Non-Planck thermal emission from two-level media

Thermal emission is a universal phenomenon of stochastic electromagnetic emission from an object composed of arbitrary materials 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 absorb and emit light may change with temperature, however, is often ignored. Here, we carry out a theoretical study of thermal emission from structures incorporating ensembles of 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 systems including two-level media. Thermal emission from such structures is 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.