Optical non-linearity in a dynamical Rydberg gas

We use the technique of electro-magnetically 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 58D${}_{5\mathrm{/}2}$ Rydberg state, an atom density of $1.6\times {10}^{10}$cm${}^{-3}$ and a positive frequency scan we measure a third-order non-linearity of ${\chi }^{(3)}=5\times {10}^{-7}$ m${}^2$V${}^{-2}$. For the reverse scan we observe a second order non-linearity of $\chi^{(2)}=5\times 10^{-6}$mV$^{-1}$. The contrasting behaviour can be explained in terms of motional effects and resonant excitation of Rydberg pairs.