Nonreciprocal Thermal Emission Using Spatiotemporal Modulation of Graphene

We present a nonreciprocal thermal emitter based on the dynamic modulation of graphene. A graphene ribbon grating situated on a dielectric slab is designed to excite high-quality resonances in the long-IR region. We show that upon space-time modulation of the Fermi energy of graphene, asymmetric modal splitting results in large nonreciprocity, leading to a strong violation of Kirchhoff's law of thermal radiation. We further show that the graphene system allows the creation of "absorptivity holes" and "emissivity holes" in the spectrum that are asymmetric. By changing the modulation frequency, the location of these holes can be adjusted, giving rise to a new dimension of tunable thermal emission in nonreciprocal systems. In this system, while Kirchhoff's law is violated, we numerically observe certain symmetry properties between absorption and emission. We establish that these symmetry properties follow compound symmetry considerations.