Subradiance and superradiance-to-subradiance transition in dilute atomic clouds

We experimentally study subradiance in a dilute cloud of ultracold rubidium (Rb) atoms where n lambda(3)(a) approximate to 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.