Dynamics of Low-Density Rydberg Gases

A state dependent stimulated emission probe was used to investigate the coherent and dynamic properties of cold Rydberg atoms. $^{87}$Rb atoms were excited to various nl Rydberg states from a MOT via continuous two-photon excitation. A stimulated emission probe laser was then used to bring the Rydberg atoms down to the 6P$_{3/2}$ state, allowing detection of decay photons as a Rydberg atom detection method. Phase-matched four-wave mixing was also achieved and the angular dependence investigated. This coherent process is optimized when detuned from the Rydberg state, giving as much as 40{\%} phase-matched light. In addition, the stimulated emission probe technique shows that radiative processes dominate the Rydberg population dynamics on a time scale much faster than the natural radiative lifetime. Modeling suggests superradiant emission may be the dominant factor [T. Wang \textit{et al., }Phys. Rev. A \textbf{75}, 033802 (2007)].