Phys. Rev. A 103, 063710 (2021)
A. Tebben, C. Hainaut, A. Salzinger, S. Geier, T. Franz, T. Pohl, M. Gärttner, G. Zürn, and M. Weidemüller
We experimentally investigate the nonlinear transmission spectrum of coherent light fields propagating through a Rydberg EIT medium with strong atomic interactions. In contrast to previous investigations, which have largely focused on resonant control fields, we explore here the full two-dimensional spectral response of the Rydberg gas. Our measurements confirm previously observed spectral features for a vanishing control-field detuning, but also reveal significant differences on two-photon resonance. In particular, we find qualitative deficiencies of mean-field models and rate-equation simulations as well as a third-order nonlinear susceptibility that accounts for pairwise interaction effects at low probe-field intensities in describing the nonlinear probe-field response under EIT conditions. Our results suggest that a more complete understanding of Rydberg EIT and emerging photon interactions requires to go beyond existing simplified models as well as few-photon theories.