FIG. 1.

The real part of the electric field is given for (Incident, black) an incident ${\text{TEM}}_{00}$ Gaussian beam with maximum field strengths on $Z=0$ of (a) 25 kV/m, (b) 50 kV/m, and (c) 100 kV/m, for the field after passing through a thin Kerr medium with parameters from Ref. [8] as approximated (Exact, blue) using the saturable nonlinear susceptibility from a two-level system. (Z, orange) denotes the approximation of the $Z$-scan method, and (K, green) denotes an ideal Kerr susceptibility. The labels are further defined in Table I. For this system, $s=1$ corresponds to 38 kV/m.