Narrow-line absorption at 689 nm in an ultracold strontium gas

We analyse the spectrum on the narrow-line transition arrow at 689 nm in an ultracold gas of $^{88}$Sr via absorption imaging. In the low saturation regime, the Doppler effect dominates in the observed spectrum giving rise to a symmetric Voigt profile. The atomic temperature and atom number can accurately be deduced from these low-saturation imaging data. At high saturation, the absorption profile becomes asymmetric due to the photon-recoil shift, which is of the same order as the natural line width. The line shape can be described by an extension of the optical Bloch equations including the photon recoil. A lensing effect of the atomic cloud induced by the dispersion of the atoms is also observed at higher atomic densities in both the low and strong saturation regimes.