Infrared radiation sources
Nanophotonics devices for thermal emission

Thermal emission from matter is one form of light-matter interaction. Using "Einstein coefficients" reasoning on a simple two-level system, E₁ and E₂ at temperature T, in the presence of a set of photons whose spectral volume energy distribution is characterized by Φ_eq(ν₁₂,T) at frequency ν = ν₁₂ (figure 1a), we can place thermal emission in a framework common to three kinds of interaction: absorption, stimulated emission, and spontaneous emission.

The aim of this section is to take up the skeleton of reasoning on a system comprising matter and a "gas" of photons, both at thermodynamic equilibrium.