The chemical potential of radiation

TLDR

Electromagnetic radiation of any kind is treated thermodynamically, providing a general framework for its description. By assigning a chemical potential to photons, the authors distinguish thermal from non‑thermal radiation, replacing an effective temperature with the emitting material’s real temperature, and apply this framework to LEDs, two‑level systems, and lasers. The resulting extension of Planck’s law enables calculation of absorption coefficients, emission spectra, and radiative recombination lifetimes, and theoretical spectra for GaAs LEDs agree excellently with experimental data.

Abstract

In a thermodynamic treatment electromagnetic radiation of any kind is described. The difference between thermal and non-thermal radiation is accounted for by introducing the chemical potential of photons. Instead of an effective temperature all kinds of radiation have the real temperature of the emitting material. As a result Planck's law for thermal radiation is extended to radiation of any kind. The concept of the chemical potential of radiation is discussed in detail in conjunction with light-emitting diodes, two-level systems, and lasers. It allows the calculation of absorption coefficients, of emission spectra of luminescent materials, and of radiative recombination lifetimes of electrons and holes in semiconductors. Theoretical emission spectra are compared with experimental data on GaAs light-emitting diodes and excellent agreement is obtained.