**2. Technique**

The usual "closed aperture" (CA) Z-scan technique (i.e. aperture in place in the far field) for nonlinear refraction is shown in **Figure 1** (sample displaying self-focusing). Sample transmittance of polarized Gaussian beam through the aperture is monitored by far field as a function of the Z position of NL in the vicinity of the linear optics focal position [10]. The scan range for the examination of the sample depends on the beam parameter and thickness of the sample L. A significant limitation is the diffraction length of the focused beam defined as a polarized Gaussian beam. A "thin" sample has a thickness of L. Even though all the information is theoretically contained within a scan range of Z direction, it is preferable to scan the sample for approximately 5 Z to determine the linear transmittance [2]. If the subjected material surface is rough or imperfect, it leads to background noise in the output. A reference detector can be used to monitor the normalized transmittance. To eliminate the possible noise due to spatial beam variations, this reference arm can be further to include a lens and an aperture identical to those in the nonlinear arm [11] as shown in **Figure 1**.
