Interaction of light with a dense cloud of ultracold atoms is an open-system many-body problem. Even within the linear optics regime, interesting effects beyond effective medium theory can arise from near-field interaction and multiple scatterings. On the other hand, when subjecting to strong optical excitions, the coupled quantum dynamics involves long-range dipole correlations defined in an exponentially large Hilbert space, and can become impossible to simulate by any classical computers. Therefore, comparing with experimental investigation of linear optical response, nonlinear optical response of cold atoms can be even more intriguing for uncovering emergent many-body effects in optics. In this work, we develop an experimental technique to study the dynamics of laser-cooled atomic ensembles subjected to strong resonant excitations. By temporally shifting the wavevector of the strongly excited dipole spin-wave, the colletive forward emissions are separated from the much stronger excitation pulse itself, fascilitating a precise measurement of the collective emission dynamics deeply in the nonlinear excitation regime, for the first time to our knowledge. In contrast to an exponential superradient decay in the linear regime, we find the nonlinearly excited spin waves decay much slower initially, for certain time, before the recovery of exponential decay characteristics. The measured decay curves deviate substantially from simple theoretical predictions by nonlinear couple dipoles that ignore many-body dipole correlations.
|Presenter name||Ji Lingjing|
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