Light Propagation in Optically Pumped Alkali Vapors

The propagation of light in an optically pumped vapor is determined semiclassically by the expectation value of a dielectric susceptibility operator. We present explicit formulas for the dielectric susceptibility of a vapor of alkali atoms. The qualitative features of the susceptibility are illustrated for the specific case of ${\mathrm{Rb}}^{87}$. The theory of this paper is essential for a clear understanding of phenomena such as the birefringence of alkali vapors and the microwave modulation of light by alkali vapors. Some experimental observations of birefringence in rubidium vapor are interpreted with the semiclassical theory.