On the applicability of Kirchhoff’s law to the lasing regime

In 1860, Kirchhoff showed that the radiance emitted by a hot body is the product of its absorptivity by a universal function of temperature and frequency which he defined as blackbody radiance. With the advent of semiconductors, it has been shown that Kirchhoff's law can be extended to account for electroluminescence and photoluminescence in a cavity. Hence, the question of the validity of Kirchhoff's law in the lasing regime arises naturally. In this work, we show that Kirchhoff's law contains the lasing condition for an arbitrary cavity. It also enables the modelling of important features of the lasing regime, such as the frequency, directivity and polarization of the first lasing mode. Finally, we show that it enables to recover the Schawlow-Townes spectral linewidth. In summary, Kirchhoff's law appears to be a versatile tool to model light emission from thermal to the lasing regime.