Optical non-linearity in a dynamical Rydberg gas

We use the technique of electromagnetically induced transparency (EIT) to probe the effect of attractive dipole-dipole interactions in a highly excited Rydberg gas. The transient character of the EIT response is investigated by rapidly scanning the probe laser through resonance. We characterize the resulting cooperative optical non-linearity as a function of probe strength, density and scan direction. For the 58D5/2 Rydberg state, an atom density of 1.6 x 1010cm-3 and a positive frequency scan we measure a third-order non-linearity of χ(3) = 5 x 10-7 m2V-2. For the reverse scan we observe a second order non-linearity of χ(2) = 5 x 10-6 mV-1. The contrasting behaviour can be explained in terms of motional effects and resonant excitation of Rydberg pairs.