Non-Planck thermal emission from two-level media

Thermal emission is a universal phenomenon of stochastic electromagnetic emission from absorbing bodies at elevated temperatures. A defining feature of this emission is the monotonic and rapid growth of its intensity with the object's temperature for most known materials. This growth originates from the Bose-Einstein statistics of the thermal photonic field. The fact that the material's ability to emit light may change with temperature, however, is often overlooked. Here, we carry out a theoretical study of thermal emission from structures incorporating two-level media. We investigate this effect in a range of geometries including thin films and compact nanoparticles and establish the general dependencies in the evolution of thermal emission from such systems. Thermal emission turns out to be essentially non-Planckian and exhibits a universal asymptotic behavior in the limit of high temperatures. These results might have important implications for the design of thermal energy harvesting and thermal vision systems.