Experimental study of nonlinear focusing in a magneto-optical trap using   a Z-scan technique
  We present an experimental study of Z-scan measurements of the nonlinear response of cold Cs atoms in a magneto-optical trap. Numerical simulations of the Z-scan signal accounting for the nonuniform atomic density agree with experimental results at large probe beam detunings. At small probe detunings a spatially varying radiation force modifies the Z-scan signal. We also show that the measured scan is sensitive to the atomic polarization distribution. 
Yingxue Wang, M. Saffman