Nonreciprocal scattering and implications for thermal emission control on a mid-infrared spatiotemporally modulated metasurface

Lorentz reciprocity fundamentally limits the performance of photonic systems by enforcing reciprocal energy exchange between source and detector, which implies a symmetric scattering matrix. In the context of thermal radiation, Lorentz reciprocity manifests as Kirchhoff's law-the equality of the spectral directional emissivity and absorptivity of a surface. Breaking this reciprocity is important for advancing photonic devices for energy conversion, radiative cooling and mid-infrared sensing and imaging. Here, we report the demonstration of spatiotemporally modulated nonreciprocal metasurfaces operating at mid-infrared frequencies. We design and fabricate a graphene-based integrated photonic structure and experimentally demonstrate nonreciprocal scattering 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, indicating a breakdown of the spectral directional version of Kirchhoff's law of thermal radiation. Together, our scattering experiments and theory imply effective decoupling of absorption and emission channels by breaking time-reversal symmetry at thermal wavelengths.