Removal of the aperture, i.e., collecting all the light on detector, which is referred to as an open-aperture Z-scan, will result in a flat response for a purely refractive nonlinearity. It is because the sensitivity to nonlinear refraction is entirely due to the aperture, and its removal completely eliminates the effect. However, if nonlinear absorption is also present, then it will be reflected as a transmission variation in the open aperture scan. Multi-photon absorption suppresses the peak and enhances the valley while saturation of absorption produces the opposite effect in the closed-aperture Z-scan. Thus, apart from the magnitude of the nonlinearity (both real and imaginary parts), this technique provides a direct measurement of the sign of nonlinearity also. The sign of the nonlinearity is an important parameter for the practical realization of optical signal processing devices. This information cannot be obtained by the other commonly used techniques such as DFWM and THG.
Theory:
When matter is subjected to an intense laser radiation, it exhibits intensity dependent index of refraction given by (Lecture 20)
(26.1)
where is the linear refractive index and is the intensity dependent change in the refractive index. is called the refractive index intensity coefficient and is related to the real part of by
is the linear refractive index and 
is the intensity dependent change in the refractive index. 
is called the refractive index intensity coefficient and is related to the real part of 
by 