Broadband Nonreciprocal Thermal Emission

The reciprocity between thermal emission and absorption in materials that satisfy the Lorentz reciprocity places a fundamental constraint on photonic energy conversion and thermal management. For approaching the ultimate thermodynamic limits in various photonic energy conversions and achieving nonreciprocal radiative thermal management, broadband nonreciprocal thermal emission is desired. However, existing designs of nonreciprocal emitters are narrowband. Here, we introduce a gradient epsilon-near-zero magneto-optical metamaterial for achieving broadband nonreciprocal thermal emission. We start by analyzing the nonreciprocal thermal emission and absorption in a thin layer of epsilon-near-zero magneto-optical material atop a substrate. We use temporal coupled-mode theory to elucidate the mechanism of nonreciprocal emission in the thin-film emitter. We then introduce a general approach for achieving broadband nonreciprocal emission by using a gradient epsilon-near-zero magneto-optical metamaterial. We numerically demonstrate broadband nonreciprocal emission in gradient-doped semiconductor multilayer, as well as in a magnetic Weyl semimetal multilayer with gradient chemical potential. Our approach for achieving broadband nonreciprocal emitters is useful for developing broadband nonreciprocal devices for energy conversion and thermal management.