Eric Van Stryland at CREOL, at the University of Central Florida in the USA
invented the Z scan technique for NLO materials characterization. So I suggest that you look up his papers on that (check his website), and email him directly for further questions. He may be reached via email
at ewvs[at]creol.ucf.edu also you may look him up through www.creol.ucf.edu Faculty. Jannick Rolland
The z-scan technique is a popular method for measuring optical nonlinearities, in which we translate a nonlinear sample about focal point of Gaussian laser and base on changes in transmittance through a finite aperture or beam radius to determine the nonlinear refractive index and nonlinear absorption coefficient of sample.