Giant Kerr effect in closed degenerate two-level transitions

Extremely large optical Kerr non-linearities (n2) and (n4) were measured in two closed degenerate two-level transitions in the caesium D2 line when a probe and a coupling laser simultaneously probed and coupled the hyperfine structure in an atomic beam. The magnitude of the measured non-linearities in both transitions were found to be a function of the coupling laser intensity and of the probe detuning from the two-photon resonance. In a figure of merit, the ratio between the non-unitarian part of the refractive index and the absorption coefficient measured in conditions of electromagnetically induced transparency showed a maximum value for a probe detuning of 160 kHz from the two-photon resonance and resulted in being 105 times larger than in the absence of the coupling laser field.