Single-shot reflection <Emphasis Type="Italic">Z</Emphasis>-scan for measurements of the nonlinear refraction of nontransparent materials

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. PACS 32.70.Jz; 42.50.Gy; 42.65-k