Modeling non-equilibrium thermal radiation phenomena using a direct simulation method*

Thermal radiation is a field of physics that finds rich applications in energy efficiency, energy conversion, and related topics. It is well-understood for bulk materials in thermal equilibrium through the application of Kirchoff’s law. Nonetheless, the recent emergence of nanoscale materials with exotic physical properties (such as violation of detailed balance) reveals that these systems can in general depart from thermal equilibrium, particularly over short time scales. These findings give rise in turn to two challenging questions: (1) How can we describe non-equilibrium radiative transport far from equilibrium (i.e., in a non-perturbative regime)? and (2) what are the fundamental limits, if any, on cooling in this regime? To begin to address both of these questions, we develop a first principles-based model of thermal radiation, based on the fluctuation-dissipation theorem. After verifying that it reproduces Kirchoff’s law in thermal equilibrium in the far field, then apply it to radiative transfer in the near field. This provides limits on radiative transport which are distinct from the equilibrium analysis. The potential effects of the presence of nonlinear media (e.g., Kerr media) are also investigated using this framework. Furthermore, we find that the presence of non-equilibrium states also significantly impacts the physics of related problems, such as the Casimir force.