Investigating the nonlinear refraction index of Rb vapor by scanning the laser frequency through the atomic resonances.

We have observed self-focusing and defocusing in rubidium vapor by scanning the laser frequency through atomic resonances. These phenomena originate from the third order electric susceptibility, and have been used to measure the nonlinear refractive index of materials with the well-known technique Z-scan. However, few experimental measurements have been carried out near the resonances of atomic systems. In our experiment we scan the frequency of a diode laser through the resonances of rubidium vapor, contained in a fixed cell. The transmittance of the beam as a function of frequency detuning is detected after a small aperture, for different laser intensities and aperture radius. The theoretical model is a two-level system considering velocity distribution. A reasonably good fit of the experimental data is obtained, indicating that it is possible to measure the intensity-dependent term of the refractive index with this technique.