprocity is important for advancing photonic devices for energy conversion, radiative cooling, and mid-infrared sensing and imaging. Driven by the growing need for photonic platforms to overcome reciprocity constraints, we report the first demonstration of spatiotemporally modulated nonreciprocal metasurfaces operating at mid-infrared frequencies enabling the violation of the Kirchhoff’s law at room temperature. We fabricate a graphene-based integrated photonic structure and experimentally demonstrate nonreciprocal reflection from a metasurface modulated at gigahertz frequencies. We further develop a theoretical framework to relate nonreciprocal scattering under spatiotemporal modulation with unequal absorptivity and emissivity for violation of the spectral directional Kirchhoff’s law. Together, our experiment and theory imply effective decoupling of absorption and emission channels by breaking time-reversal symmetry at thermal wavelengths, thereby representing an indirect demonstration of breakdown of Kirchhoff’s law of thermal radiation.