Nonlinear index of refraction of a series of organic solvents, measured by the nonlinear imaging (z-scan) technique, is compared with the same values derived from optical third-harmonic generation (THG). The systematic differences in the values obtained from the z-scan measurements and from THG are attributed to three factors: the value of the standard used in THG measurements, important rotational contribution in z-scan measurements, and the difference in dispersion for n2 derived from Kerr susceptibility and those derived from THG measurements. Using the recently proposed standard silica susceptibility value, one obtains a very good agreement for silica and significant differences for the solvents, particularly for CS2 and chlorobenzene. A correction based on a three-level model for centrosymmetric media is applied for dispersion.
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