Low Saturation Intensities in Two-Photon Ultracold Collisions

Abstract

Interactions between ultracold atoms are of considerable interest because of the appearance of novel lightinduced dynamics at low temperature and the onset of quantum behavior [1]. Ultracold collisions proved to be crucial in attaining Bose-Einstein condensation, and optical control of ultracold collisions appears to provide a promising technique for producing large ensembles of ultracold molecules [2]. In this Letter, we report on a study of light-induced ultracold atomic collisions which, among other striking observations, display saturation intensities for two-photon processes that are much lower than observed for single-photon processes. Any realistic theoretical treatment of excited state collisions must include spontaneous emission as collision times are comparable to excited state lifetimes [3]. Furthermore, at ultracold temperatures— unlike thermal collisions —the hyperfine interaction plays an important role in the collision dynamics [4], though the analysis of virtually all experiments to date has ignored hyperfine interactions. For the two-photon experiment reported here, we analyzed the measurements using a semiclassical model incorporating all hyperfine energy levels. This allows us to elucidate the interplay between the various processes of flux depletion, spontaneous emission, optical shielding, and Franck-Condon overlap. Furthermore, we can quantitatively explain the striking observation of very low saturation intensities in the two-photon collision process.