Dephasing of ultracold cesium $80D_{5/2}$-Rydberg Electromagnetically Induced Transparency

We study Rydberg electromagnetically induced transparency (EIT) of a cascade three-level atom involving 80$D_{5/2}$ state in a strong interaction regime employing a cesium ultracold cloud. In our experiment, a strong coupling laser couples 6$P_{3/2}$ to 80$D_{5/2}$ transition, while a weak probe, driving 6$S_{1/2}$ to 6$P_{3/2}$ transition, probes the coupling induced EIT signal. At the two-photon resonance, we observe that the EIT transmission decreases slowly with time, which is a signature of interaction induced metastability. The dephasing rate $\gamma_{\rm OD}$ is extracted with optical depth OD = $\gamma_{\rm OD}t$. We find that the optical depth linearly increases with time at onset for a fixed probe incident photon number $R_{\rm in}$ before saturation. The dephasing rate shows a nonlinear dependence on $R_{\rm in}$. The dephasing mechanism is mainly attributed to the strong dipole-dipole interactions, which leads to state transfer from $nD_{5/2}$ to other Rydberg states. We demonstrate that the typical transfer time $\tau_{0(80D)}$ obtained by the state selective field ionization technique is comparable with the decay time of EIT transmission $\tau_{0({\rm EIT})}$. The presented experiment provides a useful tool for investigating the strong nonlinear optical effects and metastable state in Rydberg many-body systems.