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 $_{5/2}$ Rydberg state, an atom density of $1.6\times10^{10} $cm$ ^{-3}$ and a positive frequency scan we measure a third-order non-linearity of $χ^{(3)}=5\times 10^{-7}$ m $^2$ V $^{-2}$ . For the reverse scan we observe a second order non-linearity of $χ^{(2)}=5\times 10^{-6}$ mV $^{-1}$ . The contrasting behaviour can be explained in terms of motional effects and resonant excitation of Rydberg pairs.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$_{5/2}$ Rydberg state, an atom density of $1.6\times10^{10} $cm$^{-3}$ and a positive frequency scan we measure a third-order non-linearity of $χ^{(3)}=5\times 10^{-7}$ m$^2$V$^{-2}$. For the reverse scan we observe a second order non-linearity of $χ^{(2)}=5\times 10^{-6}$ mV$^{-1}$. The contrasting behaviour can be explained in terms of motional effects and resonant excitation of Rydberg pairs.

arXiv:1102.0669Journal of Physics B Atomic Molecular and Optical PhysicsSep 14, 201126 citations

_{5/2}

Rydberg state, an atom density of $1.6\times10^{10}

cm

^{-3}$ and a positive frequency scan we measure a third-order non-linearity of

χ^{(3)}=5\times 10^{-7}

^2

^{-2}

. For the reverse scan we observe a second order non-linearity of

χ^{(2)}=5\times 10^{-6}

^{-1}

. The contrasting behaviour can be explained in terms of motional effects and resonant excitation of Rydberg pairs.