Investigation of the influence of finite aperture size on the Z-scan transmittance curve

For the nonlinear materials, not only the ratio rho of the imaginary to the real part of the third-order nonlinear susceptibility but also the finite aperture size S exert a great influence upon the appearance of a peak on the Z-scan transmittance curve. This article investigates the rho dependence of the critical aperture size S-c, which determines whether a transmittance peak occurs or not, and the dependence of the critical value rho(c) on the parameter S for the thin-sample and low irradiance limits with a Gaussian beam. These results may be applied to the adjustment of experimental conditions at all times and the assessment of the correctness of data analysis. The experiment with CS2 seeking the critical value S-c has verified above results.