Generalization of Kirchhoff’s Law: The inherent relations between quantum efficiency and emissivity

Planck’s law of thermal radiation depends on the temperature, \(T\), and the emissivity, \(\epsilon\), which is the coupling of heat to radiation depending on both phonon-electron nonradiative-interactions and electron-photon radiative-interactions. In contrast, absorptivity, \(\alpha\), only depends on the electron-photon radiative-interactions. At thermodynamic equilibrium, nonradiative-interactions are balanced, resulting in Kirchhoff’s law of thermal radiation, \(\epsilon =\alpha\). For non-equilibrium, Quantum efficiency (QE) describes the statistics of photon emission, which like emissivity depends on both radiative and nonradiative interactions. Past generalized Planck’s equation extends Kirchhoff’s law out of equilibrium by scaling the emissivity with the pump-dependent chemical-potential \(\mu\), obscuring the relations between the body properties. Here we theoretically and experimentally demonstrate a prime equation relating these properties in the form of \(\epsilon =\alpha \left(1-QE\right)\). At equilibrium, these relations are reduced to Kirchhoff’s law. Our work lays out the evolution of non-thermal emission with temperature, which is critical for the development of lighting and energy devices.