Planck’s law of thermal radiation depends on the temperature, T, and the emissivity, ε, which is the coupling of heat to radiation depending on both phonon-electron nonradiative-interactions and electron-photon radiative-interactions. In contrast, absorptivity, α, only depends on the electron-photon radiative-interactions. At thermodynamic equilibrium, nonradiative-interactions are balanced, resulting in Kirchhoff’s law of thermal radiation, ε=α. At non-equilibrium, Quantum efficiency (QE) describes the statistics of photon emission, which like emissivity depends on both radiative and nonradiative interactions. We theoretically and experimentally demonstrate a prime equation relating these properties in the form of:

ε=α(1-QE), which is reduced to Kirchhoff’s law at equilibrium.

Read more… “NATURE Phot – Generalized Kirchhoff Law”