A general Z-scan theory

A novel Z-scan theory based on the solution of the nonlinear paraxial wave equation, completed by the Huygens–Fresnel principle is introduced. This theory is valid for the general case, i.e. for thick samples and large nonlinearities including both nonlinear refraction and absorption. In both limiting cases of thin sample and weak nonlinearity, predictions of this model are in good agreement with theories not using parabolic approximation for the beam profile. It is shown that the widely used parabolic approximation leads to inadequate results when evaluating Z-scan measurements.