FIG. 4.

The Kerr coefficient is calculated ($n_2^{(\text{exact})}$, blue) as a function of the peak intensity for a two-level system for the conditions given in the text; ($n_2^{(\text{Zscan})}$, green) is determined by finding the $Z$-scan curve given the intensity-dependent Kerr coefficient presented here, finding the peak-valley difference on this curve, interpreting this peak-valley distance using low-intensity $Z$-scan theory, multiplying by constant factors following the procedure of McCormick et al. [8] for determining $n_2^{(s)}$. The inset shows the systematic error of this procedure using $\frac{n_2^{(\text{Zscan})}}{n_2^{(\text{exact})}}-1$. The saturation intensity is marked at $I=I_{\text{sat}}$ which corresponds to $s=1$.