Two-Photon Cooperative Absorption in Colliding Cold Na Atoms

Two-photon cooperative absorption is common in solid-state physics. In a sample of trapped cold atoms, this effect may open up new possibilities for the study of nonlinear effects. The experiment described herein starts with two colliding Na atoms in the $S$ hyperfine ground state. The pair absorb two photons, resulting in both a ${P}_{1/2}$ and a ${P}_{3/2}$ atom. This excitation is observed by ionization using an external light source. A simple model that considers only dipole-dipole interactions between the atoms allows us to understand the basic features observed in the experimental results. Both the pair of generated atoms and the photons originating from their decay are correlated and may have interesting applications that remain to be explored.