We experimentally study subradiance in a dilute cloud of ultracold rubidium (Rb) atoms where $n{\lambda }_a^3\approx {10}^{-2}$ ($n$: atomic density; ${\lambda }_a$: excitation wavelength) and the on-resonance optical depth of the cloud is of order unity. We show that, in the strong excitation regime, the subradiant time scales depend on the excitation fraction of the cloud; i.e., to the intensity of the excitation pulse. In this regime, the decay dynamics are highly complicated and there is not a single decay time constant. Instead, the decay time constant varies during the dynamics. Specifically, we were able to observe signatures of superradiant-to-subradiant transition; i.e., initially the decay rate is faster than independent decay (superradiant emission), while at later times it transitions to slower (subradiant emission). We also discuss a theoretical model whose numerical results are in good agreement with the experiments.

• Received 24 January 2020
• Accepted 21 September 2020

DOI:https://doi.org/10.1103/PhysRevA.102.043708