Theoretical study on the closed-aperture Z-scan curves in the materials with nonlinear refraction and strong nonlinear absorption

The features of the closed-aperture (CA) Z-scan transmittance curves have been studied in detail when the third-order nonlinear refraction and absorption, both of which can be independently positive or negative, are simultaneously present. We found that whether the peak-valley or valley-peak structure exists or not, depends on the ratio rho of the imaginary part chi ((3))(1) to the real part chi ((3))(R) of the third-order nonlinear susceptibility chi ((3)) which are caused by nonlinear refraction and absorption respectively. This communication also gives the rho dependences of the distance between the peak position and valley position, the difference between peak transmittance and the valley transmittance, the ratio of peak height to the valley depth for CA Z-scan, and especially the exact critical value rho (c) which determines whether the peak-valley or valley-peak structure exists or not, with Gaussian beams for the first time to our knowledge. (C) 2001 Elsevier Science B.V. All rights reserved.